Embryology-all basic definition,Stage wise development of fetus,development of Zygote stage ,development of Embrionic Stage ,development of Fetus Stage all are according week development,Amnione,chorion,Fetal layer, Umbilical Cord development

Embryology-all basic definition,Stage wise development of fetus,development of Zygote stage ,development of Embrionic Stage ,development of Fetus Stage all are according week development,Amnione,chorion,Fetal layer, Umbilical Cord developmentmade By sonal Patel

1. Embryology

2. INTRODUCTION Embryology: In its widest sense is applied to the various changes which take place during the growth of an animal from the egg to the adult condition. It is, however, usually restricted to the phenomena, which occur before birth. Embryology may be studied from two aspects (1) that of ontogeny which deals with the development of the individual; and (2) that of phylogeny, which concerns itself with the evolutionary history of the animal kingdom. Developmental Periods: Human development is divided into two: Prenatal (before birth): the main developmental changes occur during prenatal phase. Postnatal: (after birth). Embryological Terminology: The following terms are commonly used in discussions of developing humans: Oocyte (ovum, egg): the female germ or sex cell produced in the ovaries. When mature, the oocyte is called a secondary oocyte or mature oocyte . Sperm (spermatozoon): Refers to the male germ cell produced in the testes (testicle). Zygote: This cell results from the union of an oocyte and a sperm during fertilization. A zygote is the beginning of a new human being. Cleavage: This is the series of mitotic cell divisions of the zygote that result in the formation of early embryonic cells – blastomeres . The size of the cleaving zygote remains unchanged because at each succeeding cleavage division the blastomeres becomes smaller.

3. Morula : This solid mass of 12 to about 32 blastomeres is formed by cleavage of a zygote. Blastocyst : After the morula enters the uterus from the uterine tube (fallopian tube), a fluid filled cavity– develops inside it. This change converts the morula into a blastocyst . Implantation: The process during which the blastocyst attached to the endometrium and subsequently embeds in it. Gastrula: During gastrulation (transformation of a blastocyst into a gastrula), a three layered embryonic disc forms (third week). The three germ layers of the gastrula ectoderm, mesoderm and endoderm, subsequently differentiate into the tissues and organs of the embryo. Neurula : The early embryo during the third and fourth weeks, when the neural tube is developing from the neural plate. Embryo: The developing human during its early stages of development. The end of the eighth week ( 56 days. Conceptus : The embryo and associated membranes. Primordium : The beginning or first discernible indication of an organ or structure. Fetus: After the embryonic period (eight weeks), the developing human is called a Fetus: During the fetal period (ninth week to birth), differentiation and growth of the tissues and organs formed during the embryonic period occur. Abortion (miscarry): A premature stoppage of development and expulsion of a Conceptus from the uterus. Trimesto : A period of three calendar months during a pregnancy. The most critical stages of development occur during the first trimester (13 weeks). Congenital Anomalies or Birth Defects: Abnormalities of development that are present at birth (born with).

4. Postnatal period : The changes occurring after birth – the development of teeth and breasts, for example, are more or less familiar to most people. Infancy: Refers to the earliest period of extra uterine life; roughly the first year after birth – An infant aged 1 month or less is called a new born or neonate. Childhood: Is the period from about 13 months until puberty. Puberty: Is the period, usually between the age 12 and 15 years in girls and 13 and 16 years in boys. Adolescence: Is the period from 11 to 19 years of age, which is characterised by rapid physical and sexual maturation. Adulthood: Attainment of full growth and maturity – is generally reached between the ages of 18 and 21 years. Ossification and growth are virtually completed during early adulthood. States Of Development About 280 days after fertilization a new individual is born. These 10 lunar months are divided into 3 periods (or stages). 1. Zygote (fertilized ovum) stage: The first two weeks. 2. Embryonic stage: From the beginning of the third week to the end of the second month (eighth week) 3 rd week – birth What is the value of studying Embryology? Bridges the gap between prenatal development and obstetrics, prenatal medicine, paediatrics, and clinical anatomy. Develops knowledge concerning the beginnings of human life and the changes occurring during prenatal development. Is of practical value in helping to understand the causes of variations in human structure.

5. Illuminates gross anatomy and explains how normal and abnormal relations develop. It is important to pathology, because abnormalities which may occur during development result in malformations which appear in the adult. Branches of Embryology: Developmental anatomy: Is the field of embryology concerned with the changes that cells, tissues, organs and the body as a whole undergo from a germ cell of each parent to the resulting adult. Teratology: Is the division of embryology and pathology that deals with abnormal development (birth defects): This branch of embryology is concerned with various genetic and/or environmental factors that disturb normal development and produce birth defects. Pregnancy Symptoms: What to expect during the first trimester. The first few months of pregnancy are marked by an invisible – yet amazing transformation. Knowing what pregnancy symptoms to expect can help you face the months ahead with confidence. Your Body: Within two weeks of conception, hormones triggers your body to begin nourishing the baby – even before tests and a physical examination can confirm the pregnancy. Here are some common physical changes you may notice early on: Tender breast: Increased hormone production may make your breasts unusually sensitive. Your breasts will probably feel fuller and heavier wearing a more supportive bra or a sport bra may help. Bouts of nausea: Many women have queasiness, nausea or vomiting in early pregnancy – probably due to normal hormonal changes. Nausea tends to be worse in the morning, but it can last all day. Unusual fatigue: You may feel tired as your body prepares to support the pregnancy your heart will pump faster and harder. Make sure you are getting enough iron and protein. Include phsical activity in your daily routine.

6. The three stages of development the first two weeks are known as the ZYGOTE STAGE. At this stage all cells look like each other From the beginning of the third week to the end of the second month( eighth weeks) 3 rd week- 2 nd month is called EMBRYONIC STAGE, at this stage the tissues, organs and systems are formed The period from the beginning of the third month till the time of birth is called THE FETAL STAGE, at this stage the body grows in length and in weight

7. Increased urination: You may need to urinate more often as your enlarging uterus presses on your bladder during the first few months. The same pressure may cause you to leak urine when sneezing, coughing or laughing. Dizziness: Normal circulatory changes in early pregnancy may leave you feeling a little dizzy, stress, fatigue and hunger also may play a role. Your Emotions: Pregnancy may leave you feeling delighted, anxious, exhilarated and exhausted – sometimes all at once. It’s natural to .worry about your baby’s health, your adjustment to motherhood and and the financial demands of raising a child. You may wonder how the baby will affect your relationship with your partner or what type of parent you will be. Your relationship with your partner: Becoming a mother takes time away from other roles and relationships. You may lose some of your psychological identity as a partner and lover but good communication can help you keep intimacy a live. Be honest. Be patient. Be supportive.

8. THE BIGINNING OF HUMAN DEVELOPMENT First Week Human development begins at fertilization when a male gamete (or sperm) unites with a female gamete (or ovum) to form a single cell – a zygote. Sperm + Ovum zygote fertilization The zygote marked the beginning of each of us a unique individual it contains chromosomes and genes that are derived from the mother and father. The unicellular zygote divides many times and becomes progressively transformed into a multi cellular human being through cell division , migration, growth and differentiation. Although development begins at fertilization, the stages and duration of pregnancy described in clinical medicine are calculated from the commencement of the mothers last normal menstrual period (LNMP), which is about 14 days before conception occurs. Gametogenesis: Gametogenesis (gamete formation): Is the process of formation and development of specialized generative cells (gametes). This process involving the chromosomes and cytoplasm of the gametes, prepares these sex cells for fertilization. During gametogenesis, the chromosome number is reduced by half and the shape of the cells are altered. Gametogenesis (gamete formation) Spermatogenesis (sperm formation Oogenesis (egg formation)

9. What is the purpose of spermatogenesis? Two things : 1. To reduce the number of chromosomes from 46 to 23. 2. To change the shape of the male sex cell in order to become ready for fertilization. Site: Spermatogenesis takes place in the somniferous tubules of the uterus. occurrence: Spermatogenesis occurs continuously from puberty to old age. Stages: Spermatogenesis is divided into two stages: A. Spermato - Cyto - genesis B. Spermio - geneis Spermato-cyto-genesis = Spermatogonium Primary spermatocyte Secondary spermatocyte Spermatid Spermio – genesis = Spermatid Sperm Spermatogonium Primary Spermatocyle Seconday spermatocyle Spermotids Sperm

10. Spermatogonia, which have been dormant in the seminiferous tubules of the testes since the fetal period begin to increase in number at puberty. After several mitotic divisions, the spermatogonia grow and undergo changes that transform them into primary spermatocytes, the largest germ cells in the seminiferous tubules. Each primary spermatocyte subsequently undergoes a reduction division – the first meiotic division to form two haploid secondary spermatocytes, which are about half the size of primary spermatocytes. Subsequently the secondary spermatocytes undergo a second meiotic division to form four haploid spermatids, which are about half the size of secondary spermatocytes. The spermatids are gradually transformed into four mature sperms by a process known as spermiogenesis. N.B: The entire process of spermatogenesis takes about two months. When spermiogenesis is completed the sperms enter the lumina of the seminiferous tubules. Sertoli cells lining the seminiferous tubules support and nurture the germ cells and may be involved in the repulation of spermatogenesis.

11. . Oogenesis Oogenesis (ovogenesis): Is the sequence of events by which oogenia are transformed into mature oocytes. This maturation process begins before birth and is completed after puberty (12-15 years) and continues to menopause. Prenatal Maturation of Oocytes: During early fetal life, oogonia proliferate by mitotic division. Oogonia enlarge to form primary oocytes before birth. As a primary oocyte forms, connective tissue cells surround it and form a simple layer of flattened follicular epithelium cells. The primary oocyte enclosed by this layer of cells constitute a primordial follicle. As the primary oocyte enlarges during puberty, the follicular epithelium cells become cubical in shape and columnar, forming primary follicle. The primary oocyte soon becomes surrounded by a covering of amorphous a cellular glyco-protein material – the zona pellucida. When the primary follicle has more than one layer of follicular cells, it is called a secondary follicle. Primary oocytes begin the first meiotic division before birth but completion of prophase does not occur until adolescence (11-19 years). The follicular cells surrounding the primary oocyte are believed to secrete a substance, oocyte maturation inhibitor (OMI), which keeps the meiotic process of the oocyte arrested.

12. Postnatal Maturation of Oocytes Beginning during puberty usually one follicle matures each month and ovulation occurs, except when oral contraceptives are used. As a follicle matures, the primary oocyte increases in size and, shortly before ovulation, completes the first meiotic division, however, the division of cytoplasm is unequal – the secondary oocyte receives almost all the cytoplasm and the first polar body receives very little. The polar body is a small, non functional cell that soon degenerates. At ovulation, the nucleus of the secondary oocyte begins the second meiotic division, but progresses only to metaphase, when division is arrested. If a sperm penetrates the secondary oocyte, the second meiotic division is completed and most cytoplasm is again retained by one cell, the fertilized oocyte. The other cell, the second polar body also a small non-functional cell, soon degenerates. There are about two million primary oocytes in the ovaries of a new born female, but many regress during childhood so that by adolescence nor more than 40 thousand remain of these only about 400 become secondary oocytes and are explelled at ovulation during the reproductive period

13. The Female Genital Organs Uterus: The uterus (womb) is a thick-walled, pear-shaped muscular organ that varies considerably in size. The uterus averages 7 to 8 cm in length, 5 to 7 cm in width at its superior part, and 2’ to 3cm in thickness. The uterus consists of two major parts: 1. Body: The expanded superior two-thirds. 2. Cervix: The cylindrical inferior third. The body of the uterus narrows from the fundus (the rounded, superior part of the body) to the isthmus (the 1cm long constricted region between the body and cervix (neck). The cervix of the uterus is the tapered vaginal end that is nearly cylindrical in shape. The lumen of the cervix, the cervical canal has a constricted opening at each end. The internal os communicates with the cavity of the uterine body and the external os communicates with the vagina. The walls of the body of the uterus consist of three layers: 1. Perimetrium: The thin external layer. 2. Myometrium: The thick smooth muscle layer. 3. Endometrium: The thin internal layer.

14. Uterine (Fallopian) Tubes: The uterine tubes, 10 to 12 cm long and 1 cm in diameter, extend laterally from the horns of the uterus. The tubes carry oocytes from the ovaries and sperms entering from the uterus to reach the fertilization site in the ampulla of the uterine tube. The uterine tube also conveys the cleaving zygote to the uterine cavity. Each tube opens at its proximal end into the horn of the uterus and into the peritoneal cavity at its distal end. For descriptive purposes, the uterine tube is divided into four parts: 1. infundibulum:- is the part which opens in to the peritoneal cavity 2. Ampulla: is the wide and tortuous part which follows the infundibulum 3. isthmus: is a narrow and straight part and has a thick wall 4. Interstitial part : is the part which pierces the wall of uterus Ovaries: The ovaries are almond – shaped reproductive glands located close to the lateral pelvic walls on each side of the uterus. The ovaries produce estrogens and progesterone, the hormones responsible for the development of secondary sex characteristics and regulation of pregnancy. The ovaries are also responsible for producing and maintaining oocytes.

15. Uterine (Fallopian) Tubes : The uterine tubes, 10 to 12 cm long and 1 cm in diameter, extend laterally from the horns of the uterus. The tubes carry oocytes from the ovaries and sperms entering from the uterus to reach the fertilization site in the ampulla of the uterine tube. The uterine tube also conveys the cleaving zygote to the uterine cavity. Each tube opens at its proximal end into the horn of the uterus and into the peritoneal cavity at its distal end. For descriptive purposes, the uterine tube is divided into four parts: 1. Incunabulum. 2. Ampulla. 3. Uterine part (interstitial part) Ovaries: The ovaries are almond – shaped reproductive glands located close to the lateral pelvic walls on each side of the uterus. The ovaries produce estrogens and progesterone, the hormones responsible for the development of secondary sex characteristics and regulation of pregnancy. The ovaries are also responsible for producing and maintaining oocytes.

16. Female Reproductive Cycle: Commencing at puberty and normally continuing throughout the reproductive years. Females undergo monthly reproductive cycles (sexual cycles), involving activities of the hypothalamus of the brain, pituitary gland, ovaries, uterus, uterine tubes, vagina and mammary glands. These monthly cycle prepare the reproductive system for pregnancy. A gonadotropin – releasing hormone (Gn RH) is synthesized by neurosecretory cells in the hypothalamus and is carried by the hyperphysical portal system to the anterior tube of the pituitary gland. Gn RH stimulates the release of two hormones produced by this gland that act on the ovaries. Follicle: Stimulating hormone (F SH) stimulates the development of ovarian follicles and the production of estrogen by its follicular cells. Luteinizing hormone (LH) serves as the trigger for ovulation (release of secondary oocyte) and stimulates the follicular cells and corpus luteum to produce progesterone. These hormones also induce growth of the endometrium. Ovarian Cycle : FSH and LH produce cyclic changes in the ovaries Development of follicles. Ovulation. 3. Corpus luteum formation

17. Follicular Development Development of an ovarian follicle is characterized by: - Growth and differentiation of primary oocyte. - Proliferation of follicular cells. - Development of theca folliculi. As the primary follicle increases in size, the adjacent connective tissue organizes into a capsule, the theca folliculi. The theca soon differentiate into two layers, an internal vascular and glandular layer – the theca interna and a capsule like layer, the theca externa. Thecal cells are thought to produce an angiogenesis factor that promotes growth of blood vessels in the theca interna which provide nutritive support for follicular development. The follicular cells divide actively producing a stratified layer around the oocyte. The ovarian follicle soon becomes oval and the oocyte eccentric in position. Subsequently fluid – filled spaces appear around the follicular cells, which coalesce to form a single large cavity, the autrum, which contains follicular fluid. After the autrum forms the ovarian follicle is called a vesicular or secondary follicle. The primary oocyte is pushed to one side of the follicle, where it is surrounded by a mound of follicular cells, the cumulus oophorus, that projects into the autrum. The follicle continues to enlarge until it reaches maturity and produces a swelling on the surface of the ovary. The early development of ovarian follicles is introduced by FSH, but final stages of maturation require LH as well. Growing follicles produce estrogen.

18. Ovulation Around mid cycle (about 14 days in a 28 day menstrual cycle), the ovarian follicle under the influence of FSH and LH undergoes a sudden growth spurt, producing a cystic swelling or bulge on the surface of the ovary. A small a vascular spot, the stigma, soon appears on this swelling. Prior to ovulation, the secondary oocyte and some cells of the cumulus oophorus detach from the interior of the distended follicle. Ovulation is triggered by a surge of LH production. Ovulation usually follows the LH peak by 12 to 24 hours. The LH surge, elicited by the high estrogen level in the blood, appears to cause the stigma to balloon out, forming a vesicle. The stigma soon ruptures, expelling the secondary oocyte, with the follicular fluid. The expelled secondary oocyte is surrounded by the zona pellucid a and one or more layers of follicular cells, which are radially arranged as the corona radiata, forming the oocyte cumulus complex. Corpus Luteum Shortly after ovulation the walls of the ovarian follicle and theca folliculi collapse and thrown into folds: Under LH influence they develop into a glandular structure – the corpus luteum – which secretes progesterone, and some estrogen. These hormones particularly progesterone cause the endometrial glands to secrete and prepare the endometrial for implantation of the blastocyst.

19. If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of pregnancy and increases its hormone production. When pregnancy occurs, degeneration of the corpus luteum is prevented by human chorionic-gonadotropin (HCG),a hormone secreted by the syncytio-trophoblast of the blasto cyst, which is r.ch.n LH. The corpus luteum of pregnancy remains functionally active throughout the first 20 weeks of pregnancy. By this time, the placenta has assumed the production of the estrogen and progesterone that is necessary for the maintenance of pregnancy. Menstrual Cycle: The menstrual cycle is the period during which the oocyte matures, is ovulated and enters the uterine tube. The hormones produced by the ovarian follicles and corpus luteum (estrogen and progesterone) produce cyclic changes in the endomatrium. These monthly changes in the internal layer of the uterus constitute the endometrial cycle, commonly referred to as the menstrual cycle. Phases of the Menstrual Cycle: 1. Menstrual phase. 2. Proliferative phase. 3. Luteal phase

20. Menstrual phase: The first day of menstruation is the beginning of the menstrual cycle’ - The functional layer of the uterine wall is sloughed off and discharged with the menstrual flow – menses (monthly bleeding) – which usually lasts 4 to 5 days. Proliferative phase: Follicular (estrogenic) phase lasting about 9 days, coincides growth of ovarian follicles and is controlled by estrogen secreted by these follicles. - There is a two – to three – fold increase in the thickness of the endometrium and in its water content during this phase of repair and proliferation. - During this phase the surface palladium reforms and covers the endometrium. - The glands increase in number and length and the spiral arteries elongate. Luteal phase: Secretory (Progesterone) phase, lasting about 13 days, coincides with the formation, functioning and growth of the corpus luteum. - The progesterone produced by the corpus luteum stimulates the glandular epillalium to secrete a glycogen-rich material. - The glands become wide, tortuous and secular. - Endometrium increases in thickness. - Venous spaces develops. - Direct arterio-venous anastomose are prominent features of this stage.

21. Fertilization: Fertilization is a complex sequence of coordinated molecular events that begins with contact between a sperm and oocyte and ends with the intermingling of maternal and paternal chromosomes at metaphor of the first mitotic division of the zygote. Phases of Fertilization: Passage of sperm through corona radiate . Penetration of zona pellucid a Fusion of plasma membranes of the oocyte and sperm. Completion of second meiotic division of oocyte and formation of female pronucleus. Formation of male pronucleus. As the pronucli fuse in a single diploid aggregation of chromosomes, the Ootid becomes a zygote. The chromosomes in the zygote become arrayed on a cleavage spindle, in preparation for cleavage of the zygote. N.B: An early pregnancy factor (EPF), an immuno suppressant protein, is secreted by the trophoblastic cells and appears in the maternal serum within 24 to 48 hours after fertilization. EPF forms the basis of a pregnancy test during the first 10 days of development. Importance of Fertilization: Stimulates the penetrated oocyte to complete the second meiotic division. Restores the normal diploid number at chromosomes (46) in the zygote. Results in variation of the human species through mingling of maternal and paternal chromosomes. Determines chromosomal sex of the embryo. Causes metabolic activation of the ootid and initiates cleavage of the zygote.

22. Cleavage of Zygote: Cleavage consists of repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells. These embryonic cells – blastameres – become smaller with each cleavage division. First the zygote divides into two blastomeres, which then divide into four blastomeres, eight blastomeres and so on. Cleavage normally occurs as the zygote passes along the uterine tube toward the uterus. During cleavage, the zygote is within the rather thick zone pellucida that is translucent under the light microscope. Division of the zygote into blastomeres begins about 30 hours after fertilization. After the nine-cell stage, the blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells. This phenomenon – Compaction – probably mediated by cell surface adhesion glycoprotein. When there are 12 to 32 blastomeres, the developing human is called a morula Internal cells of the morula are surrounded by a layer of cells that form the outer cell layer. The spherical morula forms about 3 days after fertilization and enters the uterus.

23. Formation of Blastocyst: Shortly after he morula enters the uterus (about 4 days after fertilizaion). A fluid-filled space called the blasto cystic cavity appears inside the morula. The fluid passes from the uterine cavity through the zona pellucida to form this space. As fluid increases in he blasto cystic cavity it separates the blastomeres into two parts: 1. A thin, outer cell layer – the trophoblast – which gives rise to the embryonic part of the placenta. 2. A group of centrally located blastomeres – the inner cell mass – which gives rise to the embryo: Because it is the primordium of the embryo, the inner cell mess is called the embryoblast. About 6 days ater fertilization, the blasto cyst attaches to the endometrial epithelium, usually adjacent to the embryonic pole. As soon as it attaches to the endometrial epillalium, the trophoblast starts to proliferate rapidly and gradually differentiates into two layers: 1. An inner layer of cyto trophoblast. 2. An outer mass of syncytiotrophoblast.

24. Second Week Formation of Bilaminar Embryonic Disc: Completion of implantation and continuation of embryonic development. Implantation of the blastocyst which commenced at the end of the first week, is completed by the end of the second week. The erosive syncytiotrophoblast involves the endometrial connective tissue which supports the endometrial capillaries and glands. As this occurs, the blastocyst slowly embeds itself in the endometrium. The endometrial cells undergo apoptosis (programmed cell death), which facilitates the invasion of the maternal endometrium during implantation. Proteolytic enzymes produced by the syncytiotrophoblast, as well as COX -2 derived prostacyclin and fas ligual present at the implantation site, are involved in this process. The connective tissue cells around the implantation site accumulate glycogen and lipids and assume a polyhedral appearance, some of cells – decidual cells – degenerate adjacent to the penetrating syncytiotrophoblast. The syncytiotrophoblast engulfs these degenerating cells, providing a rich source of embryonic nutrition.

25. Formation of Amniotic Cavity Embryonic Disc and Yolk Sac: As implantation of the blasto cyst progresses, a small space appears in the embryo blast which is the primordium of the amniotic cavity. Rapid morphological changes occur in the embryo blast that result in the formation of a flat almost circular bilaminar plate of cells, the embryonic disc consisting of two layers: a) Epiblast: ( ectoderm )The thicker layer consisting of high columnar cells related to the amniotic cavity. b) Hypoblast ( endoderm) Consisting of small cuboidal cells adjacent to the exocoelomic cavity. The epiblast forms the floor of the amniotic cavity and is continuous peripherally with the amnion. The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic membrane. Exocoelomic membrane plus hypoblast forms the primary yolk sac. The embryonic disc now lies between the amniotic cavity and the primary yolk sac. Cells from the yolk sac endoderm form a layer of connective tissue, the extra embryonic mesoderm, which surrounds the amnion and yolk sac. As the amnion, embryonic disc and primary yolk sac form isolated cavities – lacunae appear in the syncytiotrophobllast. The 10-day human conceptus embryo and extra embryonic membrane is completely embedded in the endometrium. As the conceptus implants, the endometrial connective tissue cells undergo a transformation – the decidual reaction. After the cells swell because of the accumulation of glycogens and lipid in the cytoplasm, they are known as decidual cells. In a 12-day embryo, adjacent syncytiotrophoblastic lacunae have fused to form lacunar networks.

26. Development of Chorionic Sac: The end of the second week is characterized by the appearance of primary chorionic vile. Proliferation of cytotrophoblastic cells produces cellular extensions that grow into the syncytiotrophoblast. The growth of these extensions is though to be induced by the underlying extra embryonic somatic mesoderm. The cellular projections form primary chorionic vile, the first stage in the development of the chorionic vile of the placenta. The extra embryonic coelom splits the extra embryonic mesoderm into two layers: 1. Extra embryonic somatic mesoderm: Lining the trophoblast and covering the amnion. 2. Extra embryonic splanchnic mesoderm: Surrounding the yolk sac. The extra embryonic somatic mesoderm and the two layers of trophoblast form the chorionic, the chorion forms the wall of the chorionic sac, within which the embryo and its amniotic and yolk sacs are suspended by the connecting stalk.

27. Formation of Germ Layers and Early Tissue and Organ Differentiation: Third Week: Gastrulating: Formation of germ layers. Gastrulating is the formation process by which the three germ layers and axial orientation are established in embryos. During gestrulation, the bilaminar embryonic disc is converted into a trilaminar embryonic disc. Gastrulation is the beginning of morphogenesis (development of body form). Each of the three germ layers (ectoderm, mesoderm and endoderm) gives rise to specific tissues and organs. 1. Embryonic ectoderm: Gives rise to the epidermis, central and peripheral nervous systems, retina of the eye 2. Embryonic endoderm: . Is the source of the epithelial linings of the reparatory passages and gastro intestinal tract. 3. Embryonic mesoderm: Gives rise to smooth muscular coats, connective tissues and vessels associated with the tissues and organs, cardiovascular system the skeletal and striated muscles, reproductive and excretory organs. Primitive Streak: The fist signs of gastrulations is the appearance of the primitive streak. The primitive streak result from the proliferation and migration of cells of the epiblast to the median plane of the embryonic disc. As the streak elongates by addition of cells to its caudal end, its cranial end proliferates to form a primitive node. Concurrently a narrow groove – primitive groove – develops in the primitive streak that is continuous with a small depression in the primitive node – the primitive pit.

28. As soon as primitive streak appears, it is possible to identify the embryos craniocandal axis, its cranial and candal ends, its dorsal and ventral surfaces and its right and left sides. Shortly after the primitive streak appears cells leave its deep surface and form mesenchyone, a tissue consisting of loosely arranged cells suspended in gelatinous matrix. ` Mesenchymel cells are ameboidal and actively phagocytic, they form the supportive tissues of the embryo, such as most of the connective tissues of the body and the connective tissue frame work of glands. Some mesenchyone forms mesoblast (un differentiated mesoderm), which forms the intra embryonic or embryonic mesoderm. Cells from the epiblast displace the hypoblast, forming the intra embryonic or embryonic endoderm in the root of the yolk sac. The cells remaining in the epiblast form the intra embryonic or embryonic ectoderm. Fate of Primitive Streak The primitive streak actively forms mesoderm until the early part of the fourth week, thereafter, production of mesoderm slows down. The primitive streak diminishes in relative size and becomes an insignificant structure in the sacro ccygeal region of the embryo. Normally the primitive streak undergoes degeneration changes and disappears by the end of the fourth week.

29. Notochordal Process and Notochord: Some mesenchymal cells migrate cranially from the primitive node and pit forming a median cellular cord, the notochordal process. The process soon acquires a lumen, the notochondel canel. The notochordel process grows cranially between the ectoderm and endoderm until it reaches the prechondral plate, a small circular area of columnar endodermal cells where the ectoderm and endoderm are in contact. The prechordal plate is the primordium of the oropharyngeal membrane. Some mesenchyonal cells from the primitive streak migrate cranially on each side of the notochordal process and around the prechordal plate. Here they meet cranially to form cardiogenic mesoderm in the cardiogenic area, where the heart primordium begins to develop at the end of the third week. caudal to the primitive streak there is a circular area, the cloacal membrane which indicates the future site of the anus. The notochord is a cellular rod that develops by transformation of the notochordal process.

30. The notochordal development the notochordal process elongates by invagination of cells from the primitive pit The primitive pit extends in to the notochordal process, forming the notochordal canal The notochordal process is now a cellular tube that extends from the primitive node to the prechordal plate The floor of the notochordal process fuses with the underlying embryonic endoderm The fused layers gradually undergo degeneration, resulting in the formation, which brings the notochordal canal in to communication with the york sac The opening rapidly become confluent and the floor of the notochordal canal disappears

31. The remains of the notochordal process form a flattened, grooved notochordal plate Beginning at the cranial end of the embryo , the notochordal cells proliferate and the notochordal plate in folds to form the notochord The proximal part of the notochordal canal persists temporarily as the NEURENTERIC CANAL, which forms the communication between the amniotic and york sac cavities The notochord becomes detached from the endoderm of the york sac N.B . The notochord is an intricate structure around which the verterbral column forms. It extends from the oropharyngeal membrane to the primitive node The notochord functions as the primary inductor in the early embryo

32. ALLANTOIS The allantois appears on about day 16 as a small, sausage-shaped diverticulum( out pouching) from the caudal wall of the yolk sac that extends in to the connective stalk In human embryo the allantois remains very small , because the placenta and amniotic sac take over its functions The allantois is involved with the early blood formation in the human embryo and is associated with development of the urinary bladder As the bladder enlarges , the allantois becomes the URACHUS, which is represented in adults by the MEDIAN UMBILICAL LIGAMENT The blood vessels of the allantois become the umbilical arteries and veins ALLANTOIC CYSTS : Is the remnants of the extraembryonic portion of the allantois , are usually found between the fetal umbilical vessels and can be detected by ultrasonography. They are most detected in the proximal part of the umbilical cord, near its attachment to the anterior abdominal wall

33. Allantoic cyst

34. Neurulation The processes involved in the formation of the formation of the neural plate and neural folds and the closure of the folds to form the neural tube constitute neurulation These processes are completed by the end of the fourth week During the neurulation , the embryo may be referred to as a neurula As the notochord develops , the embryonic ectoderm over it thickens to form an elongated , slipperlike plate of thickened epithelial cells, THE NEURAL PLATE . The ectoderm of neural plate gives rise to the brain and spinal cord (CNS) At first the elongated neural plate corresponds in lengths to the underlying notochord. It appears cranial to the primitive node and dorsal to the notochord and the mesoderm adjacent to it As the notochord elongates, the neural plate broadens and eventually extends cranially as far as the oropharyngeal membrane

35. On the 18 th day, the neural plate invaginates along its central axis to form a longitudinal median NEURAL GROOVE, which has neural folds on each side The neural folds becomes prominent at the cranial end of the embryo and are the first signs of brain development By the end of the third week, the neural folds fuse , converting the neural plate in to a NEURAL TUBE, the primordium of the CNS

36. Neural Crest Formation As the neural folds fuse to form the neural tube , some neuroectodermal cells lying along the crest of each neural fold lose their epithelial affinities and attachments to neighboring cells As the neural tube separates from the surface ectoderm, NEURAL CREST CELLS, migrate dorsolaterally on each side of the neural tube They soon form a flattened irregular mass, the neural crest The neural crest soon separates in to right and left parts that migrate to the dorsolateral aspects of the neural tube; here they give rise to the sensory ganglia of the spinal and cranial nerves The neural crest cells also contribute the formation of pigment cells, the suprarenal medulla and several skeletal and muscular components of the head

37. NEURAL TUBE DEFECTS Neural tube defects ( NTDs) are among the most common congenital anomalies MEROANENCEPHALY: partial absence of the brain

38. The derived MESODERM will be splitted in to three parts Paraxial Mesoderm: which gives rise to SOMITES, somites will be divided in to SCLEROTOME and DERMO-MYOTOME Intermediate Mesoderm: which gives rise to the kidneys, testes, ovaries and cortex of the suprarenal glands Lateral Plate Mesoderm: it gives rise Intra-embryonic Coelomic Cavity

39. Development of somites About the 17 th day of development the paraxial mesoderm starts to be formed on either side of the notochord About the 20 th day the paraxial mesoderm starts to be divided in to little blocks( paired cuboidal bodies) called somites The division of paraxial mesoderm in to somites begins at the 20 th and continues up to the 35 th or even 40 th day Most somites are clearly seen by the end of the first months 42 to 44 somites are formed , However, the only the first 30 somites can be easily seen Because the somites are so prominent during the fourth and fifth weeks , they are used as one of several criteria for determining an embryo's age

40. FORMATION OF INTRAEMBRYONIC COELOM The coelom( cavity) with in the embryo arises as isolated spaces in the lateral mesoderm and cardiogenic mesoderm The coelomic vesicles subsequently coalesce to form a single, horseshoe- shaped cavity that eventually gives rise to the body cavity, the peritoneal cavity for example

41. Formation of blood vessels and blood Blood vessels first appear in the wall of the yolk sac, allantois and chorion They develop within the embryo shortly thereafter Spaces appear within aggregations of mesenchyme, known as BLOOD ISLANDS. The spaces soon become lined with endothelium derived from the mesenchymal cells These primordial tubules sprout and unite with other vessels to form a primordial cardiovascular system Toward the end of the third week, the heart is represented by paired endocardial heart tubes that are joined to blood vessels in the embryo and in the extraembryonic membranes ( yolk sac, umbilical cord and chor- ionic sac) By the end of the third week , the heart tubes have fused to form a tubular heart that is joined to vessels in the embryo

42. The primordial blood cells – HEMANGIOBLASTS, are derived mainly from the endothelial cells of blood vessels in the walls of the yolk sac and allantois Fetal and adult erythrocytes probably develop from different hematopoietic precursors

43. Completion of chorionic villi formation Primary chorionic villi become secondary chorionic villi as they acquire mesenchymal cores Before the end of the third week , capillaries develop in the secondary chorionic villi, transforming them in to tertiary chorionic villi Cytotrophoblastic extensions from these stem villi join to form a CYTOTROPHOBLASTIC SHELL, that anchors the chorionic sac to the endometrium The rapid development of chorionic villi during the third week greatly increases the surface area of the chorion for the exchange of oxygen and nutrients and other substances between the maternal and embryonic circulations.

44. Derivatives of the mesodermal germ layer Connective tissue Cartilage Bone Joints Striated muscles Smooth muscles The heart Blood and lymph vessels Red and white blood corpuscles Spleen Serous membrane Cortex of suprarenal glands Kidneys Tests and ovaries

45. Derivatives of the endodermal germ layer All epithelium lining of the Digestive tube Respiratory system Middle ear and pharyngotympanic tube Urinary bladder and urethra 2. The parenchyma of the Tonsil thyroids, parathyroid and thymus Liver pancreas

46. Derivatives of the ectodermal germ layer The nervous system The sensory epithelium of the sense organs The pituitary gland The epithelium of the skin including the hairs, nails and skin glands Mammary glands Subcutaneous glands Enamel of teeth

47. Organogenetic period: Fourth to Eighth Weeks The fourth to eight weeks of development constitute most of the embryonic period By the end of the Organogenetic period , the main organ systems have begun to develop; however, the function of most of them is minimal, except for the cardiovascular system As the tissues and organs form, the shape of the embryo changes and by the eighth week it has a distinctly human appearance.

48. Phases of embryonic development The first phase is GROWTH, which involves cell division and elaboration of cell products The second phase is MORPHOGENESIS; ( development of shape, size or other features of a particular organ or part or the whole of the body The third phase is DIFFERENTIATION; ( maturation of physiological processes, which results in the formation of tissues and organs that are capable of performing specialized functions

49. Folding of the embryo A significant event in the establishment of body form is folding of the flat trilaminar embryonic disc in to a somewhat cylindrical embryo Folding occur in both median and horizontal planes and results from rapid growth of the embryo At the time when the somites are developing, the embryonic disc begins to bulge in to the amniotic cavity and starts to be folded in an antero-posterior(cephalo-caudal) direction When the embryo reaches the 7 somites stage a head fold as well as tail fold are formed The head fold arises as result of growth of the embryo in a cranial direction while the tail fold arises as a result of growth of the embryo in a caudal direction While the embryo is undergoing folding in an antero-posterior direction, the embryonic disc becomes gradually lifted from the yolk sac and in this way two lateral folds are formed

50. Embryonic folds

51. FOURTH WEEK Major changes in body form occur during the fourth week At the beginning , the embryo is almost straight and has 4 to 12 somites that produce conspicuous surface elevations The neural tube is formed opposite the somites, but is widely open at the rostral( anterior and posterior NEUROPORES , By 24 day the pharyngeal arches are visible The first ( mandibular arch) and the second ( hyoid arch) are distinct Three pairs of pharyngeal arches are visible by 26 days, and the rostral neuropores are closed The forebrain produces a prominent elevation of the head, and the folding of the embryo has given the embryo a C-shaped curvature Along , curved caudal eminence( tail like structure ) is present The upper limb buds ; become recognizable by day 26 or 27 as small swellings on the ventrolateral body walls The lower limbs ; are visible by the end of the fourth week

52. Fourth week embryo

53. FIFTH WEEK Changes in body form are minor during the fifth week compared with those that occurred during the fourth week But growth of the head exceeds that of other regions Enlargement of the head is caused mainly by the rapid development of the brain and facial prominence The rapid growing second pharyngeal arch overgrows the third and fourth pharyngeal arches, forming a lateral ectodermal depression on each side the CERVICAL SINUS

54. FIFTH WEEK EMBRYO

55. SIXTH WEEK The upper limbs begin to show regional differentiation as the elbows and large hand plates develops The primordia of the digits called DIGITAL RAYS, begin to develop in the hand plates, which indicate the formation of digits The embryo at the six week show spontaneous movements, such as twitching of the trunk and limbs Development of the lower limbs occurs some what later than that of the upper limbs Several small swellings- AURICULAR HILLOCKS, develop around the pharyngeal groove or cleft between the first two pharyngeal arches This groove becomes the EXTERNAL ACOUSTIC MEATUS, ( external auditory canal) and the auricular hillocks around it fuse to form the auricle, the shell shaped part of the external ear The eyes are obvious The head is much larger than the trunk and is bend over the HEART PROMINENCE The embryo at sixth week show reflex responses to touch.

56. SIXTH WEEK EMBRYO

57. SEVENTH WEEK The limbs undergo considerable change during the seventh week Notches appear between the digital rays in the hand plates, clearly indicating the future digits The communication between the primordial gut and york sac is now reduced to a relatively slender duct, the YOLK STALK. The intestines enter the extraembryonic coelom in the proximal part of the umbilical cord This UMBILICAL HERNIATION is a normal event in the embryo The herniation occurs because the abdominal cavity is too small at this age to accommodate the rapidly growing intestine By the end of the seventh week, ossification of the bones of the upper limbs has begun

58. Seventh week embryo

59. UMBILICAL HERNIATION

60. EIGHTH WEEK At the beginning of this final week of the embryonic period, the digits of the hand are separated but noticeable webbed Notches are now clearly visible between the digital rays of the fan- shaped feet The tail like caudal eminence is still present but short The SCALP VASCULAR PLEUS has appeared and forms a characteristic band around the head By the end of the eighth week, all regions of the limbs are apparent Ossification begins in the lower limbs during the eighth week and is first recognizable in the femur The hands and feet approach each other At the end of the eighth week, the embryo has distinct human characteristics

61. Eighth week embryo

62. Estimation of embryonic age During the somite period ( i.e. between 20 and 30 days) the age of the embryo can be roughly estimated by counting the number of somites. Number of somites:- 1 4 7 10 13 16 19 22 25 28 31 Age in days: 20 21 22 23 24 25 26 27 28 29 30 Because embryos of the third and early fourth weeks are straight, measurement of them indicate the GREATEST LENGTH ( GL) The setting height or CROWN-RUMP length ( CRL) is most frequently used for older embryo The standing height , or CROWN-HEEL LENGTH ( CHL), is some times measured for 8 week embryo

63. Crown-rump length

64. Ultrasound Examination of the embryo Most women seeking obstetrical care have at least one ultrasound examination during their pregnancy for one or more of the following reasons: Estimation of gestational age for confirmation clinical dating Evaluation of embryonic growth Examination of a clinically detected pelvic mass Suspected ectopic pregnancy Possible uterine abnormality Detection of congenital abnormalities

65. THE Fetal Period( Ninth week to Birth) Nine to twelve weeks At the beginning of the ninth week, the head constitutes half the crown-heal length of the fetus Subsequently, growth in body length accelerates rapidly and by the end of the 12 weeks the CRL has more than double At the 9 week the face is broad , the eyes are widely separated, the ears are low-set and the eyelids are fused By the end of 12 weeks, primary ossification centers appear in the skeleton ,especially in the cranium and long bones Early in the 9 week, the legs are short and the thighs are relatively small By the end of the 12 weeks, the upper limbs have almost reached their final relative lengths, but the lower limbs are still not so well developed and are slightly shorter than their final relative length The external genitalia of males and females appear similar until the end of the ninth week Their mature fetal form is not established until the twelfth week Intestinal coils are clearly visible in the proximal end of the umbilical cord until the middle of the 10th week By the end of the 11 th week the intestines have returned to the abdomen

66. Nine to Twelve weeks continue At the 9 th week the liver is the major site of erythropoiesis (formation of red blood cells) By the end of the 12 th , this activity has decreased in the liver and has begin in the spleen Urinary formation begins between the 9 th and 12 th weeks, and urine is discharged in to the amniotic fluid The fetus reabsorbs some amniotic fluid after swallowing it Fetal waste products are transferred to the maternal circulation by passing across the placental membrane

67. Thirteen to sixteen weeks Growth is rapid during this period By 16 weeks the head is relatively small compared with that of the 12 week fetus and the lower limbs have lengthened Limb movements, which first occur at the end of the embryonic period , become coordinated by the 14 th week but are too slight to be felt by the mother Ossification of the fetal skeleton is active during this period and the bones are clearly visible in ultrasound images by the beginning of the 16 th week Slow eye movement occur at 14 th week Scalp hair patterning is also determined during this period By 16 th weeks the ovaries are differentiated and contain primordial ovarian follicles that contain oogonia The sex of the external genitalia can be recognized by 12 to 14 weeks in most cases By the 16 weeks the eyes face anteriorly rather than anterolaterally

68. Seventeen to Twenty Weeks Growth slows down during this period but the fetus still increases its CRL by about 50 mm The limbs reach their final relative proportions and fetal movements-QUICKENING- are commonly felt by the mother The mean time that intervenes between a mother's first detection of fetal movements and delivery is 147 days, with standard deviation of +5 or -5 days The skin is now covered with a greasy, cheese like material- VERNIX- CASEOSA , it consists of a mixture of a fatty secretion from the fetal sebaceous glands and dead epidermal cells The vernix caseosa protects the delicate fetal skin from abrasions, chapping and hardening that could result from exposure to the amniotic fluid Eyebrows and head hairs are also visible at 20 weeks The bodies of 20-week fetuses are usually covered with fine downy hair-LANUGO- which helps to hold the vernix caseosa on the skin Brown fat forms during this period and is the site of head production

69. continue By the 18 weeks the uterus is formed and canalization of the vagina has begin By the 20 weeks the testes have begun to descend, but they are still located on the posterior abdominal wall, as are the ovaries in female fetuses

70. Twenty-one to Twenty-five Weeks There is substantial weight gain during this period The skin is usually smooth and more translucent, particularly during the early part of this period At 21 weeks rapid eye movements begin and blink-startle responses have been reported By 24 weeks the secretory epithelial cells in the interalveolar walls of the lungs have begin to secrete surfactant( a surface active lipid that maintains the patency of the developing alveoli of the lungs) Fingernails are present by 24 weeks Although a 22 to 25 weeks fetus born prematurely may survive if given intensive care, it may die during early infancy because its respiratory system is still immature.

71. Twenty Six to Twenty-Nine Weeks At this stage the fetus survives if born prematurely and given intensive care The lungs and pulmonary vasculature have developed The central nervous system has matured to the stage where it can direct rhythmic breathing movements and control body temperature The eye lids are open at 26 weeks and head hairs are well developed Toe nails become visible and considerable subcutaneous fat is now present under the skin The fetal spleen is now an important site of hematopoiesis Erythropoiesis in the spleen ends by 28 weeks, by which time bone marrow has become the major site of this process

72. Thirty to Thirty –Four Weeks The pupillary light reflex of the eyes can be obtained by 30 weeks By the end of this period the skin is pink and smooth The upper and lower limbs have a overweight appearance At this stage the quantity of white fat is 8% of the body weight Fetuses 32 weeks and older usually survive if born prematurely If a normal –weigh fetus is born during this period , it is premature by date but not premature by weight

73. Thirty-Five to Thirty-Eight Weeks The central nervous system is sufficiently mature to carry out some integrative functions At this stage the circumferences of the head and abdomen are approximately equal There is a slowing of growth as the time of birth approaches By full term, most fetuses usually reach a CRL of 360 mm and weight about 3400 gm The amount of white fat is about 16% of body weight \ A fetus adds about 14 gm of fat a day during these last weeks of gestation In general , male fetuses are longer and weight more at birth than females The chest is prominent and breasts often protrude slightly in both sexes The testes are usually in the scrotum in full-term male infants Premature male infants commonly have undescended tests The head is smaller than the rest parts of the body but it is still one of the largest regions of the fetus, this is an important consideration related to its passage through the birth canal

74. Expected Day of Delivery The expected day of delivery ( EDD) of a fetus is 266 days or 38 weeks after fertilization. i.e. 280 days or 40 weeks after LNMP About 12% of babies , however, are born 1 to 2 weeks after the expected time of birth

75. The Placenta The placenta is the primary site of nutrient and gas exchange between the mother and fetus The placenta is fetomaternal organ that has two components A fetal part that develops from the chorionic sac A maternal part that is derived from the endometrium The placenta and umbilical cord form a transport system for substances passing between the mother and fetus Nutrients and oxygen pass from the maternal blood through the placenta to the fetal blood, and waste materials from the fetus to the maternal blood The placenta and fetal membranes perform the following functions Protection Nutrition Respiration Excretion Hormone production

76. decidua The decidua is name given to the endometrium after the blastocyst is completely embedded in it In other words, the decidua is the endometrium of pregnancy The decidua is divided in the following three parts Decidua basalis Decidua capsularis Decidua parietalis Decidua basalis: is the most important part of the decidua. It is the part which lie between the blastocyst and the wall of the uterus Decidua capsularis: this is thin layer of endometrium (decidua) which covers the blastocyst and forms a thin capsule for it. Decidua parietalis: this is the decidua which lines the remaining part of the uterine cavity As the embryo grows only decidua basalis develops and forms the maternal part of the placenta, while the other parts of the decidua degenerate

77. Development of placenta Placenta is formed as result of proliferation of the trophoblast and the development of the chorionic sac and chorionic villi By the end of the third week, the anatomical arrangements necessary for physiological changes between the mother and her embryo are established A complex vascular network is established in the placenta by the end of fourth week, which facilitates maternal-embryonic exchanges of gases, nutrients and metabolic waste products Chorionic villi cover the entire chorionic sac until the beginning of the 8 th week As this sac grows the villi associated with the decidua capsularis are compressed, reducing the blood supply to them, these villi soon degenerate , producing a relatively avascular bare area , the SMOOTH CHORION As the villi disappear, those associated with the decidua basalis rapidly increase in number

78. Placenta continues The fully developed placenta covers 15 to 30% of the decidua and weights about one-sixth that of the fetus The placenta has two parts The fetal part of the placenta: is formed by the villous chorion, the chorionic villi that arise from it project in to the intervillous space containing the maternal blood. The maternal part of the placenta:- is formed by the decidua basalis By the end of the 4 th month , the decidua basalis is completely replaced by the fetal part of the placenta Fetomaternal Junction:- the fetal part of the placenta ( villous chorion) is attached to the maternal part of the placenta( decidua basalis) by the CYTOTROPLASTIC SHELL- the external layer of the trophoblast

79. INTERVILLOUS SPACE The intervillous space containing maternal blood is derived from the lacunae that developed in the syncytiotrophoblast during the second week of development This large blood filled space results from the coalescence and enlargement of the lacunar networks The intervillous space of the placenta is divided in to compartments by the PLACENTAL SEPTA; however, there is free communication between the compartments, because the septa do not reach the chorionic plate The maternal blood enters the intervillous space from the spiral endometrial arteries in the decidua basalis

80. Placental Circulation The branch chorionic villi of the placenta provide a large surface area where materials may be exchanged across the very thin PLACENTAL MEMEBRANE ( barrier), inserted between the fetal and maternal circulations It is through the numerous branch villi, which arise from the stem villi. That the main exchange of material between the mother and fetus takes place The circulation between the mother and the fetus is separated by the placental barrier( placental membrane)

81. fetal placental circulation Poorly oxygenated blood leaves the fetus and passes through the UMBILICAL ARTERIES to the placenta At the site of attachment of the cord to the placenta, these arteries divide in to several radially disposed CHORIONIC ARTERIES that branch freely in the chorionic plate before entering the chorionic villi The blood vessels form an extensive ARTERIO- CAPILLARY- VENOUS SYSTEM within the chorionic villi, which brings the fetal blood extremely close to the maternal blood Large vessel called UMBILICAL VEIN carries oxygen rich blood to the fetus

82. Maternal placental circulation The maternal blood in the intervillous space is temporarily outside the maternal circulatory system It enters the intervillous space through 80 to 100 spiral endometrial arteries in the decidua basalis These vessels discharge in to the intervillous space through the gaps in the Cytotrophoblastic shell The blood flow from the spiral arteries is pulsatile and is propelled I jet like sprays by the maternal blood pressure

83. Placental membrane Placental membrane ( placental barrier): is a complex structure that consists of the extrafetal tissues separating the maternal and fetal blood Until about 20 weeks, the placental membrane consists of four layers Syncytiotrophoblast Cytotrophoplast Connective tissue of villus Endothelium of fetal capillaries

84. Functions of the placenta Metabolism( e.g. synthesis of glycogen) Transport of gases and nutrients Endocrine secretion( e.g. human chorionic gonadotropin)

85. Abnormalities Of The Placenta Bilobed or Trilobed placenta: formed of two or three lobes but having one umbilical cord only. This is not an important abnormality Succenturiate Placenta: this is small accessory placenta Twin Placenta: a placenta with two separate umbilical cords, this is due to twins or multiple pregnancy Placenta previa: it is a placenta which develops in the lower part of the uterus NEAR THE INTERNAL OS: normally implantation takes place on the posterior wall of the uterus and the normal placenta develops in the upper part of the uterus, in this case the implantation occurs at the lower part of the uterus, the placenta will then develop lower down near the internal os

86. Placenta Previa

87. Bilobed Placenta

88. Uterine Growth During Pregnancy The uterus of nonpregnant women lies in the pelvis minor( lesser pelvis) To accommodate the growing conceptus, the uterus increases in size and weight During the first trimester, the uterus moves out of the pelvis By 20 weeks it reaches the level of umbilicus By 28 to 30 weeks, the uterus reaches the epigastric region– the area between the xiphoid process of the sternum and the umbilicus The increase in size of the uterus largely results from hypertrophy of preexisting smooth muscular fibers, and partly from the development of new fibers

89. Parturition ( childbirth) Parturition: is the process during which the fetus, placenta and fetal membranes are expelled from the mother's reproductive tract Labor: is the sequence of involuntary uterine contractions that result in dilation of the uterine cervix and the expulsion of the fetus and placenta from the uterus. There are four stages of labor Dilation: is the first stage of labor it begins when there is objective evidence of progressive dilation of the cervix Expulsion: is the second stage of labor it begins when the cervix is fully dilated and ends with delivery of the baby The placental stage: is the third stage of labor it begins as soon as the baby is born and ends when the placenta and membranes are expelled Recovery: is the fourth stage of labor it begins as soon as the placenta and fetal membranes are expelled, this stages lasts about 2 hrs and prevents excessive uterine bleeding

90. FETAL MEMBRANES Fetal membranes include all the extra-embryonic structure, which are derived from the primitive blastomeres The fetal membranes are include the following structures Chorion Amnion Umbilical cord York sac

91. THE CHORION The chorion is the name given to the trophoblast after the extraembryonic mesoderm is formed from its inner surface Before implantation , the outer wall of the blastocyst is simply called the TROPOBLAST As the blastocyst begins to be implanted, the trophoblast becomes differentiated in to two layers Outer SYNCYTIO-TROPHOBLAST Inner CYTO-TROPHOBLAS As development proceeds, the cells continue to develop from the inner surface of the cyto-trophoblast to form a very loose tissue called the EXTRA-EMBRYONIC MESODERM After the extra-embryonic mesoderm is completely formed the trophoblast becomes called the CHORION and the blastocyst becomes called the CHORIONIC VESICLE

92. THE AMNION The amnion is a membrane which is continues with the ectoderm of the embryo and bounds the amniotic cavity The amniotic cavity appears during implantation of the blastocyst as small cleft between the ectoderm and the trophoplast Very early pregnancy the ectodermal cells are attached to the trophoblast At about the 8 th day small intercellular cleft appears between the ectoderm and trophoblast As the amniotic cavity enlarges , a layer of large flattened cells called the AMNIOBLAST develops from the inner surface of the trophoblast and form a small space called the ROOF of the amniotic cavity, while the FLOOR of the cavity is formed by the ectodermal germ layer of the embtyonic disc As the amniotic cavity increases in size, the layer of amnioblast loses its contact with the inner surface of the trophoblast and it becomes known as the AMNION

93. THE AMNION

94. Functions of the Amniotic Fluid Early In Pregnancy It serves as protective watery cushion, which absorbs push that may hurt the embryo It prevents the adhesion of the embryo to the amnion It is bad conductor of the heat and therefore , keeps the temperature of the embryo nearly constant It allows the embryo to move freely It provides a space where urine and mechonium can accumulate 2- Late In Pregnancy: It protects the fetus from strong muscular contractions of the uterus in the early stage of labor The fetus begins to swallow its own amniotic fluid , this helps the embryo to suckle

95. 3- At the End of Pregnancy It bulge in front of the embryo and form a bag of watery , which helps to dilate the canal of cervix At birth when the amniotic sac ruptures , the amniotic fluid washes the vagina just before the delivery of the fetus.

96. Abnormalities of the amniotic fluid The amniotic fluid is about 1.5 litters, it may be pathologically increased or decreased If the volume becomes more than 2 litres, the condition is called POLYHYDRAMNION If the volume is less than 0.5 litres, the condition is called OLIGOHYDRAMNIOS, which may lead to adhesion between the embryo and the amnion

97. The Umbilical Cord The umbilical cord is the pathway which connects the placenta with the ventral aspect of the embryo It is tortuous cord which measures from 50-60cm long and about 1cm in diameter It has a smooth surface because it is covered by a layer of amnion The umbilical cord contains three umbilical vessels( one vein and two arteries) embedded in a gelatinous material called WHARTON,S JELLY The umbilical cord passes through three stages of development Primitive umbilical ring Primitive umbilical cord Fully developed umbilical cord As the embryo grows, the embryonic disc bulges in to the amniotic cavity As result the junction between the amniotic cavity and the embryonic disc is carried in to the ventral aspect of the embryo and is called PRIMITIVE UMBILICAL RING

98. Continue By the 5 th week, the primitive umbilical ring constricts changing the primitive umbilical ring in to tubular sheath of amnion called the PRIMITIVE UMBILICAL CORD ABNORMALITIES OF THE UMBILICAL CORD Very long umbilical cord: it is very dangerous it may wind around the neck of the embryo Very short umbilical cord: it may cause premature separation of the placenta EXOMPHALOS: this is condition in which the physiological umbilical hernia is not completely reduced, the umbilical cord is distended and looks like a sac, which contains part of the midgut Double or triple umbilical cord An umbilical cord with only one umbilical artery

99. exomphalos

100. The Yolk Sac The york sac starts as a cavity which develops on the ventral aspect of the embryonic disc; its roof is formed by the endodermal germ layer of the embryonic disc While its wall is formed of a layer of flat cells to which a layer of loose extraembryonic mesoderm is later added There is primary, a secondary and definitive yolk sac The primary yolk sac also called the exocoelomic cavity is formed when the endoderm from the embryonic disc grows down on the inner surface of the trophoblast and forms a thin membrane called HEUSER,S MEMBRANE Together the endoderm and the heuser,s membrane form the wall of the primary york sac The primary yolk sac then separates it self from the trophoblast by the development of the extra-embryonic coelom (coelomic cavity)

101. 2- Secondary Yolk Sac: later in the development the terminal end of the primary yolk sac becomes cut of and the remaining part becomes the secondary yolk sac The outer surface of the secondary yolk sac is covered by a layer of mesoderm called SPLANCHNIC LAYER of the extra-embryonic mesoderm A network of vessels appears in this mesoderm and becomes connected with the vessels of the embryo by means of the VITELLINE VESSELS The secondary yolk sac forms the following structures The gut inside the embryo The yolk sac stalk Definitive yolk sac The gut enters the embryo and divides to form the foregut, midgut and hidgut.

102. Functions of the Yolk Sac It has a role in the transfer of nutrients to the embryo during the 2 nd and 3 rd weeks, when the uteroplacental circulation is established Blood development first occurs in the well-vascularized extraembryonic mesoderm covering the wall of the yolk sac beginning in the 3 rd week During the 4 th week the endoderm of the yolk sac is incorporated in to the embryo as the primordial gut Primordial germ cells appear in the endodermal lining of the wall of the yolk sac in the 3 rd week and subsequently migrate to the developing sex glands

103. The yolk stalk forms part of the primitive umbilical ; later it becomes obliterated and the gut becomes completely separated from the definitive yolk sac The definitive yolk sac itself does not grow except very slowly and never reaches a diameter more than 0.5 cm, it then shrinks and changes to a small solid structure