Embryology full note

1. GENERAL EMBRYOLOGY 1

2. THE DEVELOPING HUMAN Introduction Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male Cell division, cell migration , programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte , a highly specialized, totipotent cell, a zygote, into a multicellular human being From a single cell to a baby in 9 months, the study of the developmental processes that take place is called Embryology Note: Development does not stop at birth, Although most developmental changes occur during the embryonic and fetal periods, important changes occur during later periods of development: infancy, childhood, adolescence , and early adulthood 2

3. Terminologies Oocyte ( ovum /egg) : refers to the female germ or sex cells produced in the ovaries Sperm ( spermatozoon) : refers to the male germ cell produced in the testes (testicles ) Zygote: This cell results from the union of an oocyte and a sperm during fertilization A zygote or embryo is the beginning of a new human being Developmental Periods: can be divided into prenatal (before birth) postnatal (after birth) period. Stages of prenatal development: begins at fertilization and embryonic development ends on day 56 (8 th week ) The fetal period begins on day 57 and ends when the fetus is completely outside the mother 3

4. Prefertilization Events Sexual Reproduction Sexual reproduction occurs when female and male gametes ( oocyte and spermatozoon, respectively) unite at fertilization. Gametes are direct descendants of primordial germ cells, which are first observed in the wall of the yolk sac at 4 th week of embryonic development and subsequently migrate into the future gonad region where they arrive at the end of the 5 th week Gametes are produced by a process called gametogenesis (formation of gamete) In males, this process is called spermatogenesis In females, it is called oogenesis note : The sequence of gametogenesis is the same, but the timing of events during meiosis differs in the two sexes. 4

5. Primodial germ cells in wall of yolk sac Yolk sac 5

6. In preparation for fertilization, germ cells undergo gametogenesis . Which include: meiosis , to reduce the number of chromosomes and cytodifferentiation to complete their maturation Chromosomes A single chromosome consists of TWO characteristic regions called arms . These include: Short arm/ p arm Long arm/ q arm These two arms are separated by a centromere During meiosis I, single chromosomes undergo DNA replication , which essentially duplicates the arms. This forms duplicated chromosomes , which consist of two sister chromatids attached at the centromere . 6

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8. 1) Ploidy and N number Ploidy refers to the number of chromosomes in a cell The N number refers to the amount of DNA in a cell In humans, somatic cells ( cells of an organism other than the germ cells ) contain 46 single chromosomes The chromosomes occur in 23 homologous pairs , of which one member (homologue) of each pair is of maternal origin , and the other is of paternal origin to form the diploid number of 46 Note: The term “ diploid ” is classically used to refer to a cell containing 46 single chromosomes The 23 homologous pairs of chromosomes is made up of: 22 pairs of matching chromosomes called autosomes and one pair of sex chromosomes 8

9. If the sex pair is XX, the individual is genetically female; if the pair is XY, the individual is genetically male. One chromosomes of each pair is derived from the maternal gamete, the oocyte , and one from the paternal gamete, the sperm Thus, each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46 Meiosis is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells, respectively Meiosis requires two cell divisions: meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23 9

10. Meiosis 1 Events that occur during meiosis I include the following: synapsis homologous chromosomes align themselves in pairs, this process is called synapsis crossingover Crossovers: this involve the interchange of chromatid segments between paired homologous chromosomes Segments of chromatids break and are exchanged as homologous chromosomes separate. As separation occurs, points of interchange are temporarily united and form an X-like structure, a chiasma 10

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12. 12 Stages A – D, prophase of 1 st meiotic division E – metaphase F - Anaphase G - Telophase (of 1 st meiotic division) Cells contain 23 double-structured chromosomes Cells resulting from 1st meiotic division 2 nd meiotic division Metaphase of 2 nd meiotic division Disjunction Cells resulting from 2 nd meiotic division

13. Summary of Meiosis I Events that occur during meiosis I include the following: Synapsis : pairing of 46 homologous duplicated chromosomes Crossing over : exchange of large segments of DNA Alignment : alignment of 46 homologous duplicated chromosomes at the metaphase plate Disjunction : separation of 46 homologous duplicated chromosomes from each other; centromeres do not split. Cell division : formation of two secondary gametocytes (23 duplicated chromosomes, 2N). Meiosis II Meiosis II. Events that occur during meiosis II include the following: Synapsis : absent. Crossing over: absent. 13

14. Summary of Meiosis I Events that occur during meiosis I include the following: Synapsis : pairing of 46 homologous duplicated chromosomes Crossing over : exchange of large segments of DNA Alignment : alignment of 46 homologous duplicated chromosomes at the metaphase plate Disjunction : separation of 46 homologous duplicated chromosomes from each other; centromeres do not split Cell division : formation of two secondary gametocytes (23 duplicated chromosomes, 2N). Meiosis II Meiosis II. Events that occur during meiosis II include the following: Synapsis : absent. Crossing over: absent. 14

15. Alignment: alignment of 23 duplicated chromosomes at the metaphase plate. Disjunction: separation of 23 duplicated chromosomes to form 23 single chromosomes; centromeres split Cell division: formation of four gametes (23 single chromosomes, 1N). Note: As a result of meiotic divisions, genetic variability is enhanced through crossover, which redistributes genetic material, and through random distribution of homologous chromosomes to the daughter cells; and ( b) each germ cell contains a haploid number of chromosomes, so that at fertilization the diploid number of 46 is restored 15

16. SPERMATOGENESIS Spermatogenesis is the sequence of events by which spermatogonia are transformed into mature sperms . This maturation process begins at puberty 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 Spermatogonia are transformed into primary spermatocytes , the largest germ cells in the seminiferous tubules. Each primary spermatocyte subsequently undergoes 1 st meiotic division ( reduction division) to form two haploid secondary spermatocytes , which are approximately half the size of primary spermatocytes

17. Secondary spermatocytes then undergo a second meiotic division to form four haploid spermatids these haploid spermatids are approximately half the size of secondary spermatocytes . The spermatids are gradually transformed into four mature sperm by a process known as spermiogenesis The entire process of spermatogenesis, which includes spermiogenesis , takes approximately 2 months When spermiogenesis is complete, 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 regulation of spermatogenesis

18. Sperms are transported passively from the seminiferous tubules to the epididymis , where they are stored and become functionally mature The epididymis is the elongated coiled duct along the posterior border of the testis It is continuous with the ductus deferens (vas deferens) , which transports the sperms to the urethra Mature sperms are free-swimming, actively motile cells consisting of a head and a tail The neck of the sperm is the junction between the head and tail The head of the sperm forms most of the bulk of the sperm and contains the haploid nucleus. The anterior two thirds of the nucleus is covered by the acrosome , a caplike saccular organelle containing several enzymes

19. When released, these enzymes facilitate dispersion of the follicular cells of the corona radiata and sperm penetration of the zona pellucida during fertilization The tail of the sperm consists of three segments: middle piece principal piece and end piece The tail provides the motility of the sperm that assists its transport to the site of fertilization. The middle piece of the tail contains mitochondria, which provide the adenosine triphosphate necessary for activity

20. OOGENESIS Oogenesis ( ovogenesis ) is the sequence of events by which oogonia are transformed into mature oocytes This maturation process begins before birth and is completed after puberty Oogenesis continues to menopause , which is permanent cessation of the menses (bleeding associated with the menstrual cycles) Prenatal Maturation of Oocytes Primordial germ cells (46, 2N) migrate from the wall of the yolk sac and arrive in the ovary at 5 th week and differentiate into oogonia (46, 2N), which populate the ovary through mitotic division. A majority of oogonia continue to divide by mitosis but some of them enter meiosis I and undergo DNA replication to form primary oocytes (46, 4N) by the 5 th month of prenatal development, the total number of germ cells in the ovary reaches its maximum, which is estimated at 7 million

21. Note: All primary oocytes are formed by the 5 th month of fetal life At this time, cell death begins, and many oogonia as well as primary oocytes become atretic By the seventh month, the majority of oogonia have degenerated except for a few near the surface Note: No oogonia are present at birth All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a single layer of flattened, follicular epithelial cells The primary oocytes enclosed by this layer of cells constitutes a primordial follicle Primary oocytes begin the first meiotic division before birth, but completion of prophase does not occur until puberty 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

22. Postnatal Maturation of Oocytes Beginning during puberty, usually one follicle matures each month and ovulation occurs, except when oral contraceptives are used The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000 only approximately 40,000 are present by the beginning of puberty, and fewer than 500 will be ovulated As the primary oocyte enlarges during puberty , the follicular epithelial cells become cuboidal in shape and then columnar , forming a primary follicle The primary oocyte soon becomes surrounded by a covering of amorphous acellular glycoprotein material, the zona pellucida

23. Note: No primary oocytes form after birth in females, in contrast to the continuous production of primary spermatocytes in males The primary oocytes remain dormant in the ovarian follicles until puberty As a follicle matures, the primary oocyte increases in size and, shortly before ovulation , the first meiotic division is completed giving rise to a secondary oocyte and the first polar body. Unlike the corresponding stage of spermatogenesis, there is formation 2 daughters of unequal size, this is because the division of cytoplasm is unequal , each with 23 double structured chromosomes The secondary oocyte receives almost all the cytoplasm and the first polar body receives very little The 1st polar body is a small, nonfunctional cell that soon degenerates

24. The first polar body lies between the zona pellucida and the cell membrane of the secondary oocyte in the perivitelline space The cell then enters meiosis II but it is arrested at metaphase approximately 3 hours before ovulation 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 (zygote) , while the 2 nd polar body receive little cytoplasm If not fertilized, the cell degenerates approximately 24 hours after ovulation The other cell, the 2nd polar body , also a small nonfunctional cell, soon degenerates. As soon as the polar body is extruded, maturation of the oocyte is complete Note: The first polar may also undergoes a second division

25. Of these, only about 400-500 become secondary oocytes and are expelled at ovulation during the reproductive period Few of these oocytes , if any, are fertilized and become mature The number of oocytes that ovulate is greatly reduced in women who take oral contraceptives because the hormones in them prevent ovulation from occurring

26. Embryological terminologies Abortion This is the premature stoppage of development and expulsion of a conceptus from the uterus or expulsion of an embryo or fetus before it is viable-capable of living outside the uterus The products of an abortion is called an abortus (i.e. the embryo/fetus and its membranes) There are different types of abortion: A spontaneous abortion is one that occurs naturally and is most common during the third week after fertilization Approximately 15% of recognized pregnancies end in spontaneous abortion, usually during the first 12 weeks A habitual abortion is the spontaneous expulsion of a dead or nonviable embryo or fetus in three or more consecutive pregnancies

27. An induced abortion is a birth that is medically induced before 20 weeks (i.e., before the fetus is viable) This type of abortion refers to the expulsion of an embryo or fetus induced intentionally by drugs or mechanical means A complete abortion is one in which all the products of conception are expelled from the uterus A missed abortion is the retention of a conceptus in the uterus after death of the embryo or fetus. A miscarriage is the spontaneous abortion of a fetus and its membranes before the middle of the second trimester (approximately 135 days) Threatened abortion This might take place before the 20th week of pregnancy Some pregnant women have some vaginal bleeding with or without abdominal cramps during the first three months of pregnancy

28. When the symptoms indicate a miscarriage is possible, the condition is called a "threatened abortion." (This refers to a naturally occurring event, not medical abortions or surgical abortions Trimester : A period of three calendar months during a pregnancy. Obstetricians commonly divide the 9-month period of gestation into three trimesters. The most critical stages of development occur during the first trimester (13 weeks) when embryonic and early fetal development is occurring

29. C L I N I C A L C O R R E L A T E S Birth Defects and Spontaneous Abortions Chromosomal abnormalities May be: numerical structural note The normal human somatic cell contains 46 chromosomes; the normal gamete contains 23 Normal somatic cells are diploid, or 2 n ; normal gametes are haploid, or n Euploid refers to any exact multiple of n, e.g. diploid or triploid

30. Numerical Abnormalities Polyploidy Polyploidy is the addition of an extra haploid set or sets of chromosomes (i.e., 23) to the normal diploid set of chromosomes (i.e., 46) Triploidy is a condition in which cells contain 69 chromosomes The most common type of polyploidy is triploidy Triploid fetuses have severe intrauterine growth retardation with a disproportionately small trunk Triploidy could result from the second polar body failing to separate from the oocyte during the second meiotic division; but more likely triploidy results when an oocyte is fertilized by two sperms ( dispermy ) almost simultaneously

31. Triploidy occurs in approximately 2% of embryos, but most of them abort spontaneously Triploid fetuses account for approximately 20% of chromosomally abnormal miscarriages. Although triploid fetuses have been born alive, this is exceptional. These infants all died within a few days because of multiple anomalies and low birth weight B Tetraploidy : is a condition in which cells contain 92 chromosomes Usually due to failure of first mitotic division: chromosomes replicate and divide, but all end up in the same nucleus Tetraploid embryos abort very early , and often all that is recovered is an empty chorionic sac, which used to be referred to as a "blighted embryo."

32. Aneuploidy Aneuploidy is the addition of one chromosome ( trisomy ) or loss of one chromosome ( monosomy ). Trisomy results in spontaneous abortion of the conceptus . However, trisomy 13 ( Patau syndrome), trisomy 18 (Edwards syndrome), trisomy 21 (Down syndrome), and Klinefelter syndrome (47,XXY) are found in the liveborn population Monosomy also results in spontaneous abortion of the conceptus However, monosomy X chromosome (45,X; Turner syndrome) is found in the liveborn population Aneuploidy occurs as a result of non-disjunction during meiosis

33. In meiosis, two members of a pair of homologous chromosomes normally separate during the first meiotic division so that each daughter cell receives one member of each pair Sometimes, however, separation does not occur ( nondisjunction ), and both members of a pair move into one cell As a result of nondisjunction of the chromosomes, one cell receives 24 chromosomes, and the other receives 22 instead of the normal 23 When, at fertilization, a gamete having 23 chromosomes fuses with a gamete having 24 or 22 chromosomes, the result is an individual with either 47 chromosomes( trisomy ) or 45 chromosomes ( monosomy )

34. Note: In women, the incidence of chromosomal abnormalities, including non-disjunction, increases with age, especially at 35 years and older Trisomy 21 (Down syndrome) Down syndrome is usually caused by an extra copy of chromosome 21 (trisomy21) features of children with down syndrome include Physical growth retardation varying degrees of mental retardation (Down syndrome IQ is around 50, compared to children without the condition with an IQ of 100 (Mental retardation has historically been defined as an IQ below 70) craniofacial abnormalities, including upward slanting eyes, epicanthal folds (extra skin folds at the medial corners of the eyes), flat facies , and small ears; cardiac defects; and hypotonia

35. Single simian crease high incidences of leukemia, infections, thyroid dysfunction, Brachycephaly Clinodactyl of the fifth digit and premature aging nearly all develop signs of Alzheimer’s disease (progressive mental deterioration manifested by loss of memory) after age 35 Incidence rate The incidence of Down syndrome is approximately 1 in 2000 conceptuses for women under age 25 This risk increases with maternal age to 1 in 300 at age 35 and 1 in 100 at age 40

36. (A) Child with Down syndrome, which is characterized by a flat broad face, oblique palpebral fissures (B) broad hand with a single transverse (simian) crease A B

37. Anterior view of the faces of dizygotic male twins that are discordant for Down syndrome ( trisomy 21). The one on the right is smaller and hypotonic compared with the unaffected twin. The twin on the right developed from a zygote that contained an extra 21 chromosome. Note the characteristic facial features of Down syndrome in this infant: upslanting palpebral fissures, epicanthal folds, and flat nasal bridge

38. Trisomy 18 (Edwards syndrome) Patients with trisomy 18 show the following features : mental retardation, growth retardation prominent occiput congenital heart defects low-set ears, flexion of fingers and hands short sternum Micrognathia renal anomalies Syndactyly ( webbing or fusion of fingers or toes) and malformations of the skeletal system. incidence rate incidence rate of this condition is approximately 1 in 5000 newborns. 85% are lost between 10 weeks of gestation whereas those born alive usually die by age 2 months Approximately 5% live beyond 1 year

39. Child with trisomy 18. Note the low sets ears, small mouth, deficient mandible ( micrognathia ) flexion of the hands, and absent/ or hypoplasia of thr radius and unla

40. Photograph of child with trisomy 18. Note the prominent occiput , cleft lip, micrognathia , low-set ears, and one or more flexed fingers

41. Trisomy 13( Patau syndrome) The main abnormalities of trisomy 13 are mental retardation severe central nervous system malformations congenital heart defects sloping forehead; malformed ears scalp defects bilateral cleft lip and/or palate; Polydactyly (presence of more than 5 digits on hand or foot) eye defects, such as : Microphthalmia anophthalmia (congenital absence of tissue of the eyes) Incidence rate The incidence of this abnormality is approximately 1 in 20,000 live births, over 90% of the infants die in the first month after birth Approximately 5% live beyond 1 year

42. Child with trisomy 13, note the bilateral cleft lip, the sloping forehead, and anophthalmia

43. Trisomy 13: The syndrome is commonly accompanied by polydactyly

44. Turner syndrome Turner syndrome, with a 45,X karyotype , is the only monosomy compatible with life Even then, 98% of all fetuses with the syndrome are spontaneously aborted The few that survive are unmistakably female in appearance Characteritics features are the absence of ovaries ( gonadal dysgenesis ) short stature Webbed neck lymphedema of the extremities, skeletal deformities and a broad chest with widely spaced nipples

45. Patients with Turners syndrome (A) swelling in the hand A

46. Patients with Turners syndrome (B) prominient webbed neck and widely spaced nipples with a broad chest (C) Caused by lymphedema B C

47. Klinefelter's syndrome ( 47, XXY) found only in males The non disjucntion of XX homologues is the most causative event features small testes hyalinization of seminiferous tubules sterility often tall with disproportionately long lower limbs Intelligence is less than in normal siblings Approximately 40% of these males have gynecomastia Incidence rate : 1 in 500 males

48. Adolescent male with Klinefelter syndrome (XXY trisomy ) Note the presence of breasts; approximately 40% of males with this syndrome have gynecomastia (development of mammary glands) and small testes

49. Structural chromosome abnormalities involve one or more chromosomes usually result from chromosome breakage. Breaks are caused by: Environmental factors, such as viruses, radiation, and drugs The result of breakage depends on what happens to the broken pieces In some cases, the broken piece of a chromosome is lost, and the infant with partial deletion chromosome is abnormal A well-known syndrome, caused by partial deletion is the cri-du-chat syndrome Cri-du-chat syndrome caused by partial deletion of the short arm of chromosome 5 children have a catlike cry, microcephaly , mental retardation, and congenital heart disease

50. Microdeletions This takes place in a few contiguous genes This may result in microdeletion syndrome or contiguous gene syndrome They can be detected only by high resolution banding (high resolution banding allows detection of very small terminal deletions in a number of disorders) Examples of microdeletion include occurs on the long arm of chromosome 15 if the defect is inherited on the paternal chromosome, it is called Prader-Willi syndrome if the defect is inherited on the maternal chromosome , it is called Angelman syndrome

51. Prader-Willi syndrome affected individuals are characterized by hypotonia , obesity, mental retardation, hypogonadism , and cryptorchidism ( undescended testes) Angelman syndrome affected individuals are characterized by mentally retardation, inability to speak, exhibition of poor motor development, and are prone to unprovoked and prolonged periods of laughter

52. Patient with Angelman syndrome resulting from a microdeletion on maternal chromosome 15 Patient with prader willi syndrome resulting from a microdeletion on paternal chromosome 15

53. Uterus ( womb) introduction is a thick-walled, pear-shaped muscular organ dimension Length = approx 7-8 cm width = approx 5-7cm at its superior part thickness= approx 2-3 cm parts consists of two major parts Body, the expanded superior 2/3 Cervix, the cylindrical inferior 1/3 The body of the uterus narrows from the fundus (the rounded, superior part of the body) to the isthmus Note: the isthmus is the 1cm long constricted region between the body and cervix

54. The cervix of the uterus is its tapered vaginal end that is nearly cylindrical in shape It has a lumen called the cervical canal the cervical canal , has constricted openings at each end At the upper constricted end is the internal os , while at the lower constricted end is the external os The internal os communicates with the cavity of the uterine body ,while the external os communicates with the vagina The walls of the body of the uterus consist of three layers Perimetrium , the thin external layer Myometrium , the thick smooth muscle layer Endometrium , the thin internal layer

55. The perimetrium is firmly attached to the myometrium The layers of the endometrium It has 3 layers A thin superficial layer called the compact layer A thick middle layer called the spongy layer A thin deep layer called the basal layer . The basal layer of the endometrium has its own blood supply and is not sloughed off during menstruation The compact and spongy layers collectively are called the functional layer , This functional layer disintegrate and are shed during menstruation and after parturition (delivery of a baby)

56. Uterine tubes intro The uterine tubes extend laterally from the horns of the uterus Dimension approximately 10 cm long and 1 cm in diameter Parts the uterine tube is divided into four parts : infundibulum ampulla isthmus uterine part functions carries oocytes from the ovaries and sperms entering from the uterus to reach the fertilization site in the ampulla of the uterine tube conveys the cleaving zygote to the uterine cavity

57. Ovaries Intro: The ovaries are almond-shaped reproductive glands located close to the lateral pelvic walls on each side of the uterus functions They produce oocytes They also produce estrogen and progesterone, Estrogen and progesterone are hormones responsible for the development of secondary sex characteristics and regulation of pregnancy Female reproductive cycles females undergo reproductive cycles (sexual cycles), involving activities of the hypothalamus of the brain, pituitary gland ( hypophysis ), ovaries, uterus, uterine tubes, vagina, and mammary glands and this commences at purberty These monthly cycles prepare the reproductive system for pregnancy

58. Gonadotropin -releasing hormone is synthesized by neurosecretory cells in the hypothalamus and is carried by the hypophysial portal system to the anterior lobe of the pituitary gland Gonadotropin -releasing hormone stimulates the release of 2 hormones namely: Follicle-stimulating hormone ( FSH ) Luteinizing hormone ( LH ) These 2 hormones act on the ovaries: Follicle-stimulating hormone ( FSH ) stimulates the development of ovarian follicles and the production of estrogen by the follicular cells. Luteinizing hormone ( LH ) serves as the "trigger" for ovulation (release of secondary oocyte ) stimulates the follicular cells and corpus luteum to produce progesterone

59. Ovarian cycle These are cyclic changes in the ovaries, and these changes are produced by FSH and LH These changes include: development of follicles ovulation, and Formation of the corpus luteum Development of follicles Development of an ovarian follicle is characterized by: Growth and differentiation of primary oocyte Proliferation of follicular cells Formation of zona pellucida Development of the theca folliculi

60. As primordial follicles begin to grow, surrounding follicular cells change from flat to cuboidal in shape and then columnar , forming a primary follicle Also, follicular ( granulosa ) cells and the oocyte secrete a layer of glycoproteins on the surface of the oocyte , forming the zona pellucida Development of theca folliculi As the primary follicle continues to grow, the adjacent ( surounding ) connective tissue organizes into a capsule, called the theca folliculi cells of the theca folliculi then differenciate into: an inner layer of secretory cells, called the theca interna and an outer fibrous capsule,called the theca externa

61. As development continues, fluid-filled spaces appear between follicular ( granulosa ) cells The fluid filled space is crescent-shaped , but with time, it enlarges which coalesce to form a single large cavity, the antrum , which contains follicular fluid After the antrum 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 antrum The secondary/ vesicular follicular continues to grow and become matured having a diameter of about 25mm or more At maturity, it is called the mature vesicular/ mature secondary or Graafian follicle The early development of ovarian follicles is induced by FSH, but final stages of maturation require LH as well

62. Growing follicles produce estrogen , hormone that regulates development and function of the reproductive organs Ovulation This is the release of an oocyte from the ovarian follicle In a few before ovulation, under the influence of FSH and LH, the secondary follicle grows rapidly to a diameter of about 25 mm to become mature vesicular/ mature secondary or Graafian follicle Coincident with final development of the vesicular follicle, there is an abrupt increase in LH that causes the primary oocyte to complete meiosis I and the follicle to enter the preovulatory mature vesicular stage Meiosis II is also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovulation In the meantime, the surface of the ovary begins to bulge locally, and at the apex, an avascular spot , the stigma, appears

63. For the oocyte to be released, 2 events occur which are caused by LH surge: The high concentration of LH increases collagenase activity , resulting in digestion of collagen fibers (connective tissue) surrounding the follicle Prostaglandin levels also increase in response to the LH surge and cause local muscular contractions in the ovarian wall Those contractions extrude the oocyte , which together with its surrounding follicular ( granulosa ) cells from the region of the cumulus oophorus , this causes ovulation in which oocyte floats out of the ovary Some of the cumulus oophorus cells then rearrange themselves around the zona pellucida to form the corona radiata

64. C l i n i c a l c o r r e l a t e s During ovulation, some women feel a slight pain called mittelschmerz also known as middle pain because it normally occurs near the middle of the menstrual cycle A variable amount of abdominal pain, mittelschmerz , accompanies ovulation in some women. In these cases, ovulation results in slight bleeding into the peritoneal cavity, which results in sudden constant pain in the lower abdomen. Mittelschmerz may be used as a symptom of ovulation, but there are better symptoms, such as the slight drop in basal body temperature Some women fail to ovulate, this is called anovulation , because of a low concentration of gonadotropins In these cases, administration of an agent to stimulate gonadotropin release and hence ovulation can be employed Although such drugs are effective, they often produce multiple ovulations, so that the risk of multiple pregnancies is 10 times higher in these women than in the general population

65. Theca interna Luteal cells fibrin Blood vessels antrum Theca externa C: CORPUS LUTEUM B: OVULATION A : PREOVULATORY FOLLICLE

66. Corpus Luteum Shortly after ovulation, the walls of the ovarian follicle and theca folliculi collapse and are thrown into folds are vascularized by surrounding vessels Under LH influence, they develop into a glandular structure, these cells develop a yellowish pigment and change into lutein cells the corpus luteum , which secretes progesterone and some estrogen progesterone and estrogen , cause the endometrial glands to secrete and prepare the endometrium for implantation of the blastocyst If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of pregnancy and increases its hormone production. Degeneration of the corpus luteum is prevented by human chorionic gonadotropin The corpus luteum of pregnancy remains functionally active throughout the first 20 weeks of pregnancy

67. If the oocyte is not fertilized, the corpus luteum involutes and degenerates 10 to 12 days after ovulation It is then called a corpus luteum of menstruation The corpus luteum is subsequently transformed into white scar tissue in the ovary, a corpus albicans Except during pregnancy, ovarian cycles normally persist throughout the reproductive life of women and terminate at menopause , the permanent cessation of menstruation, usually between the ages of 48 and 55