Embryogenesis first week ovulation, fertilization and implantation

descriptive lecture about human development

1. Embryology Dr. Sabrin Sami Lec.1

2. ovum sperm

3. Embryology Is the study of intrauterine formation and development of an embryo from the stage of ovum fertilization through to the birth.

4. Terminology

5. Zygote fertilized egg cell, consists of genes from two parents, and thus it is  diploid  (carrying two sets of chromosomes) . Embryo this term is applied to the unborn child just after fertilization of the female egg cell until the end of the 8 th week. Fetus From the beginning of the 9 th week the embryo is called fetus. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs .

6. The gestational period (pregnancy) length of time that a fetus grows inside the mother’s uterus which is 9 months, divided into two periods: embryonic and fetal. The embryogenesis period (organogenesis period) is a complicated process by which the zygote develops into an embryo (the period of establishing organ primordia) extends to the end of the first 8 weeks of human development.

7. The fetal period the period from the end point of embryogenesis until birth. It begins when the embryo has a recognizable external feature. It is characterized by continues growth. morphologic differentiation. size increasing by the full development of the organ systems. Prenatal development The process encompassing the period from the formation of an embryo, through the development of a fetus, to birth (parturition).

8. Female reproductive system production and transportation of gametes. production of sex hormones. facilitates the fertilization of ova by sperm . supports the development of offspring (infant) during pregnancy and infancy. 1- Ovaries 2- fallopian tubes 3- uterus

9. Each ovary is an almond-shaped structure and consists of an outer cortex. an inner medulla. The cortex contains the ovarian follicles at different stages of maturation. Each follicle 1- primary oocytes surrounded by a layer of squamous cells called 2- follicular cells (granulosa) . This mass called the primordial ovarian follicles .

10. The ovarian follicles consists of 1- primary oocyte ( ssurrounded by a layer of squamous cells called) 2- follicular cells (granulosa) At this stage , its called a primordial ovarian follicles

11. First week of development The ovulation

12. The female monthly cycles are controlled by gonadotropins : 1- follicle-stimulating hormone (FSH) 2- luteinizing hormone (LH) both synthesized and secreted by the pituitary gland. both hormones are stimulate and control cyclic changes in the ovary.

13. mature ovarian follicle (graafian follicle) Under the influence of FSH the primordial ovarian follicles are stimulated to grow rapidly to become a mature ovarian follicle, consists of: cavity filled with follicular fluid . periphery attached oocyte surrounded by: - a protective membrane called the zona pellucida and - a layer of follicular cells called cumulus oophorus .

14. As the surface of the ovary begins to bulge locally, an avascular spot at the apex appears called the stigma .

15. Under the influence of LH : the levels of prostaglandin increase cause local muscular contractions in the ovarian wall. lead to Graafian follicle rupture the ovum released together with its surrounding cumulus oophorus out of the ovary in the process called ovulation . the cumulus oophorus cells rearrange around the zona pellucida to form the corona radiata .

16. The ovum that is released at the ovulation is picked up by the fimbriated end (finger like projections at the end of fallopian tubes) of the uterine tube and enters the ampulla .

17. Corpus luteum (yellow body ) It is formed after ovulation, with rupture of the follicle: Its wall collapses, crumbles and gets folded The follicular cells become large conical. Under the influence of LH , these cells change into lutein cells contains yellowish pigment, form the corpus luteum .

18. The corpus luteum serves as a temporary endocrine gland , secretes female sex hormones the estrogens and progesterone. It can be easily recognized as a yellowish projection on the surface of the ovary. In the absence of fertilization, the corpus luteum has a short life of 14 days , it undergoes degeneration and gets converted into the mass of fibrous tissue known as the corpus albicans .

19. Fertilization Fusion of the two mature germ cells (an ovum and the spermatozoon), takes place in the ampullary part of the uterine tube, resulting in formation of the zygote .

20. Phases of fertilization ■ Phase 1 (penetration of the corona radiate) . ■ Phase 2 (penetration of the zona pellucida): this contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte. In turn, these enzymes alter properties of the zona pellucida to become impenetrable to other spermatozoa. ■ Phase 3 (fusion of the oocyte and sperm cell membranes): after adhesion, the plasma membranes of the sperm and egg fuse.

21. The main results of fertilization Restoration of the diploid number of chromosomes: Half from the father and half from the mother. Determination of the sex of the new individual: An x-carrying sperm produces a female (xx) embryo, and a y-carrying sperm produces a male ( xy ) embryo. Initiation of cleavage: Without fertilization, the oocyte usually degenerates 24 hours after ovulation.

22. Morula Stage

23. Within 24 hours after fertilization, the zygote initiates a rapid series of cell divisions called cleavage. These divisions are not accompanied by cell growth, they subdivide the large zygote into many smaller daughter cells called blastomeres. The embryo as a whole does not increase in size during cleavage and remains enclosed in the zona pellucida. By three days, the embryo consists of 16 to 32 cells. At this stage the embryo is called a morula ( latin word morum : mulberry).

24. Some of blastomeres segregate to the center of the morula and others to the outside. The centrally placed blastomeres are now called the inner cell mass , which gives rise to the embryo proper called the embryoblast . The outside blastomeres are called the outer cell mass constitute the trophoblast ( trophe – means nutrition) which surround the embryoblast and supplies nutrition to the them . Trophoblast considered as the primary source of the fetal component of the placenta .

25. Blastulation Stage

26. Between the 4 th and 5 th day after the fertilization, the morula reaches the uterine cavity and begins to absorb fluid from the uterine cavity. As the pressure of the fluid increases the embryoblast separated from the trophoblast, and a large cavity formed within the morula called the blastocyst cavity (blastocoel) . The embryoblast cells then form a compact mass at one side of this cavity, and the trophoblast organizes into thin, single outer layer. The embryo is now called a blastocyst .

27. Implantation of blastocyst

28. The implantation occurs at the 6 th day of the first week through the second week. As the blastocyst increased in size, it hatches out of the zona pellucida and the trophoblast layer become exposed, thus enabling it to attach to the endometrium. The trophoblast gets implanted on the wall of the uterus by eroding the endometrium with proteolytic enzymes.

29. The embryoblast begins to differentiate into two layers: A distinct external (or upper) layer of columnar cells , called the epiblast layer (primitive ectoderm) . An internal (or lower) layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer (primitive endoderm) .

30. An extracellular basement membrane is laid down between the two layers as they become distinct. The resulting two layered embryoblast is called the bilaminar embryonic disc . Around 8 th day, the amniotic cavity appears as fluid begins to collect between the epiblast and trophoblast.

31. Fully implanted

32. As the embryo begins to implants, the uterine endometrium induces the trophoblast to differentiate into two sub-layers : An outer layer called the syncytiotrophoblas t , form syncytium . An inner layer called the cytotrophoblast .

33. The syncytiotrophoblast facilitates implantation by secreting proteolytic enzyme to digest the extracellular matrix of the uterine endometrium. By day 9 th , the syncytiotrophoblast ultimately comes to envelop the entire embryo. The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum .

34. The placenta develops once the blastocyst is implanted, connecting the embryo to the uterine wall .

35. The vacuoles appear in the syncytium, and fuse to form large lacunae syncytial lacunae , and this phase of trophoblast development is known as the lacunar stage . The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunae and flow through the trophoblastic system, establishing the uteroplacental circulation .

36. By the 11 th to the 12 th day of development, the blastocyst is completely embedded inside the endometrium. The blastocyst cavity gradually grows to form the yolk sac . Around the end of the 2 nd week: The embryonic disc with its yolk sac is suspended within a large cavity called chorionic cavity . Attached to the trophoblastic shell by a narrow connecting stalk , that develops lately into the umbilical cord to attach the placenta to the embryo.

37. Third week of development Gastrulation Stage

38. The most characteristic event occurring during the 3 rd week of gestation is gastrulation : The process that establishes all three germ layers in the embryo : Definitive endoderm. Intraembryonic mesoderm. Ectoderm.

39. In a 15- 16 day, the gastrulation stage begins with formation of a longitudinal midline thickening structure called primitive streak in the epiblast near the caudal end of the bilaminar embryonic disc. This streak elongates to occupy about half the length of the embryonic disc. Its continuous growth changes the embryonic disk shape from circular to ovoid-shaped .

40. Formation of the primitive streak defines all major body axes : The cranial-caudal ( head-tail ) axis. The dorsal-ventral ( back-belly ) axis. The medial-lateral axis. The left-right axis.

41. The primitive streak contains a midline narrow groove called the primitive groove surrounded by slightly bulging regions on either side. The cranial end of the primitive streak is expanded forming the primitive node , which consists of a slightly elevated area surrounding a small circular depression called the primitive pit .

42. The primitive pit and groove represent areas where cells are migrating and moving from the epiblast into the interior of the embryonic disc. This inward movement is known as invagination . The collective movement of cells through the primitive streak into the interior of the embryo to form the three primary germ layers constitutes gastrulation .

43. The process of gastrulation is complete when the formation of all three definitive germ layers of the trilaminar embryonic disc are complete throughout the disc . The epiblast cells are the source of all of the germ layers during the process of gastrulation . They will give rise to all of the tissues and organs in the embryo .

44. Formation of the germ layers

45. On day 16 , the epiblast cells lateral to the primitive streak elongate and become flask or bottle shaped, detaching from their neighbors and migrating through the primitive streak :

46. By the middle of 3 rd week, the intraembryonic mesoderm separates the ectoderm and endoderm everywhere except at: Oropharyngeal membrane cranially. Prechordal plate. Cloacal membrane caudally.

47. The endoderm derivatives: Epithelial lining of the gastrointestinal and respiratory tracts. Parenchyma of the tonsils, liver, thymus, thyroid, parathyroid, and the pancreas. The epithelium lining the auditory tube and tympanic cavity. The epithelium of the urinary bladder and urethra.

48. Intraembryonic mesoderm gives rise to: Teeth except the enamel. The connective tissue, cartilage and bone. Striated and smooth muscles, except the iris. The blood and the blood vascular and lymphatic systems. Kidneys, gonads (testes and ovaries). The cortex of the suprarenal glands. The mesothelial linings of the pericardial, pleural and peritoneal cavities. Spleen

49. The embryonic ectoderm gives rise to: The enamel of teeth. The salivary glands. The epithelium lining the nose and paranasal sinuses, the roof of the mouth, the gums and the cheeks. Skin and its appendages (hair and nails). Subcutaneous glands. The central and peripheral nervous system. The epithelium of the cornea, conjunctiva and lacrimal glands. The muscle of the iris. The neuroepithelium of the sense organs (eye, ear, and nose).

50. Growth of the embryonic disc

51. The embryonic disc, initially is flat and round, gradually becomes elongated, with a broad cranial end and narrow caudal end, due to the continuous migration of cells from the primitive streak region in a cranial direction. The primitive streak shrinks, and disappears at the end of the fourth week , when the invagination and migration of surface cells into the primitive streak are stopped.

52. In the cephalic part, germ layers begin their specific differentiation by the middle of the third week , whereas in the caudal part, differentiation begins by the end of the fourth week . The gastrulation continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalocaudally .

53. Notochord formation

54. The prenotochordal cells are derived from epiblasts. Start invaginating in the primitive node. Migrate in a midline, toward the anterior end of the embryo to form a solid rod the notochord

55. The notochord grows forward cranially and impeded by the tightly adherent endoderm and ectoderm of the prechordal plate. By day 16 , the notochord is visible throughout the length of the embryo surrounded by layered concentrations of cells, which are representing the primordia of the future vertebral bodies.

56. Notochord functions: The notochord defines the axis of the embryo and gives it some rigidity. It interacts with the overlying ectoderm to induce differentiation of the neural tube which becomes the central nervous system. Its signaling protein influences organization of somites and development of the vertebral column. Provides axial support for the embryo. In an adult, the notochord persists as the gelatinous central Nucleus pulposus of the intervertebral disks.

57. Neurulation

58. Occurs at or near the end of gastrulation during the 3 rd through 4 th week after fertilization. It is the process by which the embryo develops structures that will eventually become the nervous system, and transforms the gastrula into a neurula .

59. Neurulation occurs in four stages: Transformation of the central portion of the embryonic ectoderm into a thickened neural plate. Shaping and elongation of the neural plate. Bending of the neural plate around a medial groove followed by elevation of the lateral folds. Closure of the neural tube.

60. Formation of neural plate: On day 18 th notochord induces the overlying ectoderm to thicken and form neural plate . Cells of the plate make up the pseudostratified, columnar neuroepithelial cells neuroectoderm . The neural plate forms first at the cranial end of the embryo and then differentiates in a cranial to caudal direction. The neural plate is broad cranially and gives rise to brain , and is tapered caudally gives rise to the spinal cord .

61. Formation of the neural tube

62. During the 4 th week , the neural plate increased in length and its lateral edges elevate to form neural folds consisting of both neuroectoderm and adjacent surface ectoderm. The neural groove appears on the neural plate with the formation of neural folds. Neural folds start fusing in the middle at the cervical region and the process of fusion proceeds cranially and caudally. As a result, the neural tube is formed.

63. There will be two separate epithelial layers; the roof plate of the neural tube and the overlying surface ectoderm . Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by the cranial neuropore and caudal neuropore . Closure of the cranial neuropore occurs at approximately day 25 , whereas the posterior neuropore closes at day 28 . Neurulation is then complete , and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion.

64. Neural crest cells

65. They are a unique population of cells that originating from the crest of the neural fold as it closes during neural tube formation. These cells undergo an epithelial to mesenchymal transformation and become free mesenchymal cells. They leave the neuroectoderm by active migration to the distant parts of the body, and displacement to enter the underlying mesoderm.

66. The neural crest cells are so important and contributed to so many organs and tissues that they are sometimes referred to as the fourth germ layer . Connective tissue and bones of the face and skull. Cranial nerve ganglia . C cells of the thyroid gland . Odontoblasts . Parasympathetic ganglia of the gastrointestinal tract. Adrenal medulla . Schwann cells. Glial cells. Meninges . Melanocytes. Smooth muscle cells to blood vessels of the face and forebrain.