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FERTILIZATION, CLEAVAGE N BLASTOCYST FORMATION.pdf
- 1. Fertilization; Cleavage and Blastocyst formation; Implantation Coursetitle: Gross Anatomy Lecturer: Dr Sabiu B. SOJA Unit: Epigenetics and Neurodevelopmental Anatomy Date : 23rd September, 2024
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- 3. Introduction-Fertilization 3 • Define asa fusion of a sperm nucleus of paternal origin, with an egg nucleus, of maternal origin, to form the primary nucleus of an embryo. Or • Is the fusion of haploid gametes, egg/ovum and sperm , to form the diploid zygote • Usually occurs in ampulla of the uterine tube and occurs in three phases:
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- 5. Fertilization (Cont’d) 5 • Phase1 : Sperm penetration of corona radiata - involves the action of both sperm and uterine tube mucosal enzymes. • Phase 2:-Sperm binding and penetration of the zona pellucida • Phase 3: Fusion of sperm and oocyte cell membranes
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- 10. Cleavage 1 0 • Is therapid mitotic divisions of the zygote within 24hrs after fertilization • It is not accompanied by cell growth, the large zygote is subdivided 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
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- 12. Cleavage (Cont’d) 1 2 • 1STcleavage division divides the zygote to produce two daughter cells • The 2nd division, which is complete at about forty hours after fertilization, produces four blastomeres. • By 3days, the embryo consists of 6-12 cells, while by 4 days, it consists of 16- 32 blastosmere cells. • The embryo at this stage is called a morula
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- 14. Cleavage (Cont’d) 14 • Cleavagein humans is Holoblastic, asymmetrical and asynchronous • Which means the cells divide completely through their cytoplasm, • Daughter cells are unequal in size (i.e., one cell gets more cytoplasm than the other) at least during the first few cell divisions and only one cell will divide at a time
- 15. Cleavage (Cont’d) 15 • Thelargest daughter cell will divide next at least during the first few cell divisions • The process of cleavage eventually forms a blastula consisting of cells called blastomeres
- 16. Blastocyst formation 16 • Involvesfluid secreted within the morula that forms the blastocyst cavity • The conceptus is now called a blastocyst • 1. The inner cell mass of the blastocyst is called the embryoblast (becomes the embryo) • 2. The outer cell mass of the blastocyst is called the trophoblast
- 17. Blastocyst formation (Cont’d) 17 •The embryoblast cells are pluripotent. • Pluripotency refers to a stem cell that can differentiate into ectoderm, mesoderm, and endoderm • Zona pellucida degeneration occurs by day 4 after conception for implantation to occur
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- 19. Implantation 19 • The blastocystusually implants within the posterior superior wall of the uterus by day 7 after fertilization. • Implantation occurs in the functional layer of the endometrium during the progestational (secretory) phase of the menstrual cycle. • The trophoblast proliferates and differentiates into the cytotrophoblast and syncytiotrophoblast
- 20. Implantation (Cont’d) 20 • Failureof implantation may involve immune rejection (graft versus-host reaction) of the antigenic conceptus by the mother • If an embryo implants, the syncytiotrophoblast produce the hormone, human chorionic gonadotropin (hCG), • Which supports the corpus luteum and thus maintains the supply of progesterone
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- 23. Bilaminar Germ Disc 23 •Begins from 8- 14days • The embryoblast differentiates into two distinct cellular layers: • The dorsal epiblast layer (columnar cells) and the ventral hypoblast layer (cuboidal cells). • The epiblast and hypoblast together form a flat, ovoid-shaped disk known as the bilaminar embryonic disk. • Within the epiblast, clefts develop and eventually coalesce to form the amniotic cavity.
- 24. Bilaminar Germ Disc(Cont’d) 24 • Hypoblast cells migrate and line the inner surface of the cytotrophoblast to form the exocoelomic membrane, • which delimits a space called the exocoelomic cavity (primitive yolk sac). • The primitive yolk sac is later called the definitive Yolk Sac when a portion of exocoelomic cavity is pinches off as an exocoeloemic cavity
- 25. Bilaminar Germ Disc(Cont’d) 25
- 26. Bilaminar Germ Disc(Cont’d) 26 • The syncytiotrophoblast is the outer multinucleated zone of the trophoblast where no mitosis occurs (i.e., it arises from the cytotrophoblast) • During this period, the syncytiotrophoblast continues its invasion of the endometrium • Thereby eroding endometrial blood vessels and endometrial glands. • Lacunae form within the syncytiotrophoblast and become filled with maternal blood and glandular secretions.
- 27. Bilaminar Germ Disc(Cont’d) 27 • In addition, endometrial stromal cells (decidual cells) at the site of implantation become filled with glycogen and lipids and also supply nutrients to the embryoblast. • The isolated lacunae fuse to form a lacunar network through which maternal blood flows, thus establishing early uteroplacental circulation
- 28. Bilaminar Germ Disc(Cont’d) 28
- 29. Bilaminar Germ Disc(Cont’d) 29 • Is mitotically active as new cytotrophoblastic cells migrate into the syncytiotrophoblast • Fueling the growth of the syncytiotrophoblast. • Cytotrophoblastic cells also produce local mounds called primary chorionic villi that bulge into the surrounding syncytiotrophoblast.