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Lecture 3

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Biological Sciences
Connie Soros

1 Lecture 3 based on Chapter 28 The Reproductive System: Embryology and Human Development Introduction Development involves: Differentiation of cells Reorganization of cells Development can be characterized by different periods of time Prenatal development – fertilization/conception to delivery Involves Embryology – prenatal development focussing on the first two months after fertilization Postnatal development – development from birth to maturity Fertilization Fertilization is the joining of two haploid cells to create a diploid cell Function of the haploid cells: Spermatozoon (see chapter 27) Delivers the paternal chromosomes to the ovum o sperm has head, midsection and tail o head has an acrosome and a nucleus containing 22 haploid chromosomes and one X or Y chromosome) o midsection has mitochondria to provide energy to move flagellum o flagellum (tail) moves to allow sperm to swim Ovum o Outermost layer has loosely defined cells called corona radiata (protective layer) o Deep to corona radiata is another protective layer the jelly-like zona pellucida o Contains a polar body which is a non-functioning cell that eventually degrades Provides the maternal chromosomes (secondary oocyte that has 23 haploid chromosomes, 22 autosomes and a second X chromosome, suspended in metaphase of meiosis II) Ovum provides nourishment for embryonic development Fertilization Fertilization occurs in the ampulla of the uterine tube 200 million sperm cells enter the vaginal canal Only about 10,000 make it to the uterine tubes Less than 100 actually contact the egg Only one will fertilize the egg Fertilization Fertilization details Oocyte at Ovulation: Ovulation releases a secondary oocyte and the first polar body; both are surrounded by the corona radiata, which protects the egg. The oocyte is suspended in metaphase of meiosis II o Fertilization and Oocyte Activation: Acrosomal enzymes (hyaluronidase) from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the 2 oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis o Pronucleus Formation Begins: The sperm is absorbed into the cytoplasm, the female pronucleus develops. o Spindle Formation and Cleavage Preparation: The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division. o Amphimixis Occurs and Cleavage Begins: metaphase of first cleavage division o Cytokinesis Begins: The first cleavage divison nears completion roughly 30 hours after fertilization o Once fertilized called a zygote Prenatal Development Prenatal development is gestation period (9 months) Prenatal development is divided into trimesters First trimester (rudiments of all organs appear) Second trimester (fetus looks like a human) Third trimester (organs become functional, rapid growth) The First Trimester - 1 to 12 weeks Four events within the first trimester Cleavage (sequence of cell reproduction) Implantation (implantation into endometrial lining) Placentation (formation of the placenta) Embryogenesis (development of the embryo) Prenatal Development Cleavage and Blastocyst Formation Cell division results in the formation of blastomeres (blast – precursor) A solid ball of cells eventually develops – this is a morula (morus – mulberry, tightly packed) Some cells migrate to one “edge” of the morula creating a mass of cells and a hollow cavity called the blastocoele The ball of cells is now called the blastocyst The outer layer of the blastocyst consists of cells called the trophoblast (tropho – food, blast – precursor, these cells will provide food/nutrients to the developing embryo) The inner cell mass consists of stem cells that will develop into all the cells and cell types in the body (note: stem cells that are used for cloning are from the blastocyst stage,
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