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Lecture 3

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Biological Sciences
Connie Soros

1 BIOB33 – Lecture 3 Prof’s Speech - Purple Textbook Material - Orange The Reproductive System: Embryology and Human Development (based on Chapter 28) Introduction • Development involves: o Differentiation of cells o Reorganization of cells • Development can be characterized by different periods of time o Prenatal development – fertilization/conception to delivery  Involves Embryology – prenatal development focussing on the first two months after fertilization o Postnatal development – development from birth to maturity Fertilization • Fertilization is the joining of two haploid cells (sperm and egg) to create a diploid cell • Function of the haploid cells: • Spermatozoon (see chapter 27) o Delivers the paternal chromosomes to the ovum o sperm has head, midsection and tail o head has an acrosome (point of head) 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)  Radiating “crown” of cells o Deep to corona radiata is another protective layer the jelly-like zona pellucida  Zona pellucida = gel-like protection o Contains a polar body which is a non-functioning cell that eventually degrades  The polar body has nothing to do with fertilization o Provides the maternal chromosomes (secondary oocyte that has 23 haploid chromosomes, 22 autosomes and a second X chromosome, suspended in metaphase of meiosis II) o Ovum provides nourishment for embryonic development • Mothers always provide X chromosomes, fathers can provide either the X or Y Fertilization • An ovum is produced in one of the ovaries once a month and travels along the uterine tube to the uterus (cilia in the tube move the egg along and it takes about 4-5 days for the egg to get to the uterus) • Fertilization usually occurs in the ampulla of the uterine tube (end of tube close to the ovary) o Fertilization can also happen in the uterus • 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 2 • Sperm – “survival of the fittest”, maximum number of sperm are sent, only one fertilizes the egg • Egg – only one egg is produced, with many layers of protection • Fertilized egg attaches to the uterine wall • Zygote – develops inside of the endometrium • As the baby gets larger, the wall gets larger • Fertilization details o 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 oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis, cortical granules from inside the oocyte dissolve into the cytoplasm changing the permeability of the egg and make it impermeable to other sperm  The acrosome of the sperm contains the acrosomal enzymes  When the acrosome hits the egg, it begins to break down the corona radiata  Membrane fusion occurs between the head of the sperm and the outside of the oocyte, which changes its permeability 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  Spindle and cleavage = the mixing of male and female genetic information o Amphimixis Occurs and Cleavage Begins: metaphase of first cleavage division o Cytokinesis Begins: The first cleavage division nears completion roughly 30 hours after fertilization  Cytokinesis is the separation of the cytoplasm o Once fertilized called a zygote Prenatal Development • Prenatal development is gestation period (9 months) • Prenatal development is divided into trimesters o First trimester (rudiments of all organs appear) o Second trimester (fetus looks like a human) o Third trimester (organs become functional, rapid growth) • The First Trimester - 1 to 12 weeks • Four events within the first trimester o Cleavage (sequence of cell reproduction) o Implantation (implantation into endometrial lining) o Placentation (formation of the placenta) o Embryogenesis (development of the embryo) • Cleavage and Blastocyst Formation • Cell division results in the formation of blastomeres (blast – precursor) – identical daughter cells, get smaller with each subsequent division - Begins in the fallopian tubes after fertilization, before being implanted in the endometrium - Figure 28.2 – in the first cleavage division, the cytoplasm begins to separate 3 - 2-cell stage – cell has two blastomeres - 4-cell stage – cell has four blastomeres - Early morula (8 blastomeres)  advanced morula (32 blastomeres)  blastocyst (by day 6) - Throughout the divisions, the zygote always stays the same size, the cells within the zygote are dividing - Blast – means making new/producing new cells • 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 (inner cell mass) and a hollow cavity called the blastocoele  The blastocoele is the developing cavity • 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/nut
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