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The Reproductive System: Embryology and Human Development

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The Reproductive System: Embryology and Human Development 28 The Reproductive System: Embryology and Human Development PowerPoint® Lecture Presentations prepared by Steven Bassett Southeast Community College Lincoln, Nebraska © 2012 Pearson Education, Inc. Introduction • Development involves: • Differentiation of cells • Reorganization of cells • Development can be characterized by different periods of time • Prenatal development • Embryology • Postnatal development © 2012 Pearson Education, Inc. An Overview of Development • Development can be characterized by different periods of time • Prenatal development • Conception to delivery • Involves embryology (development during the prenatal period) • Postnatal development • Development from birth to maturity © 2012 Pearson Education, Inc. Fertilization • Fertilization is the joining of two haploid cells to create a diploid cell • Function of the haploid cells • Spermatozoon • Delivers the paternal chromosomes to the ovum • Ovum • Provides the maternal chromosomes • Provides nourishment for embryonic development © 2012 Pearson Education, Inc. 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 © 2012 Pearson Education, Inc. Fertilization • Fertilization details • When the egg is ovulated, it is surrounded by the corona radiata, which protects the egg as it is being ovulated • Numerous sperm cells release hyaluronidase, from their acrosomal cap, in an effort to decompose the corona radiata ANIMATION Fertilization and the Preparations for Cleavage © 2012 Pearson Education, Inc. Figure 28.1a Fertilization and Preparation for Cleavage A secondary oocyte surrounded by spermatozoa © 2012 Pearson Education, Inc. Figure 28.1b Fertilization and Preparation for Cleavage Oocyte at Ovulation Fertilization and Oocyte Activation Pronucleus Formation Begins Ovulation releases a secondary oocyte and Acrosomal enzymes from multiple sperm create The sperm is absorbed into the cytoplasm, the first polar body; both are surrounded by gaps in the corona radiata. A single sperm then and the female pronucleus develops. the corona radiata. The oocyte is suspended makes contact with the oocyte membrane, and in metaphase of meiosis II. membrane fusion occurs, triggering oocyte activation and completion of meiosis. Corona First polar radiata body Fertilizing Second polar Nucleus of Female spermatozoon body fertilizing pronucleus spermatozoon Zona pellucida Cytokinesis Begins Amphimixis Occurs and Spindle Formation and Cleavage Begins Cleavage Preparation The first cleavage division nears completion The male pronucleus develops, and spindle roughly 30 hours after fertilization. Further Metaphase of first fibers appear in preparation for the first events are diagrammed in Figure 28.2 cleavage division cleavage division. Male Female pronucleus pronucleus Blastomeres Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. Figure 28.1b Fertilization and Preparation for Cleavage (Part 1 of 2) Oocyte at Ovulation Fertilization and Oocyte Activation Pronucleus Formation Begins Ovulation releases a secondary oocyte and Acrosomal enzymes from multiple sperm create The sperm is absorbed into the cytoplasm, the first polar body; both are surrounded by gaps in the corona radiata. A single sperm then and the female pronucleus develops. the corona radiata. The oocyte is suspended makes contact with the oocyte membrane, and in metaphase of meiosis II. membrane fusion occurs, triggering oocyte activation and completion of meiosis. Corona First polar radiata body Fertilizing Second polar Nucleus of Female spermatozoon body fertilizing pronucleus spermatozoon Zona pellucida Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. Figure 28.1b Fertilization and Preparation for Cleavage (Part 2 of 2) Cytokinesis Begins Amphimixis Occurs and Spindle Formation and Cleavage Begins Cleavage Preparation The first cleavage division nears completion The male pronucleus develops, and spindle roughly 30 hours after fertilization. Further Metaphase of first fibers appear in preparation for the first events are diagrammed in Figure 28.2 cleavage division cleavage division. Male Female pronucleus pronucleus Blastomeres Events at fertilization and immediately thereafter © 2012 Pearson Education, Inc. 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) © 2012 Pearson Education, Inc. Prenatal Development • 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) © 2012 Pearson Education, Inc. Prenatal Development • Cleavage and Blastocyst Formation • Cell division results in the formation of blastomeres • A solid ball of cells eventually develops – this is a morula • 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 © 2012 Pearson Education, Inc. Figure 28.2 Cleavage and Blastocyst Formation Blastomeres Polar bodies 4-cell stage 2-cell Early morula stage DAY 1 DAY 2 DAY 3 Advanced DAY 4 morula First cleavage division DAY 0: Inner cell Fertilization mass DAY 6 Ovulation Blastocoele DAYS 7–10: Implantation in Trophoblast uterine wall Blastocyst (See Figure 28.3) © 2012 Pearson Education, Inc. Prenatal Development • Cleavage and Blastocyst Formation (continued) • Trophoblast cells provide nutrients to the developing embryo • The inner cell mass consists of stem cells that will develop into all the cells of the body © 2012 Pearson Education, Inc. Prenatal Development • Implantation • Upon contact with the endometrial lining, the trophoblast cells divide rapidly • The trophoblast cells “fuse” with the endometrial lining forming a syncytial trophoblast • This layer of cells releases hyaluronidase to erode away more of the endometrial lining so the mass can implant © 2012 Pearson Education, Inc. Prenatal Development • Implantation (continued) • Upon implantation, the inner cell mass separates from the trophoblast area • When the inner cell mass separates from the trophoblast, two cavities form: • Amnionic cavity • Blastocoele cavity ANIMATION Implantation © 2012 Pearson Education, Inc. Figure 28.3 Stages in the Implantation Process DAY 6 FUNCTIONAL LAYER UTERINE OF ENDOMETRIUM CAVITY Uterine glands Blastocyst DAY 7 Trophoblast Blastocoele Inner cell mass DAY 8 Cellular Syncytial trophoblast trophoblast DAY 9 Hypoblast Blastodisc Epiblast Developing primary villi Amniotic cavity Lacuna © 2012 Pearson Education, Inc. Prenatal Development • Formation of the Blastodisc • A layer of cells forms between the amnionic cavity and the blastocoele cavity • The layers are called • Epiblast • Hypoblast © 2012 Pearson Education, Inc. Figure 28.4a Blastodisc Organization and Gastrulation ENDOMETRIUM Syncytial trophoblast Amniotic cavity Cellular trophoblast Epiblast of blastodisc Hypoblast of blastodisc Lacunae Yolk sac Blastocoele Uterine lining DAY 10 The blastodisc begins as two layers: the epiblast, facing the amniotic cavity, and the hypoblast, exposed to the blastocoele. Migration of epiblast cells around the amniotic cavity is the first step in the formation of the amnion. Migration of hypoblast cells creates a sac that hangs below the blastodisc. This is the first step in yolk sac formation. © 2012 Pearson Education, Inc. Prenatal Development • Gastrulation and Germ Layer Formation • Eventually some cells of the epiblast move toward the center of the blastodisc creating a primitive streak • This movement is called gastrulation • As the cells move toward the primitive streak area, they begin to migrate between the epiblast and hypoblast layers • This creates three distinct layers of cells © 2012 Pearson Education, Inc. Prenatal Development • Gastrulation and Germ Layer Formation • The three layers of cells are: • Ectoderm (derived from the epiblast layer) • Mesoderm (new layer between the epiblast and hypoblast) • Endoderm (derived from the hypoblast layer) © 2012 Pearson Education, Inc. Figure 28.4 Blastodisc Organization and Gastrulation ENDOMETRIUM Syncytial trophoblast Amniotic cavity Cellular trophoblast Epiblast of Ectoderm blastodisc Blastodisc Hypoblast of blastodisc Primitive Lacunae Yolk sac streak Blastocoele Uterine lining Mesoderm Endoderm DAY 10 DAY 12 The blastodisc begins as two layers: the epiblast, Migration of epiblast cells into the facing the amniotic cavity, and the hypoblast, exposed region between epiblast and to the blastocoele. Migration of epiblast cells around hypoblast gives the blastodisc a the amniotic cavity is the first step in the formation of third layer. From the time this the amnion. Migration of hypoblast cells creates a sac process (gastrulation) begins, the that hangs below the blastodisc. This is the first step in epiblast is called ectoderm, the yolk sac formation. hypoblast endoderm, and the migrating cells mesoderm. © 2012 Pearson Education, Inc. Prenatal Development • The ectoderm, mesoderm, and endoderm are collectively
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