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PowerPoint® Lecture Slide Presentation by Vince Austin Human Anatomy & Physiology FIFTH EDITION Elaine N. Marieb Chapter 29 Pregnancy and Human Development Part B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Placentation • Formation of the placenta from: • Embryonic trophoblastic tissues • Maternal endometrial tissues • The chorion develops fingerlike villi, which: • Become vascularized • Extend to the embryo as umbilical arteries and veins • Lie immersed in maternal blood • Decidua basalis – part of the endometrium that lies between the chorionic villi and the stratum basalis Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Placentation • Decidua capsularis – part of the endometrium surrounding the uterine cavity face of the implanted embryo • The placenta is fully formed and functional by the end of the third month • Embryonic placental barriers include: • The chorionic villi • The endothelium of embryonic capillaries • The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Placentation Figure 29.7a-c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gem Layers • The blastocyst develops into a gastrula with three primary germ layers: ectoderm, endoderm, and mesoderm • Before becoming three-layered, the inner cell mass subdivides into the upper epiblast and lower hypoblast • These layers form two of the four embryonic membranes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Embryonic Membranes • Amnion – epiblast cells form a transparent membrane filled with amniotic fluid • Provides a buoyant environment that protects the embryo • Helps maintain a constant homeostatic temperature • Amniotic fluid comes from maternal blood, and later, fetal urine Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Embryonic Membranes • Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo • Forms part of the digestive tube • Produces earliest blood cells and vessels • Is the source of primordial germ cells Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Embryonic Membranes • Allantois – a small outpocketing at the caudal end of the yolk sac • Structural base for the umbilical cord • Becomes part of the urinary bladder • Chorion – helps form the placenta • Encloses the embryonic body and all other membranes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gastrulation • During the 3rd week, the two-layered embryonic disc becomes a three-layered embryo • The primary germ layers are ectoderm, mesoderm, and endoderm • Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gastrulation • As cells begin to migrate: • The first cells that enter the groove form the endoderm • The cells that follow push laterally between the cells forming the mesoderm • The cells that remain on the embryo’s dorsal surface form the ectoderm • Notochord – rod of mesodermal cells that serves as axial support Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Primary Germ Layers • Serve as primitive tissues from which all body organs will be derived • Ectoderm – forms structures of the nervous system and skin epidermis • Endoderm – forms epithelial linings of the digestive, respiratory, and urogenital systems • Mesoderm – forms all other tissues • Endoderm and ectoderm are securely joined and are considered epithelia Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Primary Germ Layers Figure 29.8a-e Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Primary Germ Layers Figure 29.8f-h Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Organogenesis • Gastrulation sets the stage for organogenesis, formation of body organs • By the 8th week all organ systems are recognizable Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Specialization of Ectoderm • Neurulation – the first event of organogenesis gives rise to the brain and spinal cord • Ectoderm over the notochord thickens, forming the neural plate • The neural plate folds inward as a neural groove with prominent neural folds • By the 22nd day, neural folds fuse into a neural tube, which pinches off into the body • The anterior end becomes the brain; the rest becomes the spinal cord • Associated neural crest cells give rise to cranial, spinal, and sympathetic ganglia Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Specialization of Ectoderm Figure 29.9a, b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Specialization of Ectoderm Figure 29.9c, d Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings