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Parents might wonder if vampire myths originate in an unconscious recollection of this earliest mother-child encounter. Barry Werth, 2002 Placentation This section will explore the intimate relationship formed between the developing embryo and the mother. From its initial contact with the uterine wall the developing embryo becomes an endoparasite that takes advantage and maintains its own well-being at the expense of mom. Not only does the embryo tap into mom’s blood supply, but it also influences her entire hormonal regime. After this lecture you should be able to: • Describe the process of implantation. • Describe the extraembryonic membranes and their associated structures and the structure of the placenta. • Describe the fetal contribution to the placenta. • Describe the maternal contribution to the placenta. • Explain the process of the circulatory exchange within the placenta and the relation to the blood flow in the mother and the fetus. 43 Anatomy of the Uterine Wall The mammalian nest Three-layered structure Perimetrium – serosa Myometrium – smooth muscle coat Endometrium – mucosa Endometrial stroma Endometrial epithelium Functional divisions of endometrium Stratum functionalis or decidua Stratum basalis Implantation Endoparasitic invasion of the uterine wall Uterine contact When Where How E m b r y o l o g y 44 L e c t u r e M a n u a l b y M a r k N i e l s e n P l a c e n t a t i o n Differentiation and development of the trophoblast Cytotrophoblast Syncytiotrophoblast Rapid hypertrophy Timeline 8 days Enzymatic erosion of uterine wall Emergence of lacunae 9 days Disruption of endometrial vessels and glands Formation of lacunar network 10 days Endocrine activity - production of hCG Completion of implantation Closing plug – day 10 Complete regeneration of endometrial epithelium – day 12 12 days 45 Development of the Chorionic Sac Limiting membrane of the embryo Primary chorionic villi What are they? Developmental induction Timeline Extraembryonic somatic mesoderm Chorion Contributions Chorionic cavity 14 days Contents of chorionic cavity Amnionic sac over epiblast Yolk sac over hypoblast Connecting stalk E m b r y o l o g y 46 L e c t u r e M a n u a l b y M a r k N i e l s e n P l a c e n t a t i o n Allantois Development When does it appear? From what does it arise? Comparative embryology Functional significance Placenta Fetomaternal transport system Fetomaternal organ Fetal contribution Maternal contribution Decidua Decidua basalis Decidua capsularis Decidua parietalis 47 Placental development Development of the chorion Chorion and decidua basalis Formation of the villous chorion or chorion frondosum Chorion and decidua capsularis Formation of the smooth chorion or chorion laeve Expansion of villous chorion into decidua basalis Formation of the intervillous space Lacunae Lacunar network Connection with maternal vessels Contents of intervillous space E m b r y o l o g y 48 L e c t u r e M a n u a l b y M a r k N i e l s e n P l a c e n t a t i o n Formation of chorionic villi Primary stem villus Secondary stem villus Tertiary stem villus 49 Anatomy of the fully developed placenta Maternal components Endometrial septa Maternal blood in intervillous space Endometrial arteries (spiral) and veins Fetal components Chorionic plate Cytotrophoblastic shell Cotyledons Intervillous space Tertiary stem villus with branching chorionic villi Fetal capillaries E m b r y o l o g y 50 L e c t u r e M a n u a l b y M a r k N i e l s e n P l a c e n t a t i o n 51 Placental membrane Early placenta Syncytiotrophoblast Cytotrophoblast Connective tissue core Capillary endothelium Late placenta Fibrinoid material Connective tissue Capillary endothelium Placental functions Metabolism Transport Endocrine secretion E m b r y o l o g y 52 L e c t u r e M a n u a l b y M a r k N i e l s e n