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The Placenta I

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BIOL 1840U
Peter Cheung

The Placenta I. Early development a. embryo implants at blastocyst stage i. inner cell mass—becomes fetus ii. trophoblastic rim—develops into the placenta b. endometrial stromal cells become lg, w/ abundant pink cytoplasm = decidual cells c. Trophoblast Development i. proliferate & invade endometrium ii. replicating trophoblast cells are uninuclear--cytotrophoblasts iii. syncytiotrophoblasts—derived from cytotrophoblast fusion & incapable of further division d. Villi Development i. Vacuoles form in trophoblastic layer => become confluent as lacunae ii. trabeculae—pillars of trophoblast cells between lacunae iii. maternal sinusoids empty into the lacunae as the endometrium is invaded by trophoblasts iv. Primary villi—composed only of syncytio- & cyto- trophoblasts v. Secondary villi—mesenchyme layer grows into the trabeculae vi. Tertiary villi—fetal capillaries develop in mesenchyme of villi vii. Terminal villi—specialized to optimize diffusion 1. contain dilated capillaries located in periphery of villus 2. trophoblastic structure is simplified (only syncytiotrophoblasts) 3. goal is to decrease diffusion barrier viii. Histology of Villi 1. core of mesodermal fibroblastic tissue w/ fetal capillaries 2. basement membrane—separates mesodermal layer from cyto- & syncytio-trophoblasts 3. @ first, there’s a cytotrophoblast layer w/ in syncytiotrophoblast layer, but later in development the cytotrophoblast layer disappears 4. Hofbauer cell—phagocytic cell that clears cellular debris among villi II. Fetal membranes a. Decidua—portion of endometrium which undergoes morphological changes i. Decidua capsularis—thin part covering the fetus ii. Decidua basalis—between fetus & muscular wall iii. Decidua parietalis—part lining rest of uterus b. Chorion Frondosum—develops in chorion in contact w/ deciduas basalis; la
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