PLACENTATION -
Define and use properly the following words: Extraembryonic membranes, Yolk sac, Splanchnopleure, Mesodermal, Amnion, Chorion, Allantois, Classification of placentae, Choriovitelline, Chorioallantoic, Diffuse, Cotyledonary, Zonary, Discoid, Maternal-fetal junction, Villous, Labyrinthine, Placental barrier, Fetal layers, Endothelium of allantoic, Mesenchyme, Trophoblast, Cytotrophoblast, Syncytiotrophoblast, Multinucleated, Maternal layers, Uterine epithelium, Maternal endothelium, Epitheliochorial, Syndesmochorial, Endotheliochorial, Hemochorial, Non-deciduate, Deciduate, Decidual cells, Fetal circulation, Choriovitelline placenta, Chorioallantoic placenta, Areolae, Microplacentome, Endometrial cups, Hippomane, Placentome, Uterine caruncle, Cotyledon, Marginal hematoma.
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Describe and associate basic structure/function for the following: all structures listed above.
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Identify by microscopy: Extraembryonic membranes, Yolk sac, Amnion, Chorion, Allantois, Choriovitelline, Chorioallantoic, Diffuse, Cotyledonary, Zonary, Discoid, Maternal-fetal junction, Villous, Labyrinthine, Placental barrier, Fetal layers, Endothelium of allantoic vessels, Trophoblast, Cytotrophoblast, Syncytiotrophoblast, Multinucleated, Maternal layers, Epitheliochorial, Syndesmochorial, Endotheliochorial, Hemochorial, Non-deciduate, Deciduate, Uterine artery, Fetal Choriovitelline placenta, Chorioallantoic placenta, Areolae cups, Microplacentome, Endometrial cups, Hippomane, Placentome, Uterine caruncle, Cotyledon, Marginal hematoma.
26.0 PLACENTATION I. Extraembryonic membranes
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Yolk sac
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forms from splanchnopleure
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mesodermal layer contains blood vessels
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blood vessels carry nutrients from yolk into embryo
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also site of hematopoiesis
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fate: drawn into abdominal cavity and digested
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remnant is called Meckel's diverticulum of the ileum
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Amnion
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folds of somatopleure
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lift from the sides and ends of the embryo and fuse
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outer layer becomes chorion; inner becomes amnion
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connection between layers may persist in ruminant, pig, chick; breaks down in horse and carnivore
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avascular
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amnion contains fluid which protects embryo and prevents adhesion to surrounding membranes
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Chorion
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expands and lies next to uterine lining (or shell)
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avascular
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Allantois
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forms off hindgut from endoderm and splanchnic mesoderm
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blood vessels in the mesoderm form allantoic (umbilical) circulation
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combine with the chorion to form the placenta
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functions in gas and waste exchange
II. Introduction to Placentation
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prior to contacting the endometrium, the embryo is nourished by embryotrophe
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extraembryonic membranes and endometrium form the placenta
III. Classification of Placentae
---based on involvement of fetal extraembryonic membranes
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Choriovitelline
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Yolk sac
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early in bitch, queen;
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first 1/4 horse
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Inverted yolk sac
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rodents and lagomorphs
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endodermal surface contacts uterus
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Chorioallantoic
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Shape
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Diffuse
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sow, mare
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chorion is uniformly attached
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Cotyledonary
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ruminants
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isolated tufts of chorion (cotyledon) attach to uterine caruncle to form a placentome
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Zonary
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Carnivore
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band of villi around chorion
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Discoid
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primates and rodents
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Structure of maternal-fetal junction
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Folded
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Villous
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Labyrinthine
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Number of tissue layers involved in placental barrier
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Components of the barrier
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Fetal layers
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Endothelium of allantoic vessels
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Mesenchyme
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Trophoblast
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cytotrophoblast-individual cells
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syncytiotrophoblast-fused cells
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giant cells-multinucleated; may have fused with maternal cells
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Maternal layers
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Uterine surface epithelium
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Connective tissue
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Maternal endothelium
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Classification based on loss of maternal layers
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number of fetal layers remain constant; the maternal layers vary
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Epitheliochorial
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all 3 layers persist
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Syndesmochorial
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trophoblast cells fused as syncytial cells
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some of endometrium eroded
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Endotheliochorial
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uterine epithelium and c.t. gone
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Hemochorial
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all three layers gone; direct contact with blood
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Based on degree of anchoring into maternal tissue
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Non-deciduate
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little maternal tissue lost
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Deciduate
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deep invasion
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decidual cells present
IV. Vascular supply to the placenta
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Maternal
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uterine a.
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Fetal
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Choriovitelline placenta
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omphalomesenteric and vitelline vessels
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Chorioallantoic placenta
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umbilical circ.
V. Species variations
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Sow (diffuse, non-deciduate, epitheliochorial)
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Zones of chorionic surface
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Placental
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central half has folds and areolae
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areolae are cups in chorion opposite endometrial glands
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Paraplacental
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Ischemic
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Mare (diffuse, non-deciduate, villous, epithliochorial)
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Microplacentome
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focal areas of attachment
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Endometrial cups
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at junction of allantochorion and yolk sac, chorionic girdle forms
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embryo invades endometrium at 36-38 days
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large cells which produce horse chorionic gonadotrophin appear
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gone after 80 days
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Hippomane
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free-floating calcified bodies of uncertain origin in Allantois
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Cow and ewe (cotyledonary, non-deciduate, epitheliochorial)
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Placentome
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Uterine caruncle
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75-120 in cow; 80-90 in sheep
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convex in cow; concave in sheep
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induces chorion to form cotyledon
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sloughs after birth
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Cotyledon
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Bitch and queen (zonary, deciduate, endotheliochorial)
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Marginal hematoma
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maternal endometrium degenerates at periphery of zonary placenta and hemorrhages
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fetal epithelium secretes an anticoagulant so no clot forms
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brown or green due to degradation of hemoglobin
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