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