📝 STANDARD DESCRIPTION:
Placenta = organ of physiological exchange between foetus + dam. Has both foetal (chorioallantois) and maternal (endometrial) parts.
Classification by SHAPE / DISTRIBUTION OF VILLI:
• Diffuse — chorionic villi spread over entire chorion. Mare, sow.
• Cotyledonary — villi in tufts (cotyledons) attached to caruncles → placentome. Ruminants (cow, sheep, goat).
• Zonary — band of villi around middle of chorion. Bitch, queen.
• Discoid — disc-shaped villus area. Primates, rodents.
Classification by HISTOLOGY (layers between fetal + maternal blood):
• Epitheliochorial — 6 layers; minimal invasion. Mare, sow.
• Synepitheliochorial — the modern description of the ruminant interface, where trophoblast binucleate cells fuse with maternal epithelial cells while much of the uterine epithelium is retained. Cow, sheep and goat.
• Endotheliochorial — 4 layers; reaches maternal endothelium. Bitch, queen.
• Haemochorial — 3 layers; chorion bathed directly in maternal blood. Primates, rodents.
📚 DETAILED DESCRIPTION:
PLACENTAL CLASSIFICATION
The placenta is the foetal-maternal interface. Veterinary anatomy classifies it by THREE criteria simultaneously: shape of villi, histological layers, and fate at birth.
I. CLASSIFICATION BY GROSS SHAPE / VILLI DISTRIBUTION:
1. Diffuse Placenta — chorionic villi cover almost the entire chorion in a uniform velvet of small projections.
• Examples: Mare, Sow.
• In mare, the small villi cluster as microcotyledons (~1-2 mm).
2. Cotyledonary Placenta — villi grouped in discrete tufts (cotyledons) on chorion. Each cotyledon attaches to a non-glandular elevation of endometrium called a caruncle. Cotyledon + caruncle = PLACENTOME.
• Examples: Cow (~80-120 placentomes), Sheep, Goat.
• Placentome shape varies — convex in cow, concave in ewe/doe ('cup-and-saucer').
3. Zonary (Annular) Placenta — a complete or nearly complete BAND of villi around the equator of the chorion; ends free.
• Examples: Bitch, Queen, Mink, Bear.
• In bitch, the lateral edges of the band develop the marginal hematoma — green-brown blood breakdown product (uteroverdin) gives the 'green' fluid of canine parturition.
4. Discoid Placenta — villi form a single disc.
• Examples: Humans, monkeys, rodents.
• Bidiscoid — 2 discs (some monkeys).
II. CLASSIFICATION BY HISTOLOGY (LAYERS BETWEEN FETAL & MATERNAL BLOOD):
From maternal blood → fetal blood, possible layers are:
1. Endometrial endothelium (vessel wall).
2. Endometrial CT.
3. Endometrial epithelium.
4. Chorionic epithelium (trophoblast).
5. Chorionic CT.
6. Chorionic capillary endothelium.
1. Epitheliochorial — ALL 6 LAYERS PRESENT (least invasive). Foetal villus only touches maternal epithelium.
• Examples: Mare, Sow.
• Foetus depends entirely on maternal absorption + secretion — colostrum is essential for IgG transfer (no transplacental antibodies in horse).
2. Synepitheliochorial — the modern ruminant classification; trophoblast binucleate cells fuse with maternal epithelial cells, so the older five-layer syndesmochorial model is not an accurate universal description.
• Example: Cow, Sheep, Goat (older textbooks called this syndesmochorial; modern texts often call it 'synepitheliochorial' — i.e., epithelium is partially retained but with binucleate trophoblast cells fusing with maternal cells).
• Like horse, ruminants need colostrum for IgG.
3. Endotheliochorial — 4 layers; trophoblast erodes maternal CT; foetal chorion contacts maternal vessel ENDOTHELIUM.
• Examples: Bitch, Queen.
• Some IgG transfer prenatally; colostrum still important.
4. Haemochorial — 3 layers; maternal blood directly bathes the chorion (no maternal endothelium, CT, or epithelium).
• Examples: Primates (human, monkey), Rodents (rat, mouse), Rabbit.
• Generous IgG transfer in utero — primates receive substantial prenatal maternal IgG, although neonatal immunity remains developmentally immature.
5. Haemoendothelial — only foetal endothelium between blood streams (extremely intimate).
• Example: Some rodents.
III. CLASSIFICATION BY FATE AT BIRTH:
1. Non-deciduate (Adeciduate) — foetal villi simply slip out of crypts; minimal maternal tissue lost; minimal bleeding.
• Examples: Mare, Cow, Sow.
2. Deciduate — placenta tears off endometrial layers with it; bleeding ('lochia') follows for days/weeks.
• Examples: Bitch, Queen, Primates.
IV. SUMMARY TABLE:
• Mare: Diffuse + Epitheliochorial + Adeciduate.
• Sow: Diffuse + Epitheliochorial + Adeciduate.
• Cow/Sheep/Goat: Cotyledonary + Synepitheliochorial + Adeciduate.
• Bitch/Queen: Zonary + Endotheliochorial + Deciduate.
• Human/Primates: Discoid + Haemochorial + Deciduate.
// COMPARATIVE ANALYSIS
| COW |
Cotyledonary; ~80-120 placentomes; convex shape. Retained fetal membranes are generally defined at about 24 hr postpartum. |
| MARE |
Diffuse microcotyledonary. Placenta examined post-foaling — the 'F-shape' must be complete; missing tags = retained = septic metritis emergency. |
| BITCH |
Zonary with marginal hematoma → green discharge before the first puppy may indicate placental separation and requires prompt assessment if a puppy does not immediately follow. |
CLINICAL RELEVANCE
Failure of Passive Transfer (FPT): No transplacental IgG in horse/cow → newborn must receive colostrum within 6-12 hr. Placentitis: Bacterial infection of placenta → premature foaling/calving; common in horse (Streptococcus, E. coli). Hydrops: Excess allantoic/amniotic fluid — life-threatening for dam.
