- Overview
- Chronology
- Endometrial Cups
- Examination
- Villi
- Allantochorion
- Amnion
- Umbilical Cord
- Allantoic Fluid
- Amniotic Fluid
BASICS
- Shape—diffuse; normal placenta covers entire endometrial surface; see exceptions in section Villi
- Origin—allantochorionic; fusion of fetal allantois and chorion
- Degree of invasion—epitheliochorial; fetal tissue directly apposes maternal endometrium. Endometrial cups only fetal extensions into maternal tissue
- Vascular structure—microcotyledonary/villous; maternal and placental vessels in near apposition
- Degree of attachment—adeciduate; no loss of maternal tissue during placental formation or expulsion
- Conceptus mobile to day 16 post conception (day 0); spherical until ≈day 35; thereafter ellipsoid
- Endometrial cup formation by days 36–38
- Pregnancy maintenance by allantochorionic placentation experimentally from day 70
- Placenta contacts entire endometrial surface by ≈day 77; development complete by day 150
- Fetal trophoblast invades endometrium—days 36–38; peak function ≈day 70
- Cups necrose day 120–150, slough and form allantochorionic pouches
- Produce equine chorionic gonadotropin; support formation/function of accessory corpus luteum
- Placenta ruptures at cervical star; allantochorion usually passed with allantois exposed
- Complete examination requires observing chorionic surface, amnion, and umbilical cord
- Identify body and uterine horns; lay out as capital F
- Confirm horn tips are present
- Where torn, match allantoic blood vessels
- Tip of nonpregnant horn most likely part retained—identify in all examinations
- Cervical star—site of fetal exit; examine remnants
- Allantochorionic thickening/exudates should be sampled
- Assess amnion—uniformity, color, fecal staining
- Assess umbilical cord—length, degree of twisting, signs of vascular compromise
- Healthy Thoroughbred—placenta ≈11% of foal birth weight
- Up to 5 normal chorionic avillous areas
- Endometrial cup sites (may be absent at term)
- Cervical star
- Ostium (horn tips)
- Site of umbilical cord attachment
- Invaginated/redundant folds from umbilical cord traction
- Pathologic avillous areas
- Placental apposition (twin pregnancy)
- Placentitis—allantochorionic detachment from endometrium, cervical star (ascending placentitis), body, and pregnant horn (nocardioform sp.)
- Endometrial fibrosis—degenerative change prevents villous formation
- Endometrial cysts—degenerative change prevents allantochorionic apposition to endometrium
- Chorion has “red velvet” appearance due to diffuse microvilli over entire surface
- Tip of pregnant horn thick and edematous compared with nonpregnant horn
- Abnormal if:
- Thickened
- Edematous
- Exudate
- Ascending placentitis
- Cervical star, adjacent placental body
- Nocardioform placentitis
- Localized changes—placental body, horn base
- Demarcation may be prominent.
- Other causes usually more diffuse inflammatory changes
- Gross edema or thickening—vascular disturbance, fescue toxicosis
- Completely separate from allantochorion
- White; translucent; highly vascular
- Focal proliferative areas, small discrete plaques may occur
- Length
- >100 cm—increased risk of fetal strangulation, torsion
- <30 cm—excessive traction on fetal body wall; increased risk of fetal umbilical/urachal abnormalities, hemorrhage
- Torsion—hypoperfusion; congestion, thrombosis, mineralization of allantochorion
- Abortion—autolysis, vascular damage, thrombi (abnormal, excessive twisting), urachal tearing
- Neonate—patent urachus, elongated umbilical remnant from excessive traction in utero
- Clear/amber—hypotonic urine; fetal excretions
- Hippomane (allantoic calculus)—concentric layers of cellular debris; rubbery; brown, green, tan
MISCELLANEOUS