Placentae Classification and Types

Placentae (Specific)

• Placentae are classified by the distribution of chorionic villi
• Diffuse: even distribution across the entire chorionic surface
• Cotyledonary: discrete round patches of villi forming cotyledons on the chorion, connected to endometrial caruncles
• Zonary: prominent round transfer zone of villi as a central band, edges form pigmented area with haemorrhage and necrosis
• Discoid: discrete disc of villi on one side

Placental Distribution

• Placentae are also classified by their invasiveness, which determines the number of tissue layers between the fetal and maternal circulation (fetal and maternal separation)
• Epitheliochorial: 6 layers separating maternal and fetal circulation.
• Endotheliochorial: 5 layers separating maternal and fetal circulation.
• Hemochorial: 3 layers separating maternal and fetal circulation.

Placental Types by Species

• Cattle (cows):

  • Cotyledonary
  • Epitheliochorial
  • Convex caruncles
  • 70-120 cotyledons, increasing in size through gestation. Placentome = fetal cotyledon + maternal caruncle

• Sheep (ewes):

  • Cotyledonary
  • Epitheliochorial
  • Concave caruncles.

• Horses (mares):

  • Diffuse
  • Epitheliochorial
  • "F" shaped placenta
  • Microcotyledons
  • Endometrial cups
  • Placental cells invade endometrium
  • Transient structures (~d40-150).
  • Cervical star
  • Large, white avilous region.

• Pigs (sows):

  • Diffuse
  • Epitheliochorial
  • Simple chorionic villi
  • Smooth surface

• Dogs (bitches):

  • Zonary
  • Endotheliochorial
  • Highly invasive villi in the transfer zone
  • Marginal band of haematoma
  • Relatively avascular rest of the chorioallantois

• Cats (queens):

  • Zonary
  • Endotheliochorial
  • Highly invasive villi in the transfer zone
  • Relatively small band of haematoma
  • Relatively avascular remainder of chorioallantois

• Fetal/maternal blood NEVER mixes directly

• Fetal blood entering maternal circulation can be treated as a foreign antigen, causing sensitization and neonatal isoerythrolysis for offspring.

Histology of Placenta

• Ruminants: have both regular and binucleated trophoblasts, invade the endometrium and produce placental lactogens and placental growth factors
• Chronic trophoblast cells are typically cuboidal-columnar

Histopathology

• Examples of histopathology include necrosis (debris) and fungal hyphae, congestion in the chorioallantois, fibrinous exudate, neutrophilic infiltrate and Gram +ve bacteria.

Explore the various classifications of placentae based on chorionic villi distribution and invasiveness. This quiz covers different placental types found in species including cattle, highlighting their unique characteristics. Test your knowledge on the complexities of placental structures!