VAN 211: Veterinary Histology and Embryology (2+2)

CLASSIFICATION OF PLACENTA

• The placenta is of two varieties:
  • Deciduate: in which the foetal part of the placenta is very closely associated with the maternal part and hence part of endometrium also is shed off after parturition, e.g., man and primates;
  • Non – deciduate: in which the association between the parts is not so close and hence the maternal endometrium is not shed, e.g. all domestic mammals.
  • The non – deciduate placenta are of three types:
    • Cotyledonary: The chorionic villi are grouped in well marked prominent projection called cotyledons which are separated by stretches of smooth chorion and these foetal cotyledons are received into the maternal caruncles e.g., ruminants
    • Diffuse: The chorionic villi are diffusely scattered all over the chorion e.g. mare and sow
    • Zonary: The chorionic villi occupy a girdle-like band about the middle of the chorionic sac and these are received into crypts of endometrium around a circular zone which is shed off at parturition. Hence it is partly deciduate, e.g., carnivores.

  

Degrees of placental union

• All mammals, above the egg laying monotremes have placenta, but the degree of development and the intimacy of placental union vary. The ascending series with the names of the different types of placental structure follows in a brief form.
  • Temporary imperfect placenta: (Marsupials, such as Opossum and Kangaroo) Epithelium of chorion makes short duration contact with epithelium of endometrium.
  • Epithelio–Chorial placenta: (non–ruminant ungulates such as horse and pig) chorionic villi occupy pits in endometrium.
  • Syndesmochorial placenta: (Ruminants such as cow, sheep and goat) Epithelium of endometrium destroyed in local areas. Epithelium of chorion comes into contact with connective tissue core of the endometrium.
  • Endothelio–chorial placenta: (carnivores such as dog and cat). Epithelium of villi in contact with endothelium of blood vessels.
  • Haemo–chorial placenta: (Man, monkey, and lower rodents). Endometrial blood vessels open, forming sinus of maternal blood which bathes the epithelium of the villi. 
  • Haemo endothelial placenta: (higher rodents, such as rat, guinea pig and rabbit). Epithelium of villi is destroyed. Endothelium of chorionic capillaries bathed by maternal blood in sinus.