Chorioallantoic placentae vary widely in shape, size, and general appearance. They have been described according to the final distribution of the chorionic villi over the endometrial surface of the fetal membranes (Fabricius, 1604, cited by Steven). Fabricius recognized four main placental types, now known as Diffuse, Cotyledonary or multiplex, Zonary, and Discoid (Table 2).
In the diffuse placenta, most of the outer surface of the chorion is covered with small villi or folds, which lie in intimate contact with corresponding depressions or sulci in the uterine epithelium. This type of placenta is found in swine, peccaries, lemurs, llamas, hippopotamuses, camels, musk deer, American moles, pangolins, oriental and African Lorisidae, cetacean, and certain other mammals. The horse, a commonly cited example of this type, has also been classified as having a microcotyledonary placenta.
The cotyledonary, or multiplex placenta, is found in the majority of ruminants. The chorionic villi are restricted to a number of well-defined circular or oval areas of the chorionic sac, which are separated by less specialized areas of relatively smooth chorion. Fetal cotyledons develop normally only in those parts of the chorion overlying specialized areas of the uterus known as caruncles. Fetal cotyledons and uterine caruncles together form placental units known as placentomes. Familiar examples of this type of placentation are the cow, sheep, goat, deer, elk, moose, bison, buffalo, and giraffe.
The number of placentomes in cattle may vary from 40 to 150 (typically 80 to 100), whereas in the goat and giraffe typical ranges are 160 to 180, with a preponderance of functional placentomes occurring in the gravid horn. In sheep, the total number of placentomes varies
Table 2 Classification of chorioallantoic placentae based primarily on their shape
Multiplex or Cotyledonary (vilus epitheliochorial) Polycotyledonary Oliogocotyledonary Zonary or Annular Broad (villous, epitheliochorial) Complete
(often called diffuse) Interrupted Narrow (labyrinthine, endotheliochorial) Complete Interrupted Discoid
Pileate (labyrinthine, hemochorial) Thick (labyrinthine, hemochorial) Thick (trabecular, hemochorial)
Thick (villous, hemochorial)
Thick (labyrinthine, endotheliochorial)
Reniform (labyrinthine, hemochorial)
Spheroid (labyrinthine, hemochorial)
Double discoid (villous, hemochorial)
Llamas, camels, peccaries, horse (microcotyledonary; Equidae)
Cattle, bison (Bovidae) Deer, moose (Cervus)
Prairie mole (Scalopus)
Dog (Canis) Raccoon (Procyon)
Pocket gopher (Geomys)
Human (Homo) Bear (Ursus)
covered with columnar phagocytic cytotrophoblast that apparently absorb secretions of large endometrial glands opening around the base of the caruncle. Invasive trophoblastic giant cells are common.
The zonary placenta is characteristic of the carnivores, although it is also found in quite unrelated groups. In this type, the chorionic villi, or lamellae, of the placental labyrinth are aggregated into a band of placental tissue that encircles the equatorial region of the chorionic sac. Such placental girdles may be complete, as in the dog, cat, and spotted hyena, or incomplete, as in the ferret, polar bear, brown bear, raccoon, mink, and certain Pinnipedia. Zonary placentae are also found in a limited number of noncarnivores such as the manatee and elephant.
Discoid placentae are the most localized of the placental types and are found in humans and other primates, rodents, and bats, among others.[4,5] The disk, or plate, may be single, as in humans, or double, as in the rhesus monkey.
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