Classification Based On Membrane Layers

Historically, Gosser's system (summarized in Table 1), based on the morphology of the interhemal membrane, is the most generally used classification system (Gosser, 1909, 1927 as cited by Mossman[1]). Gosser, using the name of the maternal tissue contiguous with the chorion as the denominator, distinguished four types of placentae: epitheliochorial, syndesmochorial, endotheliochorial, and hemochorial. This system has been challenged and modified numerous times.[2-4] Because no species having the syndesmochorial type as a major portion of a chorio-allantoic placenta has been identified, this type has typically been deleted. Gosser's system became a guide for functional interpretations. Placentae having fewer numbers of layers were thought to be more efficient in interchange processes. Greater understanding of the complex nature of placental metabolism and transfer has largely dispelled that idea. The functional attributes of the placenta impact the rate at which interchange can occur; however, it is questionable whether the number of layers, or thickness, has much significance with regards to rate or efficiency of exchange. Although Gosser's system is a much battered concept, it continues to be used extensively.

Epitheliochorial placentation, observed in most hoofed mammals, moles, and lemurs, is achieved by development and growth of endometrial epithelium-lined crypts and chorioallantoic villi that fit into them. The two tissues,

Table 1 Classification of chorioallantoic placentae based primarily on their interhemal membranes

Epitheliochoral 3 maternal layers (epithelium, connective tissue, endothelium), 3 fetal layers (endothelium, mesenchyme, chorionic epithelium)

Avillous (smooth chorioallantois) extensive accessory areas in carnivores

Villous

Diffuse (often a very broad annulus)

Simple villi swine, American mole, hippopotamus, a few ruminants

Complex villi horses, whales, lemurs Cotyledonary or multiplex

Polycotyledonary cattle, goat, sheep, bison, antelope Oliocotyledonary deer, elk, moose Syndesmochorial 2 maternal layers (loss of epithelium), 3 fetal layers

Unknown as the major portion of a chorioallantoic placenta Endotheliochorial 1 maternal layer (loss of epithelium and connective tissue), 3 fetal layers

Labyrinthine carnivores, most bats, sloths, American anteaters Hemochorial maternal layers absent (except free blood), 3 fetal layers Labyrinthine

Hemomonochorial squirrels, guinea pig, chinchilla Hemodichorial rabbits, beaver Hematrichorial mice, rats Trabecular to villous

Hemomonochorial human, primates, and armadillos fetal and maternal, cooperate in the process. An increase in thickness results from growth of both maternal and fetal tissues. The trophoblast and crypt epithelium usually have interdigitating cytoplasmic processes and microvilli. Both the trophoblast and epithelium become very thin locally where a maternal and fetal capillary are adjacent to one another, and trophoblastic cells (e.g., giant cells) may migrate into the uterine epithelium where they become functional cells of the endometrial cups.

Endotheliochorial placentation, such as that of carnivores and bats, typically starts with symplasmic degeneration of surface and crypt epithelium and penetration into this symplasma by chorioallantoic villi. The symplasma rapidly disappears. The trophoblast then engulfs the adjacent endometrial capillaries and becomes continuous with that of neighboring villi, thus forming a labyrinthine placenta containing maternal blood vessels lined by maternal endothelium. Further growth of both fetal and maternal tissues results in the typical thick endothelio-chorial labyrinthine placenta.

Hemochorial labyrinthine placentation of superficially embedding species, such as rabbits and squirrels, starts much like that of endotheliochorial placentation. However, the endothelium of the subepithelial maternal capillaries soon ruptures, allowing the maternal blood to contact the trophoblast directly. The latter then differentiates an elaborate tubular system in which the maternal blood circulates. Invasion of the growing trophoblastic labyrinth by vascular mesodermal allantoic villi usually begins almost simultaneously with the development of the trophoblastic tubule system for maternal blood. Hemo-chorial villous placentation, such as that in anthropoids and primates, including humans, has a basic sequence similar to that just described, except that a greater area, two separate areas, or the entire chorion may be involved in providing the preplacental mass. A fairly well-developed circulatory system for maternal blood is formed in the preplacental trophoblast before allantoic villi appear. Placental thickening is the result of growth of the fetal trophoblast and mesodermal villi.

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