The allantoic and amniotic epithelia of the pig: SEM and TEM studies

Abstract

The epithelial layers of the allantoamnion of pig embryos and fetuses during various gestational ages were studied utilizing SEM, TEM, and light microscopic histochemistry. The allantoic endoderm exhibits a gland-like secretory activity and thereby differs greatly from that of other mammals. On the surface of this unilaminar cuboidal epithelium, the majority of the cells exhibit characteristic short vermiform ridges, while some protruding cells display larger individual microvilli. These two cell types are also distinct in thin sections. The more common “granular cells” with short and blunt microvillous projections and a lobated nucleus are characterized by small, Golgi-derived secretory granules in the apical cytoplasm showing a positive PAS-reaction. They contain vast glycogen deposits. Extensive regions of lateral interdigitations are found. In the younger stages, membrane thickenings of the apical plasmalemma resemble those of the urinary bladder. The cytoplasm harbors many more interwoven filaments than organelles. The second cell type, the “mitochondria-rich cells”, bearing longer apical microvilli in many cases, only constitutes up to 3% of the mucosal cell population. They are frequently flask-shaped, heavily reactive to oxidoreductases, and rich in lysosomes but have smaller glycogen deposits. Mitochondria-rich cells lack secretory granules but have light apical tubules, probably of endocytotic character. These cells can be found in different functional states. The amniotic epithelium is simple squamous in the younger stages and largely resembles that in other mammals exhibiting cells with few organelles but rich in filaments. Each terminal bar consists of a zonula occludens only which may open toward the end of gestation. In older fetuses, small stratified areas of cells sloughing off into the lumen appear as blisters which contain large vacuolated cells.