Ultrastructure of the amnion and amniotic plaques of the white‐tailed deer

The ultrastructure of the aminon and aminotic plaques of the white‐tailed deer was studied throughout pregnancy. The single layered amniotic epithelium had short, blunt, club‐shaped microvilli, tortuously folded lateral plasma membranes enclosing an intercellular space, and basal foot‐procsses. Wherever lateral plasma membrane processes abutted, desmosomes occurred. Hemidesmosomes were observed only in association with the foot‐processes. The epithelium contained granular endoplasmic reticulum, rod‐shaped mitochondria, free ribosomes, Golgi complexes, tonofibrils, tonofilaments and glycogen granules. The epithelium was supporte by a basal lamina. The subjacent connective tissue layer contained mesenchymal cells, fibroblasts and collagenous fibers. The amniotic plaques varied in size from microscopic to 4 mm in diameter. The small plaques were formed by mitotic activity of the aminion cells. their sites of formation appeared randomly distributed. Each microscopic plaque had one to three layers of basal columnar to cuboidal cells, and a single layer of covering cells next to the amniotic cavity. Further mitotic activity of basal columnar cells led to the formation and differentiation of large plaques which contained over 15 cell layers. The main mass of a large plaque showed four zones: stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Cytoplasmic organelles were plentiful in the basal and spinous cells, but were sparse in the granular and cornified layers. Keratohyalin granules and dense tonofibrils were usually found in the granular cells, but only occasionally in cornified cells. Each plaque contained varying amounts of PAS‐positive glyucogen granules. The tortuously folded lateral plasma membranes enclosed intercellular spaces which extended from basal to cornified layers. Small portions of amniotic cells and layers of cornified cells sloughed throughout pregnancy and thus contributed to the composition of amniotic fluid. The homology of the plaques with mammalian skin and their distribution in the eutherian mammals is discussed.