Developmental changes in the fine structure of rhesus monkey amnion

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Abstract
The purpose of this study was to examine developmental changes in the amnion with regard to: (1) the fine structure of the epithelial cells, (2) the nature and permeability of the intercellular junctions, and (3) the structure of extracellular matrix components in the subepithelial compartment. Comparisons were made between amnion from 26‐72 and 146‐170 days of gestation. Early in gestation, epithelial cells were squamous and had few apical microvilli; late in gestation, cells were cuboidal with numerous microvilli and a well‐developed glycocalyx. Epithelial cells from early gestation had large glycogen stores, well‐developed Golgi apparatus, granular ER, and relatively few 10 nm cytoplasmic filaments. Later, cells were similar except for less glycogen, somewhat more lipid droplets, and numerous 10 nm cytoplasmic filaments. Lateral cell borders were complexly interdigitated at both ages and desmosomes were the predominant type of cell junction. Occasional tight junctions were observed early in gestation; no tight junctions were observed near term. After exposure to peroxidase in vitro, reaction product was present in the intercellular spaces, and the epithelial cells demonstrated very limited pinocytosis. Early in gestation, the basal cell surface was smooth, had no specialized junctions, and was underlain by a complete basal lamina (BL). Late in gestation, the basal surface was thrown into numerous folds, had hemidesmosome‐like junctions, and an incomplete BL. Early in gestation, the BL contained ruthenium red‐positive particles; later, these particles were absent. Early in gestation, the BL had a layer of interstitial collagen fibrils beneath it; late in gestation, a thick layer of 12 nm microfibrils was interposed between the BL and the interstitial collagen. The microfibrillar layer appeared at about 72 days of gestation, when it was apparently formed between the basal cell surface and the partially detached basal lamina.The results support the view that amniotic epithelial cells remain metabolically active throughout gestation and that the intercellular pathway is important in the movement of certain large molecules across the epithelium. The microfibrillar layer may provide structural strength for the amnion late in gestation.