The origin of equine endometrial cups. III. Light and electron microscopic study of fully developed equine endometrial cups

Equine endometrial cups are localized endometrial outgrowths which develop in mares during early pregnancy and secrete pregnant mares serum gonadotrophin (PMSG). The principal cell‐type found in endometrial cups is large, regular and epithelioid in appearance. These cup cells have one or two large euchromatic nuclei with dense, characteristic nucleoli. Their cytoplasm is filled with irregular profiles of endoplasmic reticulum containing pale flocculent material. Lipid droplets and a few mitochondria are also present. Golgi material is not prominent and the absence of membrane‐bound secretory granules suggest that the Golgi apparatus does not package a secretory product. The plasmalemma is mostly smooth in outline but is modified in some places to form short microvilli or complex intercrescent folds.