Cytology of the endometrium of delayed and early implantation with special reference to mice and mustelids.

Although it is apparent that the uterus is involved in the control of embryonic diapause, in no instance do we know the precise nature of that control. The most direct control would be through synthesis of proteins or other macromolecules by the uterus itself. Whereas luminal contents have been shown to vary significantly near the time of implantation, neither the role of the luminal fluid constituents, nor the cellular origins of these molecules are fully known, nor have the mechanisms of storage and secretion of proteins in the endometrium been fully established. Cellular structures that may be indicative of functional activity include secretory granules or vesicles, increase or dilation of granular endoplasmic reticulum and increase in size, and associated vesicles of the Golgi complex. Storage of glycogen or lipid appears more indicative of lowered activity. However, despite striking changes in these cytological characteristics in different animals during delay and early implantation, there is no single pattern during the delay period, and no predictable alteration in the peri-implantation period. In general, the structure of the uterus of an animal during delay resembles a modification of that of a closely related species that does not delay more than it resembles that of other species that do show delay.