Electron microscopic studies on the endometrium of the grey seal (Halichoerus grypus) during its preparation for nidation.

The uterine luminal and glandular epithelia of two mature grey seals, a species with obligate delayed implantation, have been examined. Earlier studies of their corpora lutea had suggested that these females were in the pre-nidatory condition. Ultrastructural features of these epithelia in the animal which possessed the more mature corpus luteum indicated that the luminal epithelium had attained a state of differentiation similar to that of the pre-attachment phase in the laboratory rat, a state known to be compatible with nidation. The animal with the less well developed corpus luteum demonstrated the early stages of the early endometrial response to progesterone (the hormone which takes the uterus out of the delay condition). In the grey seal, despite the late start, a similar sequence of morphological changes to those present in the endometrium of normally implanting animals occurs. Thus, it seems that in the majority of eutherian mammals, endometrial preparation for nidation follows a relatively standard pattern, and occurs pari passu with the differentiation of the luteal tissue.