Current topic: structural responses of the primate endometrium to implantation

Morphological responses to implantation and pregnancy in the human and nonhuman primates include decidualization of the endometrial fibroblasts and accumulation of large numbers of large granular lymphocytes, formerly called endometrial granular cells, and may also include an epithelial plaque response and endothelial cell hypertrophy. Although stromal decidualization occurs in all species, it develops slower in macaques than in the human, and slower in the baboon than in the macaques. Cytologically, however, in all of these species there is extensive modification of the decidual cells by midgestation. Hypertrophy of luminal and gland neck epithelial cells is common in most monkeys and is also seen in the baboon, but has not been reported in humans. Large granular lymphocytes undergo changes in morphology during the first week after implantation in the rhesus monkey, the only species in which they have been studied in the immediate postimplantation period. Later in pregnancy many of the large granular lymphocytes are surrounded by decidual cells in this species. All of the responses can be elicited by trauma and appropriate hormonal conditions, but the epithelial plaque response forms first in the rhesus monkey and baboon. It is suggested that more complete fine structural and immunocytochemical studies of the different decidual regions at different gestational ages, combined with studies of the synthetic and antigenic nature of the different cell types, would allow determination of whether or not there are subpopulations of decidual cells in primates as well as suggesting possible functions of the cells in pregnancy.