Development and mitotic activity of the metrial gland in rat uterus during mid-pregnancy

Development and mitotic activity of metrial gland in rat uterus were investigated focusing on the point of implantation during mid-pregnancy. In decidua basalis, the granulated metrial gland (GMG) cells slightly increased in number during days 8-12 of pregnancy and then decreased, while the cells appearing in the mesometrial triangle on day 9 continued to increase until day 16, forming the metrial gland in situ. By contrast, the mitotic change in the mesometrial triangle was similar to that in the other 2 regions, decidua basalis and the inner layer of myometrium. In all 3 regions, mitotic rate rose drastically on day 9, and then gradually decreased until day 16. However, most of the mitotic cells lacked the PAS-positive cytoplasmic granules in any regions. Mitotic activity in GMG cells was very low and almost unchanged throughout the gland development. These results suggest that the rapid rise of mitotic rate is due to the active mitosis of PAS-negative cells, presumably GMG precursor cells, existing widely in the implantation point of rat uterus during the early development of the gland. Immunocytochemical study revealed that progesterone receptors were localized in decidual and muscle cells and fibroblasts, but not in GMG cells during the development of the gland. The present study suggests that the metrial gland cells are proliferated, differentiated and maintained by the progesterone-sensitive stromal cells, such as decidual cells and/or fibroblasts.