EVIDENCE AGAINST A SIGNIFICANT ROLE FOR MAST-CELLS IN BLASTOCYST IMPLANTATION IN THE RAT AND MOUSE

Rats were treated with the highly potent stabilizer of mast cells, FPL 55618, before and during the first seven days of pregnancy to establi sh whether stabilization of mast cells resulted in impaired blastocyst implantation. There was no significant reduction in either the number of ovulations or the number of implantation sites in treated rats com pared with controls; 11 of 15 treated rats were pregnant compared with 5 of 6 control rats. The distribution of mast cells was examined in u terine tissues, implantation sites and interimplantation sites in both rats and mice using highly sensitive immunohistochemical techniques. Virtually all of the mast cells in rat uterine tissue stained for rat mast cell protease-I (RMCP-I; connective tissue type), whereas few sta ined for RMCP-II (mucosal type). Most of the mast cells were present i n the myometrium with very sparse distribution in the endometrium and there were no differences in numbers of mast cells between implantatio n and inter-implantation sites on Day 7 of pregnancy. In tissue sectio ns of mouse uteri sampled from Day 1 to Day 8 of pregnancy there were virtually no mast cells in the endometrium or deciduum adjacent to imp lantation sites. Mouse uterine mast cells also stained predominantly f or the connective tissue-type mast cell protease MMCP-4, the murine eq uivalent of RMCP-I. Thus, mast cells and their products appear to play little, if any, role in blastocyst implantation in murid rodents. Sin ce mast cells are a prominent feature of human endometrium, this study emphasizes the important consideration of species differences when ch oosing animal models for implantation studies.