Granulated metrial gland cells: pregnancy specific leukocytes?

Granulated metrial gland (GMG) cells and their analogous cells are a prominent feature of the maternal placenta in rodents and in primates, including the human. Evidence which relates to the origin of these cells is reviewed and shows that there is now conclusive evidence that GMG cells are a type of leukocyte. Studies to define the type of leukocyte have been inconclusive overall, although several studies have suggested that GMG cells are a type of NK cell. However, it is proposed that attempting to define GMG cells as a type of leukocyte which is already known, like NK cells, is inappropriate and GMG cells should be considered as a separate leukocyte population. Granulated metrial gland cells are closely associated with pregnancy but technically cannot be described as pregnancy specific. The function of GMG cells remains to be established, but the possibility that GMG cells and their analogous cells are controlling the invasion of trophoblast into the uterine wall and/or the removal of aberrant trophoblast is currently under consideration.