Progesterone promotes a massive infiltration of the rat uterine cervix by the eosinophilic polymorphonuclear leukocytes.

Rat uterine cervices contained an eosinophilic infiltrate during labor, whereas samples of cervices obtained from nonpregnant controls had no infiltrate. A heavy eosinophilic invasion, which closely resembled that of the cervix uteri at term, could be reproduced in ovariectomized rats following progesterone administration, but not in spayed rats injected with estrogen. Light and electron microscopic evidence showing a widespread collagenolysis in the cervical stroma at term is presented. This collagenolysis follows the eosinophilic invasion. On the basis of these observations, a possible role for the eosinophilic infiltrate of the uterine cervix is discussed.