Changes in uterine estrogen and progesterone receptors during delayed implantation and early implantation in the spotted skunk

Although the exact cause(s) of embryonic diapause in the western spotted skunk and other carnivores remains unknown, it has been hypothesized that it may be due to levels of ovarian hormone secretion that are insufficient to promote a uterine environment conducive to continuous embryonic development and implantation. Immunocytochemistry was used to determine whether changes in abundance or distribution of estrogen receptors (ER) and progesterone receptors (PR) may be associated with the cessation or renewal of embryonic development. Thirty pregnant skunks were killed during delayed implantation and periimplantation periods. ER and PR were detected in luminal and glandular epithelium, endometrial stroma, vasculature, and myometrium of the uterus during the period of delayed implantation. There was a significant reduction of both ER and PR receptors during the periimplantation period. The most pronounced change was the complete loss or reduction in staining intensity for PR and ER in the luminal epithelium during the first 2-3 days after implantation. These findings suggest that the failure of skunk blastocysts to undergo continuous development and implant without a prolonged period of diapause is not the result of an insufficient number of ER or PR in the uterus. The data also indicate that renewed embryonic development and implantation is not associated with an increase in these uterine steroid receptors.