Reproduction Abstracts

Background: Menstruation, the cyclical breakdown of the superficial endometrial layer in the absence of pregnancy, occurs in 1.5% of mammals. There is no obvious phylogenetic link between species known to menstruate: humans, Old World monkeys, some bats and the elephant shrew, and true menstruation has never before been reported in rodents. Observations of blood at the vaginal opening in some females, led us to examine the possibility that the spiny mouse (Acomys cahirinus) menstruates.

Methods: Virgin spiny mice (n=14, 12–16 weeks) were samples by daily vaginal lavage for two complete cycles. Stage-specific collection of reproductive tissues and plasma was used for comparative histology, and ELISA assay for plasma progesterone. Decidualised endometrial stromal cells were detected using prolactin immunohistochemistry.

Results: Blood was present in vaginal lavages of all females (14/14) during the transition frm the luteal to the follicular phase in both cycles. Mean cycle length was 8.70.4 days with red blood cells seen in the lavages over 3.00.2 days. The endometrium was thickest during the luteal phase, when plasma progesterone peaked at ~102 ng/ml and the optical density for prolactin immunoreactivity was strongest. Immunopositive endometrial cells were absent during the follicular phase, and shed at the time of vaginal bleeding. Blood and endometrial shedding were seen in the uterine lumen at the conclusion of each infertile cycle. These menstrual changes occurred in association with regression of the corpora lutea in the ovary.

Discussion: The spiny mouse is the first rodent to show spontaneous decidualisation and menstruation. This discovery contradicts existing opinion that rodents do not menstruate, and challenges the evolutionary theories of menstruation. The spiny mouse provides a novel research species to advance our understanding of human menstrual and endometrial pathophysiology, and perhaps menopause.