Retained placentas in a colony of mice.

Twenty mice were killed 1-6 days after parturition, and the uterine contents were examined grossly and microscopically. Uteri of 10 animals had retained one or more placentas, each of which was firmly attached to the uterus and usually exhibited fetal membranes and an umbilical cord. Histologic preparations revealed that the fetal blood channels in the labyrinth region were collapsed and fibrosed while some maternal blood channels were open and contained blood cells. In the junctional zone and decidua, necrosis and leukocytic invasion were evident. In median sections, a stalk of fibrous tissue extended from the uterus to the placenta.