Comparative development of ruminant placentomes.

Histological examination of placentomes from cows, sheep, deer, and several antelope species revealed a common pattern of development of the utero-placental junction. Chorionic membrane in contact with the uterine caruncles developed "milky patches" composed of a thick trophoblastic epithelium and multiple allantoic blood vessels, while caruncles formed simultaneously a network of crypts. The milky patches formed chorioallantoic villi that penetrated into the caruncular crypts usually simultaneously with both the villi and crypt formation but partial delay between the villi/crypt formation and penetration had no apparent detrimental effect on the fetus. The villi penetrated into caruncles in a row until they reached the dense basal layer separating caruncular mass from adjacent glandular endometrium. Further placentome growth continued by increasing the length, diameter, branching, and surface corrugation of the villi. Placentomes in different stages of development coexisted at different locations within the uterus throughout the pregnancy. During placental release after parturition, entire villi or only the villi mainstems can pull out of the maternal crypts, or the entire placentome mass can separate from the uterine wall. The remaining maternal portions of the placentomes are destroyed and sloughed down to the basal layer, leaving only a narrow band of the caruncular tissue for the regeneration of caruncles. The bare, wrinkled caruncular surface is then covered with a new epithelium and ultimately becomes smooth.