Embryo Resorption and Placental Scar Formation in the Rat

Abstract

resorption. suggestion that placental scars are formed at the sites where embryos have been resorbed has been made by various workers. Davis and Emlen (1948) reported a study of multiparous laboratory rats in which the number of placental scars was compared with the number of young born. In 10 of 21 females there were more uterine scars counted than young born. This they ascribe to production of scars by resorbed embryos. Osborn (1953) found an average of 3.37 placental scars per female in 19 beaver (Castor canadensis) uteri. Embryo counts of 22 pregnant uteri gave an average of 2.91 embryos per female. Resorption involving one or more embryos was noted in six of the 22 uteri. He suggested that the resorption of embryos might account for the higher scar count in this species. Very little direct information has been presented on the relationship of embryo resorption to placental scarformation. Brambell and co-workers (1948) make the following statement concerning resorption in the rabbit (Oryctolagus cuniculus): The reabsorption sites remain recognizable as such macroscopically for 9-10 days after death from stilboestrol administration at either 11 or 15%3 days post-coitum; although the sites remain visible after that time, especially in the 15% series, it is not then possible to distinguish them from old post-partum placental sites. They made no further observation of the relationship between placental scars and embryonic resorption. Kerr (1947) in a study of the histology of early stages in the resorption of mouse (Mus musculus) embryos mentions that as resorption of 10-day-old embryos continues and pigment are found much as in the pregnant spots which mark the position in normal uteri of previous placentae. Both of the above quoted statements indicate that resorption of embryos in the rabbit and mouse may result in the formation of scars. However no detailed studies of this subject have been made. placental scars of the rat appear as yellow to black pigmented areas along the utero-mesometrial border. Their origin seems identical with that of the scars in the mouse which have been studied by Deno (1937). He found that scars are produced by accumulations of hemosiderin in the cells of the reticulo-endothelial system that are associated with the involuting metrial gland. In both the rat and mouse, the metrial gland is a prominent structure at the base of the placenta, characterized by large glandular appearing cells. Presumably it is formed by an extension of the decidual response into the connective tissue of the myometrium. pigment-laden cells are concentrated in this area between the longitudinal and circular muscle layers although some are found in the deeper stroma of the endometrium. As the age of the scar increases the pigmented area may decrease