Similar histological and histochemical changes occur in the uteri of pregnant and pseudopregnant ferrets. The pre-implantation period is characterized by general growth of the tissue and moderate secretory activity. At the time of implantation, the luminal epithelium undergoes hypertrophy and hyperplasia and shows much secretory activity. This area of altered activity sweeps progressively toward the gland fundi and the epithelium behind it becomes greatly enlarged and forms symplasmic masses. Symplasma formation occurs in all regions of the uterus, in contrast to other carnivores where it occurs only next to trophoblast and in response to uterine trauma.
The principal uterine secretions are mucoproteins and glycoproteins. There is some evidence that pseudopregnant uteri, but not pregnant uteri, secrete acid mucopolysaccharides. Glycogen is present in moderate amounts at estrus, declines during the pre-implantation period, and is absent during gestation. A light to moderate diffuse sudanophilia is seen during pregnancy and pseudopregnancy, becoming more pronounced toward the end of the gestation period. Occasional cholesterol crystals are seen in macrophages in the stroma of pregnant, but not pseudopregnant, uteri. Macrophages containing hemosiderin were seen in all uteri examined, being most abundant in estrous and early pre-implantation uteri. Uterine mast cells were abundant at estrus, declined during the pre-implantation period and decreased markedly at the time of implantation. The results are compared with data from other carnivore species.