Behavioural pattern of tumour cells in mouse uterus.

The behavioural pattern of transplantable mouse fibrosarcoma (MFS) cells infused into the pregnant, pseudopregnant and nonpregnant mouse uteri was studied histologically. In some nonpregnant and pseudopregnant uteri, the tumour cells, without colonizing inside the uterus, traverse the endometrium and within 5 days of treatment form tumour nodules outside the myometrium, indicating the presence of an initial and temporary tumour rejection mechanism in these uteri. In later stages MFS cells form large necrotic tumours in these uteri. In the pregnant uterus no tumour nodules are formed outside the myometrium even after 20 days of treatment. Furthermore, the size of the tumour formed in the endometrium of the pregnant uterus is quite small and the tumour cell growth rate is lower than that of the tumour cells in the nonpregnant and pseudopregnant uteri. These results indicate that, in the pregnant mouse uterus, fibrosarcoma cells display an altered pattern of invasion and decreased growth rate compared to those in nonpregnant and pseudopregnant uteri.