Transplants of the Harding-Passey melanoma will implant in the mouse uterus only if transferred on days 3–6 of normal pregnancy or pseudopregnancy. Should pregnancy or pseudopregnancy be accompanied by lactation, implantation of melanoma transplants is not delayed (though blastocyst implantation is delayed for about four days) but the period during which the transplants may implant is extended to at least day 8. Autografts of body-wall muscle tissue will implant in the uterus within the same period as will melanoma transplants.
Tumour cells injected in suspensions through the cervix tended to segregate into clumps more or less spaced out and implanted along the uterus. When the transplanted tissues or cell suspensions implanted, they did so invariably in the antimesometrial mucosa, at the site of normal blastocyst implantation.
A decidual reaction was elicited only when the transplanted tissues remained free, or relatively so, in the uterine lumen (between days 3–6). If extensive invasion of the uterine mucosa occurred then there was no decidual reaction even though there was a strong inflammatory reaction to invasion.
Decidual tissue “en bloc” is resistant to invasion by melanoma cells. Concurrently with melanoma invasion, pigment granules, from melanoma cell debris, were ingested by epithelial cells immediately adjacent to the invasion sites.