In the period 1937-1950, a number of publications appeared dealing with the stimulatory effect of hypophyseal isografts on the mammary glands of mice, leading, at least in certain strains, to mammary tumour development. Pioneers in this field were Loeb et al. (1937Loeb et al. ( , 1944, Loeb and Moskop Kirtz (1939) and Wolfe et al. (1940), and some of the results were confirmed by Silberberg (1949, 1950). The hormonal changes induced by the hypophyseal isografts were not clearly defined. Both groups of investigators assumed without further proof that the grafts stimulated ovarian oestrogen production, which in turn led to mammary tumour formation. Positive results were obtained only in strains carrying the mammary tumour virus (MTV).
Efect of hypophyseal isografts on the mammary gland in miceIn 1951 it was decided at the Netherlands Cancer Institute that the experiments described above should be repeated on a large scale, with two main objectives in mind: 1) to determine whether the hypophyseal isograft technique is also successful in MTV-free animals; 2) to analyse more thoroughly the hormonal situation in the graft-bearing female mice.In 1959 the results obtained at that time were described in our first major publication on this subject (Miihlbock and Boot, 19596
167BOOT strains and hybrids tested, except in C57BL mice. Vaginal smear studies disclosed that the grafts produced prolactin continuously throughout the life span of the recipients, starting immediately after transplantation. This hormone is endowed with a double function in mice: it has luteotrophic activity, stimulating the ovarian corpora lutea to produce progesterone, causing a di-oestrous vaginal smear; and it acts directly on the mammary glands (mammotrophic activity). The mammary tumours thus develop in glands exposed to abnormally high amounts of prolactin and progesterone. A number of other parameters studied indicated that the levels of the other hypophyseal hormones, oestrogens, corticosteroids, etc., were about normal.The changes in the vaginal smear pattern allowed not only a qualitative but also a semiquantitative estimation of the amount of prolactin produced by the grafts. In animals with a limited number of subcutaneous grafts the progesterone phases were interrupted by short oestrous periods at 10-to 1Cday intervals. With an increasing number of grafts the oestrous peaks became less pronounced and occurred at longer intervals or disappeared completely.On the basis of the vaginal oestrous cycle changes described above, it was concluded that hypophyseal isografts at other sites (renal, ovarian, splenic or intraperitoneal) also produced prolactin continuously and even more efficiently than subcutaneous grafts. This observation was elaborated upon by Boot et al. (1962) in a subsequent paper, in which variables such as sex and age of the donors were analysed as well, also with respect to mammary tumour induction potential. The kidney proved to be the most favourable implantation site and hypophyses of male donors were at least as effecti...