Extensive Breeding as an Adjunct to Mammary Gland Carcinoma Susceptibility in Mice

Abstract

Susceptibility to carcinoma of the mammary gland in mice of the F₁ generation, following an outcross, is usually determined by the female parent, that is, if a female mouse of a cancer susceptible strain be used in an out-cross to a male mouse of a resistant-to-cancer strain the F₁ females develop spontaneous tumors; whereas, if the reverse cross be made, female mice of the F₁ generation do not usually develop spontaneous tumors of mammary tissue. It is Bittner's opinion that the “milk influence” explains this peculiar maternal inheritance of cancer susceptibility. The “forced breeding” principle of Bagg also plays a role in cancer susceptibility but apparently only upon a suitable genetic background.

In the present experiment, consisting of 2 parts, female mice of the JK strain (cancer resistant) were mated to male mice of the C₃H strain (cancer susceptible). The ensuing F₁ individuals were mated either inter se or back-crossed to either mice of the original C₃H or JK strains. Mice were continued as breeders for their entire life span, the sexes being kept together. This procedure differs from the “forced breeding” technic of Bagg in that the mice were permitted to nurse their own young, whereas in “forced breeding” the young were discarded within 24 hours of birth. The cyclic changes within the mammary gland are considerably different therefore in the two systems of breeding. Following sexual maturity, the female mice were kept, in this experiment, in an almost continuous reproductive activity—nursing one litter while the next one was developing in utero. This regime was greatly aided by maximal nutritional conditions. This reproductive activity was also enhanced by the fact that F₁ individuals show the phenomenon of heterosis, which influences health and vigor of mice as well as of other animals and plants.