Effects of dietary fat on mammary development relative to age and hormones in BALB/c mice

Earlier studies reported that mammary ducts grew faster if the 10% fat in the diet was composed of oils containing polyunsaturated fatty acids (corn oil: CO) compared to hydrogenated cottonseed oil (HCTO), which is devoid of such fatty acids. These experiments were primarily carried out in immature mice and left unanswered questions regarding the effects of dietary fats on more differentiated stages of mammary development. The use of transplanted ducts permitted the study of mammary growth rates in adult mice. If the diet was started when the animals were adults, there was no difference in the growth rate of those fed HCTO diet compared to those fed CO diet. However, when the diets were fed to immature mice, the mammary gland grew slower in mice fed the HCTO diet, confirming our earlier observations. The HCTO and CO diets caused no difference in the growth rate or morphology of fine ducts and alveoli that developed during pregnancy. Furthermore, no differences were seen in female mice following 6 weeks of progesterone administration begun at 3 weeks of age. Experiments that used male mice to examine the initial stages of mammary duct growth also showed that the effect of dietary fat was not observed when estrogen (E) or E and progesterone (P) were injected. In addition, there was no effect of dietary fat in ovariectomized 3-week-old females when any dose of E was administered from 0.01 to 1 microgram/day. Examination of the ovaries from mice fed either HCTO or CO diets from 3 to 9 weeks or 3 to 13 weeks of age showed that mice fed HCTO diet did not develop corpora lutea, while those fed CO diet had normal appearing ovaries. The HCTO diet inhibits normal maturation of the follicle and corpus luteum formation. We conclude that the effect of the dietary fat on the developing mouse is on the maturation of the ovary and subsequently on mammary growth.