Nutritional and somatotropin regulation of the mitogenic response of mammary cells to mammary tissue extracts.

Our objective was to investigate the mitogenic response of primary mammary epithelial cells to extracts of mammary parenchyma from 24 prepubertal Friesian heifers treated with placebo or growth hormone at either a low or a high feeding level. The mitogenic responses to mammary extracts were tested by using primary mammary epithelial organoids obtained from prepubertal heifers cultured for 4 to 5 d in collagen gels in serum-free medium supplemented to 5% concentration of the mammary extracts. Cell proliferation was determined using [methyl-3H]thymidine incorporation as a measure of DNA synthesis. High feeding level reduced DNA synthesis in response to mammary extracts. At low feeding level, growth hormone treatment decreased DNA synthesis in response to mammary extracts whereas, at high feeding level, growth hormone increased DNA synthesis in response to mammary extracts. These results suggest that locally produced growth factors are involved in the regulation of mammary development when mammary growth is modulated by feeding level and growth hormone treatment.