Casein Secretion and Cytological Differentiation in Mammary Tissue from Bulls of High or Low Genetic Merit

We evaluated tissue content and secretion of milk proteins by mammary parenchymal explants prepared from tissue of mature Holstein bulls as a possible index for genetic merit. Explants from eight selection-line and eight control-line bulls were cultured for 48 or 96h in medium with 5% fetal bovine serum with or without the addition of lactogenic hormones. Four selection-line and four control-line bulls were also treated for 7 d with estrogen and progesterone or placebo before tissue was removed on d 15 of the experiment. Overall, tissue content and secretion of casein were increased approximately twofold in selection-line bulls. Differences between genetic lines were evident only with the addition of lactogenic hormones. However, treatment of bulls with steroids was not necessary for casein secretion or detection of differences between genetic lines. Averaged for both lines, 96-h media concentrations of casein were 89-fold greater for explants cultured with added lactogenic hormones. Epithelial cells were classified as non-secretory, droplet-containing, or degenerated. The appearance of droplet-containing cells was markedly increased in cultures supplemented with lactogenic hormones, and the cells were most often located within epithelial folds or pockets. Only occasionally did we observe areas of alveolar-like tissue. The data demonstrate that it is possible to induce the secretion of casein in mammary explants prepared from sexually mature bulls, that lactogenic hormones markedly stimulate secretion, and that differences in genetic merit may be expressed.