Prolactin regulation of cytological differentiation of mammary epithelial cells in periparturient cows.

The influence of periparturient secretion of PRL on cytological differentiation of mammary epithelial cells was studied in 17 multiparous, monotocous Holstein cows. Mammary tissue was obtained 10 days prepartum and 10 days postpartum from untreated cows and from cows treated with CB154 (2-Br-alpha-ergokryptin) or CB154 plus PRL. CB154 was administered from 12 days before expected parturition through 10 days postpartum to reduce serum PRL concentrations, whereas exogenous PRL was administered for 6 days during the periparturient period to mimic the normal periparturient surge of the hormone. On day 10 postpartum, mammary epithelial cells of cows given CB154 alone were classified 18% undifferentiated, 65% intermediately differentiated, and 18% fully differentiated. In contrast, there were no undifferentiated epithelial cells in either postpartum controls or cows treated with CB154 plus PRL, and 73% and 79% of epithelial cells, respectively, were fully differentiated. Ultrastructural analysis demonstrated a relative lack of cellular differentiation in cows given CB154 alone. Specifically, the rough endoplasmic reticulum occupied 24% and 27% of the epithelial cell area in postpartum controls and cows treated with CB154 plus PRL, respectively; but the rough endoplasmic reticulum occupied only 16% of the cellular area in cows treated with CB154 alone (P less than 0.01). The relative area occupied by Golgi membranes and vacuoles was approximately 11% lower (P less than 0.01) in cows treated with CB154 than in lactating controls or cows that received PRL replacement therapy. The data demonstrate that periparturient secretion of PRL is necessary for complete structural differentiation of bovine mammary alveolar epithelium.