Dynamics of mammary cell division during pregnancy and lactation in dairy cows

Binucleated mammary epithelial cells (MEC) contribute to milk production in mice, yet their prevalence and function in species including dairy cattle remain unclear. The objective of this study was to determine the incidence and contribution of binucleated MEC in the bovine mammary gland during lactation. In a first study, we induced lactation in nulliparous Holstein heifers while concurrently labeling dividing MEC with intramammary nucleotide analogs (EdU and BrDU) in the presence or absence of the aurora kinase inhibitor, alisertib, to block binucleation. In a second study, we determined the abundance of binucleated MEC in the udder of multiparous Holstein cows that had increased milk production in response to unilateral frequent milking of one udder half. Contrary to data for mice, binucleated MEC comprise only 1% to 3% of the total MEC population in the bovine udder. Interestingly, the rear quarters of the udder contained a higher proportion of binucleated MEC (1.1%) versus front quarters (0.7%). Intramammary alisertib during mammogenesis tended to reduce milk yield and was associated with increased abundance of somatic cells and MEC in milk at the start of lactation. Cellular proliferation during hormone-induced lactation was consistently low during both the growth phase and lactation, where EdU or BrdU-labeled MEC were retained in the lactating udder up to 39 d postlabeling. In response to increased milking frequency, the incidence of binucleated MEC tended to increase from 2% to 3.2% of the epithelium, and the incidence of Ki-67-positive MEC decreased from 1% to 0.5%. Taken together, these data indicate that binucleated MEC constitute a small but potentially significant population in the bovine mammary gland.