Investigating the Process of Mammary Epithelial Cell Loss Into Milk and Its Relationship With Lactation Stage, Milk Production, and Lactation Persistency

Abstract

Mammary epithelial cells (MEC) are the milk-secreting cells of the mammary gland and are lost into milk throughout a cow's lactation. Because milk yield is a function of the number and productivity of MEC, understanding when and by which mechanisms cells are lost into milk can help us better understand local regulation of milk production and how it relates to milk yield and lactation persistency. The objective of this thesis was to investigate the relationship between MEC loss into milk and lactation persistency and better understand cellular mechanisms behind MEC loss into milk. This thesis consisted of two studies. The first study evaluated loss of MEC into milk longitudinally across seven stages of lactation and the relationship of MEC loss into milk with four corresponding persistency estimates for each stage. There was an inverse relationship between milk yield and MEC loss into milk but no relationship between any persistency estimate and MEC loss into milk. Mammary epithelial cell loss into milk declined in early lactation to a minimum shortly after peak and increased thereafter. Persistency parameter calculations were uncertain in early stages of lactation but converged and remained consistent in the later stages. The second study used an immortalized line of bovine mammary epithelial cells, BME-UV cells, to evaluate the relationship between apoptosis, epithelial crowding, and cellular extrusion. There was an association between apoptosis and extrusion, with increasing actin ring accumulation around cells as they progressed later into apoptosis. However, inhibition of canonical extrusion signaling pathways involving sphingosine-1-phosphate (S1P) and Rho-associated kinase (ROCK) did not disrupt extrusion. Increased epithelial crowding promoted extrusion of non-apoptotic cells, suggesting that crowding induces live cell extrusion. Taken together, the results of both studies suggest that MEC detected in milk may originate, at least in part, from apoptotic and crowding-induced extrusion processes that function to maintain tissue homeostasis. However, the signaling pathways involved in these extrusion processes in BME-UV cells may differ from canonical S1P-ROCK-mediated pathways ob-served in other epithelial models. Additionally, while MEC loss into milk is inversely related to milk yield, persistency estimates show that milk yield declines at a constant rate post-peak production. This suggests that the mechanisms responsible for MEC loss from the mammary gland, including loss into milk and other pathways, may shift across lactation. This work provides insight into local regulation of milk production in the mammary gland, an important consideration when evaluating milk production and lactation persistency.