Apoptosis and proliferation in Staphylococcus aureus-challenged, nonlactating mammary glands stimulated to grow rapidly and develop with estradiol and progesterone.

Bovine mastitis is a common and costly disease in the dairy industry and is known to negatively affect the amount of epithelium in nonlactating mammary glands. Despite this recognition, an understanding of the mechanisms contributing to reductions in epithelium is lacking. The objective of this study was to evaluate cellular apoptosis and proliferation in uninfected and Staphylococcus aureus-infected mammary glands that were stimulated to rapidly grow and develop. Estradiol and progesterone injections were administered to 18 nonlactating dairy cows to induce mammary growth, and 2 quarters from each animal were infused with saline or Staph. aureus. Mammary tissues were collected at 5 (n = 9) and 10 d (n = 9) postinfusion and examined using quantitative bright field and florescent immunohistochemistry. Staphylococcus aureus mammary glands tended to have a greater number of mammary epithelial cells undergoing apoptosis than saline quarters. In the stromal compartment, challenged quarters contained a lower proportion of cells undergoing apoptosis than saline quarters overall; however, cell types undergoing apoptosis were differentially affected. Staphylococcus aureus quarters contained a lesser percentage of apoptotic fibroblasts while also containing more nonapoptotic immune cells than saline quarters in the intralobular stroma compartment. A similar number of proliferating epithelial cells were present in Staph. aureus and saline mammary tissues, but more proliferating cells were present in the intralobular stroma compartment of Staph. aureus-infused quarters than those infused with saline. When these cellular responses are considered together, it indicates that changes in cellular apoptosis and proliferation contribute to changes in the gland structure by potentiating the expansion of the intralobular stromal compartment, via cellular accumulation, and limiting the amount of epithelium due to increases in cellular apoptosis in affected glands. Reductions in mammary epithelium are expected to reduce future milk yields and productive herd life.