Cellular proliferation and apoptosis in Staphylococcus aureus-infected heifer mammary glands experiencing rapid mammary gland growth

Intramammary infections in nonlactating mammary glands are common and can occur during periods of rapid mammary epithelial cell (MEC) accumulation, which may ultimately reduce total MEC numbers. Reduced MEC numbers, resulting from impaired MEC proliferation and increased cellular apoptosis, are expected to reduce future milk yields. The objective of this study was to measure the degree of cellular proliferation and apoptosis in the epithelial and stromal compartment of uninfected and Staphylococcus aureus-infected mammary glands hormonally induced to grow rapidly. Nonpregnant heifers (n = 8) between 11 and 14 mo of age were administered supraphysiological injections of estradiol and progesterone for 14 d. One mammary gland of each heifer was randomly selected and infused with Staph. aureus (CHALL) while another mammary gland was designated as an uninfected control on d 8 of injections. Mammary tissues were collected on the last day of hormonal injections from center and edge parenchymal regions and subject to proliferation assessment via Ki-67 staining and apoptotic assessment via terminal deoxynucleotidyl transferase dUTP nick-end labeling. Differences in cellular proliferation between CHALL and uninfected control quarters were not apparent, but proliferation of MEC was marginally greater in edge parenchyma than in center parenchyma. Coincidently, CHALL quarters experienced a greater percentage of apoptotic MEC and lower percentage of stromal cells undergoing apoptosis than uninfected control quarters. This study also provides the first insight into the mechanisms that allow the mammary fat pad to be replaced by expanding mammary epithelium as edge parenchyma contained a greater percentage of apoptotic stromal cells than center parenchyma. When taken together, these data suggest that Staph. aureus intramammary infection impairs mammary epithelial growth through reductions in MEC number and by preventing its expansion into the mammary fat pad. These factors during periods of rapid mammary growth are expected to impair first lactation milk yield.

Organization of mammary blood vessels as affected by mammary parenchymal region and estradiol administration in Holstein heifer calves

Mammary blood flow is central to mammary growth, development, and productivity, but the development of the vasculature network is poorly understood. The objective of this study was to determine how the vascular system adapts to mammary growth by inducing different levels of mammary growth and examining 2 regions of mammary parenchymal tissue. Holstein heifer calves (n = 12) received daily injections on the days immediately preceding euthanasia at 82 d of age. Treatments were control (CON), short-term estradiol (STE), and long-term estradiol (LTE). The CON calves received corn oil injections, the STE calves received 9 injections of corn oil followed by 3 injections of estradiol, and the LTE calves received 12 estradiol injections. Mammary tissues were collected from the center and edge parenchymal regions of all right rear mammary glands to quantify the tissue area of various tissue structures, the percentage of proliferating epithelial cells, and the number and form of blood vessels. Results showed that LTE calves had a greater tissue area occupied by epithelium than CON and STE calves, and the epithelial area in CON and STE calves was similar. Edge parenchyma had a greater percentage of proliferating epithelial cells than center parenchyma across all treatment groups. In the edge region, LTE calves had the greatest percentage of proliferating epithelial cells, coinciding with greater epithelial area. The number of blood vessels per unit of tissue area was greater in center than in edge parenchyma; the corresponding vessel surface area per unit of tissue area followed the same pattern. Mammary blood vessel measures were not markedly influenced by estradiol treatment. These results highlight the marked difference in the number and organization of blood vessels in different mammary parenchyma regions but indicate that the effects of estradiol on stimulating mammary epithelial proliferation does not directly translate to increasing numbers of blood vessels.

Effects of Staphylococcus aureus intramammary infection on the expression of estrogen receptor α and progesterone receptor in mammary glands of nonlactating cows administered estradiol and progesterone to stimulate mammary growth

Intramammary infections (IMI) are prevalent in nonlactating dairy cattle and are known to alter mammary structure and negatively affect the amount of mammary epithelium in the gland. Mechanisms responsible for the observed changes in mammary growth during an IMI are poorly understood, yet the importance of the key mammogenic hormones driving mammary growth is well recognized. This study's objective was to characterize the expression of estrogen receptor α (ESR1) and progesterone receptor (PGR) in mammary glands stimulated to grow and develop in the presence or absence of an IMI as well as preliminarily characterize myoepithelial cell response to IMI. Mammary growth was stimulated in 18 nonpregnant, nonlactating dairy cows using subcutaneous estradiol and progesterone injections, and 2 culture-negative quarters of each cow were subsequently infused with either saline (n = 18) or Staphylococcus aureus (n = 18). Mammary parenchyma tissues were collected 5 d (n = 9) or 10 d (n = 9) postchallenge and examined using immunofluorescence microscopy to quantify positive nuclei and characterize staining features. There tended to be a greater number of ESR1-positive nuclei observed across 8 random mammary parenchyma fields of view in saline quarters than in Staph. aureus quarters (201 vs. 163 ± 44 nuclei). Saline quarters also contained a greater number of PGR-positive nuclei (520 vs. 440 ± 45 nuclei) and myoepithelial cells (971 vs. 863 ± 48 nuclei) than Staph. aureus-challenged quarters. However, when ESR1, PGR, and myoepithelial nuclei counts were adjusted for Staph. aureus quarters containing less epithelium, differences between quarter treatments abated. The examined ESR1 and PGR staining characteristics were similar between saline and Staph. aureus quarters but were differentially affected by day of tissue collection. Additionally, nuclear staining area of myoepithelial cells was greater in Staph. aureus quarters than in saline quarters. These results indicate that IMI had little effect on the number or staining characteristics of ESR1- or PGR-positive nuclei relative to epithelial area, but myoepithelial cells appear to be affected by IMI and the associated inflammation in nonlactating mammary glands that were stimulated to grow rapidly using mammogenic hormones. Accordingly, reductions in mammary epithelium in affected glands are not suspected to be resultant of alterations in the number or staining characteristics of ESR1- or PGR-positive mammary epithelial cells.