Mammary Epithelial Proliferation and Estrogen Receptor α Expression in Prepubertal Heifers: Effects of Ovariectomy and Growth Hormone

Abstract

The objectives of this study were to determine the effects of ovariectomy and growth hormone on mammary epithelial cell proliferation and estrogen receptor alpha (ER alpha) expression within the bovine mammary gland. Two experiments were performed. In the first experiment, eight Holstein heifer calves aged between 1 and 3 mo were ovariectomized, while six calves served as controls. At 6 mo of age, calves were treated with bromodeoxyuridine (BrdU) to label proliferating cells and sacrificed 2 h later. Coinciding with reduced mammary mass (304 +/- 25 vs. 130 +/- 21 g), proliferation of mammary epithelial cells was significantly lower in ovariectomized heifers compared to control heifers (2.24 vs. 0.25%). ER alpha expression was restricted to mammary epithelial cells and was not observed within intra-lobular stroma of parenchymal tissue. The proportion of ER alpha positive cells was significantly higher in ovariectomized heifers than in controls (36.1% +/- 2.2 vs. 46.7% +/- 2.4). In the second experiment, mammary biopsies were taken from five 6-mo-old heifers, immediately preceding and 7 d following a single injection of bovine growth hormone. Mammary epithelial cell proliferation (assessed by incorporation of 3H-thymidine) was increased by growth hormone. The proportion of ER alpha positive mammary epithelial cells was not increased by growth hormone. In conclusion, reduced mammary epithelial cell proliferation following ovariectomy was associated with an increase in ER alpha expression, whereas increased proliferation caused by bovine growth hormone was not associated with changes in the proportion of ER alpha positive cells.

Effect of staged ovariectomy on measures of mammary growth and development in prepubertal dairy heifers

“…There was a 10-fold decrease in epithelial proliferation along with an increase in the proportion of ERa-positive epithelial cells in ovariectomized heifers when ovaries were removed between 1 and 3 months of age and samples collected at 6 months of age (Berry et al, 2003a). Even though the age at which surgeries were performed overlap between the two studies, we failed to show a reduction in cell proliferation in our study.…”

“…Wallace, 1953;Purup et al, 1993;Ballagh et al, 2008) or utilized animals with a broader range of starting ages (e.g. Berry et al, 2003a). We also made the decision to use a laparoscopic approach knowing that in these very young heifers the reproductive tract is positioned in a relatively extreme caudal position and has not achieved sufficient rostral elongation to easily facilitate the laparoscopic ovariectomy approach we utilized.…”

“…When ovaries were removed at birth, there was almost complete blockage of mammary development in calves (Wallace, 1953) and up to a 90% reduction in mammary parenchymal growth when heifers were ovariectomized at 2.5 months of age (Purup et al, 1995). When ovaries were removed from heifers between 2.5 and 3.0 months of age, there was a reduction in mammary development but not complete cessation (Berry et al, 2003a). These observations suggest that the magnitude of mammary gland response to ovariectomy depends on the age at which ovaries are removed.…”

“…Moreover, literature shows that a daily dose of , 0.1 mg/kg body weight (BW) estradiol is required to elicit positive responses in mammary epithelial cell (MEC) proliferation (Woodward et al, 1993;Capuco et al, 2002;Meyer et al, 2006a). On the other hand, a large increase in the expression of estrogen receptor-a (ERa) was noted in conjunction with a significant reduction in MEC proliferation in older ovariectomized heifers (Berry et al, 2003a). This suggests there are locally active interacting mechanism(s) involving estrogen, estrogen receptor expression, cell proliferation pathways and likely other signaling elements that coordinate and control peripubertal mammary development in heifers and other ruminants.…”

Interactions between the ovary and the local IGF-I axis modulate mammary development in prepubertal heifers

“…This supports our observations that IGF-I mRNA is lower in mammary tissue from ovariectomized animals. However, in the bovine mammary gland, it is unlikely that estrogen directly stimulates IGF-I transcription through the estrogen receptor (ER ), because IGF-I expression is exclusive to stromal cells and the ER is found only in epithelial cells (Capuco et al 2002, Berry et al 2003. However, regulation of IGF-I mRNA expression by estrogen may be controlled through interactions between parenchymal and stromal mammary tissues.…”