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Expression in Prepubertal Heifers: Effects of Ovariectomy and Growth Hormone
,1
* Department of Dairy Science, Virginia Tech, Blacksburg 24061
Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg 24061
Gene Evaluation and Mapping Laboratory, USDA-ARS, Beltsville, MD 20705
Corresponding author:
R. M. Akers; e-mail:
rma{at}vt.edu.
The objectives of this study were to determine the effects of ovariectomy and growth hormone on mammary epithelial cell proliferation and estrogen receptor 
(ER) 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 expression was restricted to mammary epithelial cells and was not observed within intra-lobular stroma of parenchymal tissue. The proportion of ER 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 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 expression, whereas increased proliferation caused by bovine growth hormone was not associated with changes in the proportion of ER positive cells.
Key Words: estrogen receptor • growth hormone • mammary • ovariectomy
Abbreviation key: BrdU = bromodeoxyuridine, E = estrogen, ER = estrogen receptor , GH = growth hormone, IGF-I = insulin like growth factor I, OVX = ovariectomized, TDU = terminal ductular unit
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