Abstract
Previous studies in prepubertal heifers suggest that the magnitude of reduction in mammary parenchymal growth in response to ovariectomy varies with the age at which surgery is performed. We hypothesized that ovarian secretions are essential for initiating mammary development but not required to maintain allometric mammary growth in prepubertal dairy heifers. The objectives of this study were to determine the effect of staged ovariectomy during the prepubertal period on mammary growth and tissue composition and the expression of selected genes. Prepubertal Holstein heifers at 2, 3 or 4 months of age were randomly assigned to one of two treatments, ovariectomized (OVX; n = 12) or sham operated (INT; n = 12). Mammary parenchyma (PAR) and fat pad (MFP) were harvested 30 days after surgery. Proximate composition of PAR and MFP (DNA, protein and lipid) as well as expression of the selected estrogen-responsive genes stanniocalcin1 (STC1), tissue factor pathway inhibitor precursor (TFPI) and proliferating cell nuclear antigen (PCNA) were determined in PAR and MFP by quantitative real-time PCR. The relative amount of epithelium and proportion of epithelia cell nuclei expressing the proliferation marker Ki67 were determined by histological and immunohistochemical analyses, respectively. MFP mass was not impacted by treatment but was decreased with age as was lipid content and concentration (P ⩽ 0.01). The mass of mammary PAR was reduced in OVX and increased with age (P ⩽ 0.01). Parenchymal tissue tended to have less total DNA, protein and lipid in OVX heifers. Parenchymal tissue concentrations of protein and DNA were increased with age and there was an age × treatment interaction. Treatment had no effect on either the Ki67 labeling index or percent epithelial area. The relative abundances of STC1, TFPI and PCNA mRNA in PAR were reduced in OVX. We did not find a significant impact of ovariectomy on mRNA expression when surgery was performed at 2 months compared with surgery at 3 or 4 months of age. However, having nearly undetectable PAR in two heifers ovariectomized at the earliest period (2 months of age) suggests that early ovariectomy is especially detrimental to subsequent parenchymal development.
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