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Short Communication: Intramammary Infusion of IGF-I Increases Bromodeoxyuridine Labeling in Mammary Epithelial Cells of Prepubertal Heifers^*

¹ Department of Animal Production and Nutrition, Faculty of Veterinary Science, University of São Paulo, Brazil
² Department of Animal Science, Michigan State University, East Lansing 48824

Corresponding author: Michael J. VandeHaar; e-mail: mikevh{at}pilot.msu.edu.

When dairy heifers are fed to gain more than 900 g of body weight/d, they have less mammary parenchymal DNA at puberty but more insulin-like growth factor-I (IGF-I) in serum. This negative relationship between serum IGF-I concentration and mammary epithelial cell proliferation is in disagreement with the extensively reported role of IGF-I as a stimulator of mammary epithelial cell proliferation. Despite the large body of evidence suggesting that an increase in IGF-I concentration should lead to an increase in mammary epithelial cell proliferation of prepubertal heifers, it had not been previously tested. Our objective was to determine if intramammary infusions of IGF-I would stimulate mammogenesis in prepubertal heifers in vivo. After 7 d of treatment, bromodeoxyuridine was infused intravenously and heifers were slaughtered 3 h later. Samples from 3 regions of the mammary parenchyma were collected, fixed, sliced, and incubated with bromodeoxyuridine monoclonal antibody to identify cells in the S-phase of the cell cycle. Intramammary infusion of IGF-I increased the percentage of epithelial cells in the S-phase by 52% (6.4 vs. 4.2%, ± 0.3%). Proliferation was similar in all 3 parenchymal regions, and the response to IGF-I was similar in each region. We conclude that local IGF-I increases proliferation of mammary parenchymal epithelial cells in prepubertal heifers.

Key Words: bovine • heifer • insulin-like growth factorI • mammary gland

Abbreviation key: BrdU = bromodeoxyuridine

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