Growth of mammary epithelium is regulated by endocrine signals, local factors and sensitivity of epithelial cells to mitogens and inhibitors. This study was designed to identify how these factors interact in regulating mammogenesis and determine if breed differences exist among these factors. Mammary epithelial cell organoids were isolated from five pairs of beef and dairy heifers (7 to 9 months of age) and cultured within collagen gels. Sera were collected from all heifers and pooled within breed for use in culture. At slaughter, explants of mammary fat pad were also prepared for culture. Culture treatments were arranged in a 2x2x3x4 factorial design. Factors included source of cells (beef or dairy), addition of growth factors (10 ng/ml IGF-I + 50 ng/ml EGF or control), source of mammary fat pad explants (beef, dairy or none) and source of serum added to culture media at 5% final concentration (beef, dairy, foetal calf serum, or none). Mammary growth was assessed as incorporation of [3H]thymidine into DNA. Mammary epithelial cells from beef heifers grew more rapidly than dairy cells (2518 vs 1192) cpm/ug; P<0.01). Growth of epithelia was enhanced by addition of growth factors (2265 vs 1445 cpm/ug; P<0.01). Sera from both beef and dairy heifers stimulated growth 2-fold compared to no serum (P<0.01), but addition of mammary fat inhibited epithelial growth (P<0.05). Cells from beef heifers were also more responsive to addition of serum and growth factors. These findings demonstrate that multiple factors interact to control mammary growth. Mammary fat pad of ruminants, unlike rodents, appears to inhibit epithelial growth.
Proceedings of the New Zealand Society of Animal Production, Volume 53, , 91-94, 1993
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