Title: DEVELOPMENTAL AND NUTRITIONAL REGULATION OF THE PREPUBERTAL BOVINE MAMMARY GLAND: II. ALLOMETRIC GROWTH, EPITHELIAL CELL PROLIFERATION, AND THE INFLUENCE OF AGE AT SLAUGHTER ON PARENCHYMA DEVELOPMENT
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 17, 2006
Publication Date: November 1, 2006
Citation: Meyer, M.J., Capuco, A.V., Ross, D.A., Lintualt, L.M., Van Amburgh, M.E. 2006. Developmental and nutritional regulation of the prepubertal bovine mammary gland: ii. allometric growth, epithelial cell proliferation, and the influence of age at slaughter on parenchyma development. Journal of Dairy Science. 89:4298-4304.
Interpretive Summary: Elevated nutrient intake by prepubertal heifers is known to reduce overall mammary development at puberty. This effect was previously thought to be brought about by a direct negative effect of elevated energy intake on mammary epithelial cell proliferation and therefore overall mammary growth. Data presented herein demonstrate that the single greatest determinant of mammary epithelial growth is the length of time between birth and the given body weight at which mammary development is assessed, which has traditionally been puberty. Plane of nutrition affects the length of time necessary to reach this given body weight, but appears to have no direct influence on the local or systemic controls that coordinate the rate of parenchyma accretion, mammary epithelial cell proliferation, or total parenchyma mass.
It is well documented that elevated nutrient intake prior to puberty reduces prepubertal mammary development in the bovine. The objectives of this experiment were to evaluate the effects of increased nutrient intake from early in life on 1) mammary epithelial cell proliferation, 2) mammary parenchyma DNA accretion rates, and 3) the dynamics of prepubertal allometric parenchyma growth. Holstein heifers (n = 78) were fed from 45 kg body weight (BW) either elevated (E) or restricted (R) levels of nutrients to gain 950 (E) or 650 (R) g/d. Six heifers per treatment were slaughtered at 50 kg increments from 100 to 350 kg BW. Heifers on the elevated plane of nutrition had less parenchyma DNA than their BW-matched cohorts on the restricted plane of nutrition. Despite this reduction in parenchyma DNA, elevated nutrient intake did not reduce mammary epithelial cell proliferation. In fact, E-heifers tended to have a higher mammary epithelial cell bromodeoxyuridine incorporation index than R-heifers. Additionally, daily parenchyma DNA accretion rate was not influenced by level of nutrient intake. Slopes of the plots of log BW vs. log parenchyma DNA were similar between the two treatment groups and illustrate that the allometric and isometric parenchyma growth rates were unaffected by plane of nutrition. Finally, we were able to describe 82% of the variation in both control and nutritionally impaired prepubertal mammary development in previously published data using our observed parenchyma DNA accretion rate and the published age at slaughter. These data demonstrate that the single greatest determinant of parenchyma DNA mass is the age of the heifer at which mammary development is assessed. Plane of nutrition affects the length of time necessary to achieve this given BW, but has little or no direct influence on the local or systemic controls that coordinate the rate of parenchyma DNA accretion, mammary epithelial cell proliferation, or total parenchyma mass.