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Effects of preweaning nutrient intake in the developing mammary parenchymal tissue.
Historically, mammary gland growth has been considered isometric the first 2 mo of life and then allometric until peripuberty. However, recent work indicated that the mammary gland might be responsive to nutrient intake preweaning. The objectives of this study were to describe the effects of nutrient intake preweaning on mammary gland development and to investigate cell specific proliferation during this phase of development. Twelve dairy heifer calves were fed either a fixed amount of milk replacer (MR; control, n = 6) or an amount of MR adjusted for BW (enhanced, n = 6). Control calves received a constant amount of a 28% crude protein, 15% fat milk MR per day that was equivalent to 2.8 Mcal of metabolizable energy intake per day; enhanced calves received 0.3 Mcal of metabolizable energy intake per kilogram of metabolic body weight (from 4.2 to 8.4 Mcal of metabolizable energy intake per day). All calves had constant access to water and a 22% crude protein commercial calf starter. Calves were killed at 54 ± 2 d. Control calves consumed 32.6 ± 2.4 kg of MR and 6.7 ± 0.5 kg of calf starter per calf, whereas the enhanced calves consumed 69.5 ± 2.4 kg of MR and 1.9 ± 0.5 kg of calf starter per calf over the 54-d period. Further, to evaluate putative stem cell proliferation, BrdU (5-bromo-2'-deoxyuridine; 5 mg/kg) was injected intramuscularly once per day between 12 to 15 d and again once per day between 24 to 27 d of life. Initial and final body weight for the control and enhanced treatments were 39.2, 61.0, 39.7, and 83.2 kg, respectively. At euthanasia, weights of liver, kidneys, pancreas, whole skinned mammary gland, and mammary parenchyma were measured. The growth rate of each organ was calculated using the concept of allometry as the difference in the change in organ weight as a percentage of body weight. The mammary glands of calves fed the enhanced diet were significantly heavier at euthanasia; when mammary parenchymal weight was analyzed, enhanced calves had 5.9 times greater mammary parenchymal mass, indicating the mammary gland was responsive to nutrient intake before weaning. Allometric growth of the mammary gland was initiated preweaning in the calves fed the enhanced treatment. Further characterization of mammary cells that retained BrdU label revealed no significant differences among the tissue slices analyzed between treatments; however, as calves fed the enhanced diet had more mammary parenchymal mass, if the number of label-retaining cells per counted slide were similar between treatments then the enhanced calves had a larger total population of putative mammary stem cells present in the mammary gland.
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