Characterization of mammary gland development in pregnant gilts

The purpose of this study was to quantify mammary gland (MG) growth during pregnancy in gilts and to determine the effect of anatomical location on gland growth. Size, composition, and histomorphology of MG were determined during gestation in 29 primigravid gilts. Gilts were allotted randomly to 6 slaughter groups: d 45 (n = 6), 60 (n = 4), 75 (n = 5), 90 (n = 4), 102 (n = 5), and 112 (n = 5) of gestation. Mammary glands were obtained at slaughter, and skin and extraneous fat pad were removed to obtain parenchymal MG tissue. Mammary glands were further separated into individual MG, and their locations were recorded. Individual MG were weighed and bisected in an approximate midsagittal section to measure cross-sectional area. Mammary glands were ground individually and pooled according to anatomical region: the first and second pairs of MG = anterior MG; the third, fourth, and fifth pairs of MG = middle MG; the sixth, seventh, and eighth pairs of MG = posterior MG. Contents of DM, CP, ether extract, and crude ash were measured. Wet weight, DM, CP, and ash content of total and individual MG increased (P < 0.01) between d 45 and 112 of gestation. Cross-sectional area of individual MG increased (P < 0.01) as gestation progressed. Percentage of CP and ash increased (P < 0.01), whereas percentage of ether extract decreased (P < 0.01) as gestation progressed. This inverse relationship between percentages of CP and ether extract (r = -0.999; P < 0.0001) was consistent with the histological shift from primarily an adipose tissue in early gestation to one containing extensive lobuloalveolar tissue in late gestation. Wet weight of middle MG was greater (P < 0.05) than that of posterior MG at d 102 and 112 of gestation, and amount of CP in middle MG was greater (P < 0.05) than that in anterior and posterior MG at d 102 and 112 of gestation, indicating that middle MG grow faster than other MG during late gestation. Rates of wet weight gain and protein accretion were accelerated (P < 0.01) after d 74 and 75 of gestation, respectively, indicating the importance of MG growth during the last trimester of gestation. The increase in rate of protein accretion after d 75 indicates a greater protein requirement for MG growth during later gestation.