Mammary Gland Development of Dairy Heifers during Their First Gestation

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Abstract
Mammary gland development was determined by analysis of udders at 13 stages of the 1st gestation of 9 pairs of identical twin dairy heifers. Stained sagittal sections of udders showed that gland parenchyma spread into and displaced adipose tissue so that total udder wt did not reveal the extent of gland development. Changes in composition of the mammary gland during gestation reflected gross and histological changes. Percentage of fat and DNA decreased while precentages of N H2O, dry fat-free tissue and RNA increased. Development of the portion of the udder which was mammary gland appeared to be a continuous exponential process through gestation, and increases in quantitative development following the general formula for organ growth, Y = Aekt (In Y = 1n A + kt), where k is the rate constant for growth by months (t). Analysis of variance showed in addition to stage of gestation that body wt and pairs, but not age, affected the regression. Adjusting for body wt only produced rate constants for growth similar to those from adjusting for pairs and wt. Development rates were .apprx. 33% per mo. for N, dry fat-free tissue and internal surface area of mammary glands, and .apprx. 25% for parenchyma wt and DNA. Average rate of increase in total udder wt was .apprx. .5 for mammary glands only.