Mammary gland development of dairy heifers during their first gestation.

Mammary gland development was determined by analysis of udders at thirteen stages of the first gestation of nine 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 weight does not reveal extent of gland development. Changes in composition of mammary gland during gestation reflected gross and histological changes. Percentages of fat and deoxyribonucleic acid decreased while percentages of nitrogen, water, dry fat-free tissue, and ribonucleic acid 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 followed the general formula for organ growth, Y = Aekt (In Y = In A + kt), in which k is the rate constant for growth by months (t). Analysis of variance showed in addition to stage of gestation that body weight and pairs, but not age, affected the regression. Adjusting for body weight only produced rate constants for growth similar to those from adjusting for pairs and weight. Rates of development were approximately 33% per month for nitrogen, dry fat-free tissue, and internal surface area of mammary glands, and approximately 25% for parenchyma weight and deoxyribonucleic acid. Average rate of increase in total udder weight was only about .5 that for mammary glands only.