Effect of Recurring Pregnancy on Mammary Gland Growth in Mice

It was shown by the increase of total desoxyribosenucleic acid (DNA) that mammary gland growth continues during the latter half of pregnancy.This observation was rather unexpected, since morphological study of lobule~alveolar growth can not distinguish between additional cell growth and the initiation of milk secretion.These data indicate for the first time that cell nlultiplication continues throughout pregnancy.The effect of recurring pregnancy on mammary gland growth was studied, employing albino mice bred successively without nursing periods.Maximum growth of the mammary glands was attained in triparous animals.A slight decline in total growth was observed in quadriparous animals.Their age at this time was estimated to be equivalent to the age of cows which begin to dec.line in yearly milk and fat yields.It was also confirmed that increased mammary gland growth in multiparous animals was more closely related to hormonal stimuli associated with recurring pregnancy and lactation than to increasing body weight.Primiparous mice after 3 too. of involution had mammary glands containing DNA equivalent to mice pregnant for six days.No variation in average litter size was observed during four consecutive pregnancies.Milk and fat secretion in dairy cattle has been shown to increase with advancing age up to about 8 yr.The age-production relation has been used extensively to formulate age-correction factors for converting milk and fat production records to their mature equivalent.Two obvious factors are involved in the increase in milk secretion: (a) the increase in body weight up to 8 yr.(7, 9) and (b) recurring pregnancy and lactation.In the analysis of these data it was suggested that recurring pregnancy (and factors associated with pregnancy and lactation) influenced the increase in lactation with age far more than did the increase in body weight.Confirmation of this relation has been provided by an analysis of milk records of Swedish dairy cattle (5).It was shown that milk yield per lactation depends not only on age up to maturity but also on the number of lactations.To reach nmximum yield the cows must, on the average, have had at least three pregnancies.The most obvious factor ill recurring pregnancy is growth of the mammary gland.While it has been inferred during the last 30 yr. that additional growth of the mammary gland was stimulated with each pregnancy up to the time of maximum milk yield, ua quantitative method of determination of mammary gland growth has been available to prove that additional growth occurred.