Effect of Recurring Pregnancy on Mammary Gland Growth in Mice

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Abstract
It was shown by the increase of total desoxyribosenucleic acid (DNA) that mammary gland growth continues during the latter hall of pregnancy. This observation was rather unexpected, since morphological study of lobule-alveolar growth cannot distinguish between additional cell growth and the initiation of milk secretion. These data indicate for the first time that cell multiplication 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 decline in yearly milk and fat yields. It was also confirmed that increased mammary gland 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 months of involution had mammary glands containing DNA equivalent to mice pregnant for 6 days. No variation in average litter size was observed during 4 consecutive pregnancies.