"Changes in the mammary function with increased number of lactations and aging"

众所周知，奶牛的产奶量随着年龄的增长和哺乳次数的增加而变化。已经对此给出了一些解释，但没有提供任何实验证据。因此，本研究旨在阐明反复妊娠和增龄导致乳腺功能改变的机制。先后在小鼠体内建立了四个哺乳期。乳汁分泌功能在第1乳低于第2乳和第3乳，但乳腺细胞数无明显差异。在第4个哺乳期，随着乳腺细胞数量的减少和摄食量的减少，产奶量减少。不同年龄的小鼠之间以及同年龄的产后和未出生的小鼠对给定剂量的雌激素和孕酮的乳房生长反应存在差异。此外，在这些生理条件下取自小鼠的分散培养的乳腺上皮细胞在有或没有松果树毒素和/或表皮生长因子的培养液中进行了不同的增殖。不同年龄以及产后和未出生的小鼠对雌激素的反应不同，脑垂体和血清催乳素水平的变化也不同。在老年大鼠中观察到垂体催乳素的异质性，而在年轻大鼠和老年大鼠中未观察到这种异质性，并且雌激素对年轻大鼠和老年大鼠都有诱导作用。这些结果可能为揭示乳房功能随增龄和催乳而改变的机制提供一些线索。

It is well known that the amount of milk produced by cows changes with advancing age and increased number of lactations. Some explanation has been given for this but no experimental evidence has been provided. Therefore, the present study was conducted to elucidate the mechanism by which mammary function changes with recurring pregnancy and aging. Four lactations were successively established in mice. Mammary secretory function was lower in the 1st than in the 2nd and 3rd lactations despite that no difference existed in mammary cell numbers. In the 4th lactation, milk yield reduces with decreased number of mammary cells and reduced food intake. Differences in mammary growth response to a given dose of estrogen and progesterone between mice at different ages and between parous and nuliparous mice at the same age were shown. Also, cultured dispersed mammary epithelial cells taken from mice under these physiological conditions proliferated differently in culture media with or without chorela toxin and/or epidermal growth factor. Pituitary and serum prolactin levels changed differently between different ages and between parous and nuliparous mice in response to estrogen. Heterogeneity in pituitary prolaction was observed in old but not young rats and was induced by estrogen administration both in young and old rats. These results may provide some clue to enlighten the mechanism of change in mammary function with aging and advancing lactations.