Developmental Stage of the Rat Mammary Gland as Determinant of Its Susceptibility to 7, 12-Dimethylbenz[a]anthracene2

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Abstract
Postnatal development of the mammary gland was studied in 80 noninbred Sprague-Dawley virgin rats ranging in age from 2 to 112 days, and the changes induced by 7,12-dlmethylbenz[a]anthracene (DMBA) were studied In 60 noninbred Sprague-Dawley virgin rats that, at the age of 55 days, were inoculated intragastrically with 10 mg DMBA/100 g body weight. To correlate the sequential structural changes in the two groups, animals of both groups were killed weekly and their mammary glands removed and processed for wholemount. Terminal endbuds (TEB), terminal ducts (TD), alveolar buds (AB), and lobules per square millimeter were counted in wholemount preparations under a stereomicroscope. During postnatal development, the mammary gland tree grew by sprouting numerous ducts ending in club-shaped TEB. The density of TEB reached a peak when the rats were 21 days old (25±2 TEB/mm2), decreased sharply until the animals were 63 days old, and then decreased slowly until they were 84 days of age. The number of TEB decreased because of their differentiation mainly into AB or their evolution to TD. AB later differentiated into lobules. AB increased in number steadily to a plateau when the animals were 70–84 days old. After DMBA was administered to the rats at 55 days of age, the number of TEB remained higher than that in the control animals and the TEB became larger and had higher mitotic activities. Such TEB were called intraductal proliferations (IDP); they evolved to adenocarcinomas. DMBA increased the number of TD and decreased the number of AB and lobules in relation to the control animals. These findings led to the conclusion that DMBA administration to 55-day-old rats alters the differentiation of TEB→AB→Iobules, inducing instead the sequence TEB→IDP→adenocarcinoma