 we investigated mammary gland differentiation and cell proliferation in rats after acute exposure to xenoestrogens . 
 pubertal female sprague - dawley rats ( six / group ) were treated for 1 week with diethylstilbestrol ( des ) , genistein , o , p'-ddt , aroclor 1221 , aroclor 1254 , 2,3,7,8-tetrachlorodibenzo - p - dioxin ( tcdd ) , or the vehicle , sesame oil . animals were killed 18 hr after the last treatment . 
 analysis of mammary whole - mounts revealed that exposure to des , genistein , and o , p'-ddt resulted in enhanced gland differentiation and increased epithelial cell proliferation as measured by proliferating cell nuclear antigen immunohistochemistry , tcdd treatment inhibited cell proliferation and gland development . 
 aroclor 1221 and aroclor 1254 treatments had slight but not statistically significant effects on cell proliferation and mammary gland development . 
 we conclude that des , genistein , and o , p'-ddt given to pubertal rats act as morphogens ; i.e. , they increase cell proliferation , which promotes maturation of the undifferentiated terminal end buds to more differentiated lobular terminal ductal structures.imagesfigure 1 . 
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