Mammary Gland Evaluation in Juvenile Toxicity Studies

Abstract

There are currently no reports describing mammary gland development in the Harlan Sprague Dawley (HSD) rat, the current strain of choice for National Toxicology Program (NTP) testing. Our goals were to empower the NTP, contract labs and other researchers in understanding and interpretation of chemical effects in this rat strain. To delineate similarities/differences between the female and male mammary gland, data were compiled starting on embryonic day 15.5 through postnatal day 70. Mammary gland whole mounts, histology sections, and immunohistochemically-stained tissues for estrogen, progesterone and androgen receptors were evaluated in both sexes; qualitative and quantitative differences are highlighted using a comprehensive visual timeline. Research on endocrine disrupting chemicals in animal models has highlighted chemically-induced mammary gland anomalies which may potentially impact human health. In order to investigate these effects within the HSD strain, 2,3,7,8-tetrachlorodibenzo p-dioxin, diethylstilbestrol or vehicle control was gavage dosed on gestation day 15 and 18 to demonstrate delayed, accelerated and control mammary gland growth in offspring, respectively. We provide illustrations of normal and chemically altered mammary gland development in HSD male and female rats to help inform researchers unfamiliar with the tissue and may facilitate enhanced evaluation of both male and female mammary glands in juvenile toxicity studies.

Relationship between morphological development and sex hormone receptor expression of mammary glands with age in male rats

“…TEBs are solid or semisolid bulbous clusters of immature epithelial cells at the ends of ducts and important sites of epithelial cell proliferation during puberty [ 24 ]. The number of TEBs showed a similar tendency to the reported data about the mammary glands of female rats [ 8 ]. The number of TEBs prominently increased between 21 and 35 days old; consistent with a previous study about male rats [ 8 ].…”

“…The number of TEBs showed a similar tendency to the reported data about the mammary glands of female rats [ 8 ]. The number of TEBs prominently increased between 21 and 35 days old; consistent with a previous study about male rats [ 8 ].…”

“…Previous studies have demonstrated that the mammary glands in adult rats showed sexual dimorphism and male rats had male-specific cells [ 4 , 8 , 9 , 11 , 21 , 22 , 32 , 33 ]. Sexual dimorphism in mammary glands has not been reported in other mammalian animals including mice [ 4 ].…”

“…The progesterone might be related to male mammary gland development at young ages, not at adult ages. On the other hand, another report showed that no PgR expressions in mammary glands of male rats were observed at from 1 to 70 days old [ 8 ]. This difference on the PgR expression from our study was considered due to the differences of primary antibody used for IHC and rat strain.…”

“…Rats have been used for the mammary carcinogenesis model [ 13 , 15 , 27 , 31 , 36 ], and several investigators have reported the development of mammary glands with age in female rats [ 19 , 20 , 24 , 29 ]. Little is known about the male mammary glands in rats except for limited information from a few reports [ 4 , 8 , 11 , 21 , 28 ]. In particular, male-specific cells in mature male rats, with abundant and eosinophilic cytoplasm, were described in some reports; however, the details remain unknown.…”