Authors
S.E. Fenton et al.

Title
Persistent abnormalities in the rat mammary gland following gestational and lactational exposure to 2,3,7,8 - tetrachlorodibenzo-p-dioxin (TCDD)

Journal
Toxicological Sciences (67):63-74, 2002

Summary
Dioxin is the common name for a family of 75 toxicants, the most potent of which is TCDD [2,3,7,8-tetrachlorodibenzo-P-dioxin]. TCDD, a halogenated aromatic hydrocarbon, is rapidly absorbed into the body and bioaccumulates in lipids and other tissues of higher vertebrates. This dioxin is a potent growth dysregulator that alters normal cell regulatory processes, such as cell proliferation and differentiation, metabolic capacity, and hormonal pathways. In general, dioxins have the potential to disrupt a large number of critical developmental events at specific developmental stages, which can lead to increased embryo and fetal mortality or impaired organ systems. In mammals, postnatal functional alterations involving the developing reproductive system appears to be the event most sensitive to perinatal dioxin exposure. The authors of the present investigation examined mammary gland development in female rat offspring following gestational and lactational exposure to TCDD exposure.

Pregnant Long-Evans rats were treated with 1 µg TCDD/kg bw on GD 15, and their female offspring were sacrificed on PND 4, 33, 37, 45, 68, and 110. Unlike control animals, the mammary tissue from TCDD-exposed females demonstrated consistent developmental defects, including significantly fewer terminal structures (i.e. terminal branches, alveolar buds) and primary branches, and migratory delays of the epithelium through the fat pad. In addition, these pups weighed significantly less at weaning and grew slower than age-matched controls. Furthermore, peripubertal animals had delayed vaginal opening and persistent vaginal threads, yet did not display altered estrous patterns.

In a study designed to determine the critical time of TCDD exposure required to induce developmental perturbations of the mammary gland, pregnant Long-Evans rats were treated with 1 µg TCDD/kg bw on GD 15 or 20 or PND 1, 3, 5, or 10. Rats exposed to TCDD on GD 15 displayed consistently underdeveloped epithelium. In contrast, pups from dams exposed on GD 20 or during lactation displayed no noticeable effects in gland development. The findings suggest that GD 15 is a critical period that may be susceptible to impaired fetal mammary gland development following TCDD exposure.

Serum concentrations of prolactin (PRL), thyroid stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) in 25-day old offspring exposed to TCDD on GD 15 were examined. Although previous studies have demonstrated alterations in these endocrine hormones in response to TCDD administration, the results of the present study do not indicate an association between serum hormone levels and developmental delays in the mammary epithelium. Baseline serum PRL levels were decreased, but remained within the normal range for a young rat. Serum TSH concentrations were significantly elevated; however, serum T3 and T4 levels did not change, suggesting a compensatory action of the thyroid.

Epithelial transplants were performed to evaluate the source of mammary gland disruption. The fat pads of control and TCDD-exposed animals (on GD 15) were cleared, and a small piece of epithelium was transplanted into the recipient (control into TCDD-exposed, and vice versa). The epithelium from a TCDD-exposed animal transferred into a control fat pad developed normally with differentiated terminal structures, proliferative ducts, and visible lobules. In contrast, control epithelium transplanted into a cleared fat pad of TCDD-exposed animal developed poorly differentiated terminal structures, irregularly formed ducts, and few lobules. These results suggest that the stroma of the mammary gland is altered by early TCDD exposure and thus can no longer support normal epithelial formation.

Susceptibility to TCDD exposure may be more complex in the mammary gland because its development occurs in multiple stages. The results of the present study suggest that GD 15 is a critical period susceptible to the adverse developmental effects of this toxicant. In addition, the effects of TCDD on the endocrine system and tissue differentiation may increase susceptibility to mammary adenocarcinomas induced by other compounds. The less complex mammary ductal branching pattern observed in TCDD treated female rats is similar in morphology to an immature gland sensitive to DNA damaging agents. Future studies are necessary to elucidate the level of TCDD exposure necessary to produce developmental delays of the mammary, as well as determining the effects of this toxicant on lactational performance which is necessary for the survival of our endangered wildlife.