Differences in the Rate of in Situ Mammary Gland Development and Other Developmental Endpoints in Three Strains of Female Rat Commonly Used in Mammary Carcinogenesis Studies

Abstract

The potential of chemicals to alter susceptibility to mammary tumor formation is often assessed using a carcinogen-induced study design in various rat strains. The rate of mammary gland development must be considered so that the timing of carcinogen administration is impactful. In this study, in situ mammary gland (MG) development was assessed in females of the Harlan Sprague Dawley (Hsd:SD), Charles River Sprague Dawley (Crl:SD), and Charles River Long Evans (Crl:LE) rat strains at postnatal day (PND) 25, 33, and 45. Development was evaluated by physical assessment of growth parameters, developmental scoring, and quantitative morphometric analysis. Though body weight was consistently lower and day of vaginal opening (VO) occurred latest in female Hsd:SD rats, they exhibited accelerated pre-and peripubertal MG development compared to other strains. Glands of Crl:SD and Crl:LE rats exhibited significantly more terminal end buds (TEBs) and TEB/mm than Hsd:SD rats around the time of VO. These data suggest a considerable difference in rate of MG development across commonly used strains, which is independent of body weight and timing of VO. In mammary tumor induction studies employing these strains, administration of the carcinogen should be timed appropriately, based on strain, to specifically target the peak of TEB occurrence.

Mammary Gland Evaluation in Juvenile Toxicity Studies

“…However, the pace of development is accelerated in this strain, as has been recently suggested in a three strain comparison of rats (Stanko et al, Toxicol Pathol, in press). By PND 33, 100% of the fourth and fifth mammary glands in the control females had already grown together (data not shown).…”

“…Around PND 21, the TEBs begin to cleave into new ducts that form clusters of 3 to 5 alveolar buds each with a centrally located lumen surrounded by a layer of cuboidal epithelial cells. Between PND 21 and PND 46, the females will have undergone vaginal opening, began regular hormone surges, and have an established estrous cycle (Miousse et al 2013; Stanko et al, in press). The males are peripubertal at PND 46, likely progressing through preputial separation a few days prior to this time point (Rocca and Pepperl 2000).…”

“…We could not determine the persistence of these differences as PND 70 was the oldest age evaluated. This strain of rat already exhibits accelerated mammary gland development compared to other strains (Stanko et al, in press), therefore it may be more difficult to quantitate accelerated development in females without the use of biomarkers such as receptor staining status or TEB counts. Further, the DES dose that was used in our study may have been too low for the effects we anticipated.…”

“…NTP and European-based guideline studies (modified one-generation reproduction studies, reproductive assessment by continuous breeding studies, and 2-year bioassays) have recently been modified to include assessing mammary gland development in their standard testing protocols (NTP 2011; Organisation for Economic Co-operation and Development [OECD] 2011). Strain differences in mammary gland development are reported (Stanko et al in press), necessitating a pictorial of normal HSD rat mammary gland development.…”

Effect of Arrabidaea chica extract against chemically induced breast cancer in animal model

“…Wistar female rats ( Rattus norvegicus ) weighing 185±23g from the Animal Science Center, UFRN, were used. Young (six week) rats, more sensitive to DMBA, were used for breast tumor induction 14 . The rats were acclimatized for 1 week prior to the start of the experiment under standard housing conditions, including room temperature 22-24°C, relative humidity 40% and 12-hour light-dark cycle in polypropylene cages (maximum 2 animals/cage).…”