Assessment of postnatal femur development in Wistar Han rats

The use of Wistar Han (WH) rats in regulatory toxicology studies, including juvenile animal studies, is increasing. The current study was performed to characterize femur development in neonatal and juvenile WH rats. Beginning on postnatal day (PND) 4 through PND 50, groups of WH rats (n = 5/sex) were evaluated for the development and ossification of the femur by Alizarin Red S staining, micro-computed tomography (micro-CT) imaging, microscopic examination, and Collagen X immunostaining. The focus of these evaluations was limited to the proximal femur including the head, neck, greater trochanter, lesser trochanter, and shaft. Through the multiple methods utilized, it was determined that the femoral neck is the first structure of the proximal femur to ossify, which occurs between PND 15 and 20. The femoral head and greater trochanter begin to separate on PND 15 with separation completing on PND 25 (both still composed of cartilage). Evidence of mineralization and ossification of these structures is apparent at PND 25, with a progressive increase with age, reaching adult-like morphology by PND 40. This work has provided a thorough characterization of postnatal femur development in WH rats to help inform bone and femur effects in juvenile toxicity studies as well as the design of investigative studies.