Growth Patterns and Body Composition evaluation in C57Bl/6J Mouse Strain by Dual Energy X-Ray Absorptiometry

Aim of the study: Growth curve models are routinely used in biomedical research to better understand overall development of the body and its components, and the pathogenetic mechanisms underlying growth and metabolism disorders. Wild type or genetically engineered C57BL/6J mouse strain is widely used in a great variety of research fields on growth and metabolism. The objectives of this study were to investigate, in a noninvasive and longitudinal way, the growth pattern and changes in body composition in this mouse strain. Materials and Methods: Body weight, surface area and composition were obtained from four (n. 20), eight (n. 20), twelve, sixteen and twenty (n.5) weeks aged female mice. Relative rate of growth between 4 vs 8 and 8 vs 20 weeks of age were defined as :RG= 100[(W2- W1)/ (t2-t1)]/ [(W2- W1)/ 2]. Dual energy x-ray absorptiometry (DEXA) imaging was used to determine in vivo lean, fat, bone mineral concentration (grams) and density (grams/cm2) in the same subject over time. Body lean, fat and mineral bone content values were normalized for body surface area (BS, in m2), according to the DuBois equation: BS (m2)=0.007184 x body weight (kg) 0.425x body length (cm) 0.725 1, to obtain the correct projected areal bone mineral density (BMD g/cm2 * m), lean mass index (LMI) and fat mass index (FMI). The small animal densitometer consists of a cone beam x-ray source generating energies of 35 and 80 keV, a flat 100×80 mm detector, and a high spatial resolution (0.18 × 0.18 pixel size), allowing accurate measurements.2, 3 DEXA acquisition takes approximately 5 minutes. All parameters were presented as mean ± standard deviation. Results : Body weight and surface area increased linearly from four to twenty weeks of age (weight range: 16,1 ± 0,6 - 23,4 ± 0,8 g; surface area interval: 51,3 ± 0,07 - 65,4 ± 0,09 cm2). Rate of growth increased by 50% between 4 and 8 weeks old mice, whereas grew only the 16,5% from 8 to 20 weeks of age. Bone mineral density peak was reached at sixteen weeks of age ( 0,68 ± 0,02 g/cm2 * m). Lean and fat mass showed inversely correlated changes describing a sigmoid curve, with an evident gain in body protein at eight (0, 278 ± 0,01 g/ cm2) and twelve weeks (0,281 ± 0,01 g/ cm2) of age, and a slow prevalence of fat mass increase from twelve (0,043 ± 0,001 g/ cm2) to twenty (0,060 ± 0,0015g/ cm2) weeks of age. Conclusions: Dual energy X-ray absorptiometry (DEXA) has been utilized in a variety of clinical and research applications in the fields of nutrition, metabolism and bone physiology. This study was designed to form the baseline for future comparison of wild-type animals with those in which altered skeletal growth, body composition and metabolism have been induced by surgical, pharmacological or genetic manipulation. DEXA appears a useful technique that permits longitudinal measurements of body composition, according to the principle of "refinement, reduction, and replacement", and improving the accuracy of experimental results. Bibliography 1. DuBois D; DuBois EF: A formula to estimate the approximate surface area if height an weight be known. Arch Int Med 1916 17:863-71. 2.Brommage R. Validation and calibration of DXA body composition in mice. Am J Physiol Endocrinol Metab 2003;285:454-459. 3.Nagy TR, Clair AL. Precision and accuracy of dual-energy X-ray absorptiometry for determining in vivo body composition of mice. Obes Res. 2000;392-398.