Photoperiod and gender affect adipose tissue growth and cellularity in juvenile Syrian hamsters

Adult Syrian hamsters are reproductively active and at their annual body and lipid mass nadirs in long 'summer-like' days (LDs), whereas they are reproductively quiescent and at their annual body and lipid mass peaks in short 'winter-like' days (SDs). Because hamsters are born in the SDs of fall in the wild, the development of the reproductive system of juvenile Syrian hamsters exposed to SDs has been studied in the laboratory, but not the development of body and lipid mass. Therefore, we tested the effect of SDs on white adipose tissue (WAT) growth and cellularity (fat cell number, FCN; fat cell volume, FCV) in 3-15-week-old male and female Syrian hamsters. SDs increased body fat in both genders. This effect was partially independent of the decline in gonadal steroids because gonadal regression was only beginning in males (Week 11) and females (Week 15) when carcass lipid content was significantly increased in males, and nearly so in females. This SD-induced increased adiposity was reflected in few regional differences in WAT growth, and then only in males (increased mesenteric and inguinal WAT masses). SDs increased FCV for all non-gonadal WAT and increased FCN only in retroperitoneal WAT. SDs blunted the LD-induced increased FCV and FCN of parametrial and epididymal WAT, respectively. For nearly all conditions and pads, FCV peaked first, followed by increased FCN, the latter accounting for nearly all WAT growth. These data appear to support the view that adipocyte proliferation is stimulated once a 'critical' FCV is reached.