FAT-BODY OF THE FROG RANA-ESCULENTA - AN ULTRASTRUCTURAL-STUDY

In the frog, the fat body is the largest body lipid deposit and is ass ociated with the gonad. The aim of the present work was to investigate the fine structure of the fat body at different periods of the annual cycle and during prolonged starvation. Results indicate that fat body cells of Rana esculenta caught in autumn and after winter hibernation resemble mammalian adipocytes of white adipose tissue and contain mar kers of adipose tissue, such as S-100 protein and lipoproteinlipase. H owever, unlike mammalian adipocytes, fat body adipocytes consistently show small lipid droplets associated with their single, large lipid de posits, a lack of a definite external lamina, and the presence of cell ular prolongations and spicula at their surfaces. Transmission and sca nning electron microscopy in association with lanthanum tracer experim ents suggest that in fat body adipocytes a vesicular-tubular system co nnects the cytoplasm and the interstitial space. In June (i.e., during the reproductive period), fat body adipocytes appear to have lost muc h of their lipid deposit and adjacent adipocytes show interdigitation of their plasma membranes and prominent Golgi complexes. In starved fr ogs, fat body cells can be almost devoid of lipid and in regression to a near-mesenchymal state. Nevertheless, these fat bodies still contai n lipoproteinlipase activity (similar to 45% of that found in lipid-fi lled ones), indicating persistent adipose differentiation of the cells therein. Results presented here show that the R. esculenta fat body i s an adipose organ undergoing reversible extreme changes in adipocyte fat content, which are associated with definite ultrastructural featur es. The fat body represents a suitable model for studying adipose tiss ue under different and extreme physiological conditions. (C) 1996 Wile y-Liss, Inc.