The pre- and postnatal development and ageing of interscapular brown adipose tissue in the rat.

Development and ageing changes in interscapular brown adipose tissue have been studied in white Wistar rats by light, fluorescence and electron microscopy. Cellular aggregation was noted in the fetal interscapular area on the 15th day of gestation and vascularised primitive lobules of brown adipose tissue became unequivocally identifiable on the 17th day in utero. Brown adipocyte precursors appeared to be derived directly from mesenchymal cells and were uniquely characterised by larger (0.8-1.7 micrometer diameter) mitochondria. Numbers of these precursors cells (pre-adipocytes) were seen in mitosis during intrauterine life. Pericapillary cells similar in appearance to embryonic pre-adipocytes were regularly observed within brown fat lobules throughout later life. Cardinal features noted in mature brown adipose tissue were parenchymal cells with a multilocular lipid distribution and numerous large mitochondria with distinctive in-parallel cristae, as well as an extensive vascular network and a dense catecholaminergic vasomotor and parenchymal innervation. Brown adipocytes generally retained a multilocular lipid distribution into old age, and although the catecholaminergic fluorescence of the nerves supplying the tissue was reduced, a widespread distribution of noradrenergic vasomotor and parenchymal nerves and of nexuses between brown adipocytes continued to be demonstrable by electron microscopy in the brown adipose tissue of senile rats.