PLASMA MEMBRANE APPOSED TO MITOCHONDRIA IN THE RAT INTERSCAPULAR BROWN FAT CELLS: A FREEZE-FRACTURE STUDY

Abstract

Close apposition of mitochondria to the plasma membrane in the rat interscapular brown fat cells was demonstrated by freeze-fracture and thin-section electron microscopy. In thin sections, some mitochondria were seen in close proximity to the plasma membrane, separated only by a thin layer of cytoplasm. Caveolae, endo- or exo-cytotic invaginations of plasma membrane, were not seen in such a region of the plasma membrane. In freeze-fracture replicas, the subplasmalemmal mitochondria formed caveola-free hillocks on the P face, and dimples on the E face of the plasma membrane. These caveola-free domains of the cell surface were preferentially located in the vicinity of the blood capillary, and bulged slightly into the pericapillary space. Apposition of mitochondria to the plasma membrane was consistently observed in the brown fat cells at various stages of postnatal development.