ELECTRON TOMOGRAPHY OF NEURONAL MITOCHONDRIA - 3-DIMENSIONAL STRUCTURE AND ORGANIZATION OF CRISTAE AND MEMBRANE CONTACTS

The structure of neuronal mitochondria from chick and rat was examined using electron microscope tomography of chemically fixed tissue embed ded in plastic and sliced in similar to 500-nm-thick sections. Three-d imensional reconstructions of representative mitochondria were made fr om single-axis tilt series acquired with an intermediate voltage elect ron microscope (400 kV). The tilt increment was either 1 degrees or 2 degrees ranging from -60 degrees to +60 degrees. The mitochondrial ult rastructure was similar across species and neuronal regions. The outer and inner membranes were each similar to 7 nm thick. The inner bounda ry membrane was found to lie close to the outer membrane, with a total thickness across both membranes of similar to 22 nm. We discovered th at the inner membrane invaginates to form cristae only through narrow, tubular openings, which we call crista junctions. Sometimes the crist ae remain tubular throughout their length, but often multiple tubular cristae merge to form lamellar compartments. Punctate regions, similar to 14 nm in diameter, were observed in which the inner and outer memb ranes appeared in contact (total thickness of both membranes similar t o 14 nm). These contact sites are known to a play a key role in the tr ansport of proteins into the mitochondrion. It has been hypothesized t hat contact sites may be proximal to crista junctions to facilitate tr ansport of proteins destined for the cristae. However, our statistical analyses indicated that contact sites are randomly located with respe ct to these junctions. In addition, a close association was observed b etween endoplasmic reticulum membranes and the outer mitochondrial mem brane, consistent with the reported mechanism of transport of certain lipids into the mitochondrion. (C) 1997 Academic Press.