Serial section analysis of mitochondrial form and membrane relationships in the neonatal rat liver cell

Abstract

Serial sections of mitochondria from osmium-fixed neonatal rat liver cells were examined by electron microscopy. In every case, dumbbell-shaped mitochondria in single sections were found to be part of approximately disk-shaped mitochondria with flattened centers. The endoplasmic reticulum closely approximated the flat site, in one case giving rise to a nipple-like protrusion into a fold of the mitochondrion. Electron dense lines could be traced between endoplasmic reticulum and mitochondria, but were not considered to be real connections because they occurred at sites of obvious membrane slope. Connections between inner and outer mitochondrial membranes (membrane reversal) were examined, and all could be explained on the basis of small slopes of mitochondrial membranes through the extent of the section rather than real continuity. Interlacing or fusion of inner mitochondrial membranes in the flat zone could also be explained on the basis of sloped membranes. Apparent location of inner and outer mitochondrial membranes within the mitochondrial matrix was termed the staircase effect; it could be explained by membranes which were alternately sloped and perpendicular to the incident electron beam through the extent of a section.