VARIATIONS IN MITOCHONDRIAL ULTRASTRUCTURE AND DYNAMICS OBSERVED BY HIGH-RESOLUTION SCANNING ELECTRON-MICROSCOPY (HRSEM)

Rat adrenal cortex was processed for high resolution scanning electron microscopy (HRSEM) to confirm tubular cristae, reported by transmissi on electron microscopy to be present in cortex mitochondria. Mitochond ria in several other tissue and cell types were also observed and thei r ultrastructure confirmed by using three-dimensional, stereo, high re solution scanning electron microscopy. The mitochondria in rat and hum an hepatocytes as well as human skin fibroblasts grown in culture cont ained tubular cristae approximately 30 nanometers in diameter. The fib roblast mitochondria proved to be long, up to 46 micrometers and branc hing, as compared to those in liver which were spherical in shape. Col d adapted brown fat cells were packed with mitochondria, these contain ing plate or shelf-like cristae. Branched, rat striated muscle mitocho ndria were observed to curve around contractile protein filament bundl es. The muscle mitochondrial cristae were found to be both tubular and plate-like, within the same mitochondrion. The ratio of tubular crist ae to plate-like cristae varied considerably between muscle mitochondr ia. In order to use ultrastructural changes in mitochondria for differ ential diagnosis, and because 3D reconstruction of mitochondria based on transmission electron microscopy serial sections is severely limite d in resolution, it is imperative to first develop a correct understan ding of tissue specific, normal mitochondrial ultrastructure based on three-dimensional, HRSEM methods.