Diameters of the circular profiles of spherical mitochondria in parenchymal cells of the zona fasciculata in rat adrenal cortex were measured for intact controls and for the regenerating adrenal cortex on electron micrographs recorded at random . The diameter data were then processed by Bach's method which deals with the sphere size distribution . The structural parameters of the mitochondria were computed with the aid of an electronic computer . The total number of mitochondria in all the parenchymal cells of the zona fasciculata were calculated . The surface area of the inner mitochondrial membrane was then determined stereologically . Biochemical parameters were obtained for the protein, the phospholipid, and the cytochrome P-450 content, per averaged mitochondrion . The number of cytochrome P-450 molecules contained in the inner membrane was determined in terms of the unit surface area and of the unit amount of phospholipid . These correlated biochemical and stereological parameters have led to the following conclusions . (a) The genesis of the mitochondria after the adrenal enucleation is almost completed within 10 days . (b) During the period of mitochondrial proliferation, the mitochondria are small in size and also immature both in the structure and in the function of their inner membrane . (c) These small and immature mitochondria grow through an increase of the phospholipid and protein, and this increase is accompanied by expansion of the area of the membrane surface . (d) An enrichment of the inner membrane with cytochrome P-450 molecules occurs, thus indicating the differentiation of adrenocortical mitochondria . The process of membrane differentiation is not tightly coupled with that of membrane growth.THE JOURNAL OF CELL BIOLOGY • VOLUME 52, 1972 . pages 5 03 -513 investigators focussed attention on the histograms of the mitochondrial frequencies which were grouped as a function of the profile diameter . The histograms seem to indicate that at the fifth and also at the 10th day in tissue regeneration, most mitochondria were small in size, but at the 20th
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