Ontogenesis of mitochondrial reticulum in rat diaphragm muscle.

Ontogenesis of mitochrondrial reticulum discovered in rat diaphragm has been studied with an electron microscope by the serial section technique. The amount of mitochondrial material in the diaphragm of rat embryos was found to be small. There are rare inclusions of single mitochondria which form columns paralleling the long axis of muscle fiber. After birth, a progressive increase in the amount of mitochondrial material takes place. This process is accompanied by the appearance of new components inherent in the unitary system of mitochondrial reticulum, namely mitochondrial junctions, as well as branches and tubules of mitochondrial material paralleling Z-discs and localized on the levels of the isotropic regions of sarcomeres. The first mitochondrial junctions were found to appear as early as on the second day, and the mitochondrial branches between the second and the ninth days of postnatal development. For a one-month-old rat, a three-dimensional model of a part of mitochondrial reticulum corresponding to one sarcomere was reconstituted. It revealed columns of mitochondrial material crossing the anisotropic region of sarcomere. Such columns connect two networks of mitochondrial profiles localized on the levels of two isotropic regions of sarcomere. Formation of mitochondrial reticulum is completed within two postnatal months.