Embryonic development of rat diaphragm. An electron-microscopic study.

Ultrastructural aspects of white Wistar rats diaphragm during part of its embryonic development (from the 13th embryonic day till birth) have been studied. The dominating structures observed in the period of the thirteenth embryonic day (ED 13) are undifferentiated cells, their cytoplasm being poor in organelles but rich in ribosomes. The close examination of these cells reveals that some of them possess a kind of thin filaments near their Golgi zones. At ED 14-15 clusters of myoblasts are readily detected (their cytoplasm containing a lot of glycogen granules and myofibrils, some of them even with Z-line material); contact sites between their cell membranes appear, somewhere forming specialized junctions; in this period the myoblasts start to fuse giving rise to the primary generation of myotubes. At ED 16-17 the quantity of myofilaments and glycogen granules increases alongside with the initiation of a basal-lamina formation; occasionally some oval, undifferentiated cells very similar to those viewed at ED 13 are found. At ED 18-19 the cytoplasm of the myotubes contains a lot of myofibrils and a well-developed endoplasmic reticulum; at some places the adjacent membranes still form deep interdigitations. At the end of the prenatal myogenesis (ED 20-21) most of the muscle cells are close to their mature morphological appearance--the sarcomers are well-organized and some nuclei present a peripheral localization; nevertheless, in this period new generations of myotubes can be also distinguished.

[Ultrastructural changes in the connective tissue of skeletal muscles following denervation]. / Ul'trastrukturnye izmeneniia soedinitel'noi tkani myshts skeleta posle denervatsii

The ultrastructural changes found in the endomysium of rats after denervation of the diaphragm and m. plantaris were studied. Within the first week after crossing the peripheral nerve in the nedomysium there appeared an increased amount of neutrophils and monocytes as well as phagocytic material in the cytoplasm of histocytes. Activization of the cytoplasm of fibroblasts which manifested itself in the appearance of numerous vesicles and multiple free and bound ribosomes was detected by the end of the second and the beginning of the third weeks after denervation. At the same period eosinophils invaded the endomysium and became closely surrounded by numerous collagenic fibres. After reparation of neuromuscular synapses these changes disappeared. On the basis of these results and others founded in previous studies of denervated and reinnervated skeletal muscles the authors consider these changes in the endomysium appearing under the above experimental conditions to be manifestations of metabolic interrelations between the endomysium connective tissue and muscle fibres.