[Light and electron microscopic studies of the skeletal muscle (M. triceps brachii) capillaries in white rats aged 2 to 20 months].

The capillarisation of the triceps muscle of white rats was investigated with light and electron microscope up to the age of 20th month. During the postnatal development the capillary to fiber ratio corresponding to the increase of the number of capillaries is well adapted to an exponential function. The rapid increase of thickness of the muscle fibers causes a decrease of the number of capillaries per area unit. After the third week of life with histochemical methods the differentiation of muscle fiber types is possible, well correlated to the beginning of motoric activity. Ultrastructural alterations of the muscle capillaries consist in a flattening of the endothelium within the first month of postnatal life, a decrease of the content of organelles, a high degree of vesiculation and a continuous thickining of the basement membrane up to more than 100 nm. The interendothelial clefts show a simple structure containing 1--2 junctional zones and only few dilatations.