[The postnatal development of capillaries in the triceps muscle of arm of the white rat. Light and electron microscopic studies].

This paper continues former ultrastructural investigations about fetal development of capillaries in the muscle anlage of the fore limb of white rats. Present investigations deal with postnatal ultrastructural changes of the capillaries of the triceps muscle of rats from the 4th up to the 23rd day p.p. Counting of the capillaries with light and electron microscope reveal a linear increase of the capillary to fiber ratio in this period proportional to the growth of the muscle. Related to a defined area however the number of capillaries remains constantly. Ultrastructural changes of capillaries consist in further flattening of the endothelial profiles, decrease in number or organelles (mitochondria and endoplasmic reticulum), while the number of pinocytotic vesicles remains highmthe endothelial joints become less interdigitated, the basement membrane shows an increase in density leading to a temporary decrease in thickness. Since the 9th postnatal day all of the capillaries including buds closely border upon the muscle fibres. The distance between endothelium and myofiber diminishes up to 0,1 mum.