The three-dimensional ultrastructure of human tendons has been studied. Epitenon and peritenon consis of a dense network of longitudinal, oblique and transversal collagen fibrils crossing the tendon fibres. The interna structure of tendon fibres is also complex. The collagen fibrils are oriented not only longitudinally but also transversel· and horizontally. The longitudinal fibrils do not run only parallel but also cross each other forming spirals (plaits) These fibril bundles are bound together by a three-dimensional collagen fibril network of endotenon. In the myotendi nous junction the surface of the muscle cells form processes. A network of tendineal collagen fibrils fills the recesses between the muscle cell processes penetrating the basement membrane of these processes. This complex ultrastructur < of human tendons most likely offers a good buffer system against longitudinal, transversal, horizontal as well as rotational forces during movement and activity.

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The development of tendolipomatosis has been studied both by light and scanning electron microscopy in the tendons of healthy persons killed in accidents. Lipid cells are rarely seen in the intact tendon. In the early stages of tendolipomatosis lipid cells are found deep and singly but in the later phases lipocytes are seen to disrupt the continuity of collagen fibres and tendon bundles and so reduce the strength of the tendon.

Alterations in the capillary density of the M. biceps brachii was studied in 20 cases during 2 months after spontaneous rupture of the tendon. The fiber typing was done using succinic dehydrogenase and myofibrillar ATP-ase at pH 9.4 for the staining, the capillaries were visualized with alkaline phosphatase and PAS reactions, and in each case 1,000 muscle fibers with their capillaries were estimated. In the intact muscles the capillary count ranged from 1,500 to 1,910 (mean 1,740) per 1,000 fibers. During the 1st month following the rupture the decrease in the capillary count of the muscle occurred rapidly, being after 4-7, 8-10, and 18-29 days 36%, 52%, and 66%, respectively, as compared to the contralateral muscles. After 2 months a decrease of approx. 70% was found in the amount of capillaries compared with the contralateral muscles. The obliteration of the capillaries was particularly pronounced in the amount of anastomoses. These results indicate that the capillary supply of the human M. biceps brachii decreases enormously during the 1st month after a tenotomy.

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