The structure of rat tail tendon.

The structure of rat tail tendon and its associated sheaths is examined. A model for the structure is presented. The epitenon and endotenon sheaths are identified by their criss-cross pattern of crimped collagen fibers. The multi-layered paratenon sheath consists of very fine collagen fibrils with a small crimp size. The paratenon collagen is aligned with the long axis of the tendon. The tendon is subdivided by the endotenon into units, delineated by the parietal paratenon, comprising one or more fascicles. Each fascicle is delineated by the visceral paratenon. A fascicle is composed of collagen fibrils incompletely subdivided by fibroblasts into 'fibers'. These collagen fibrils have a relatively large crimped appearance. The surface crimp of the fascicle is masked by the innermost layers of the visceral paratenon.