Ultrastructural evidences of a distinct axial domain within native rat tail tendon collagen fibrils

The structure of collagen fibrils of tail tendon in young and mature rats was investigated by transmission (TEM) electron microscopy after cytochemical methods associated with special procedures of dehydration and fixation. All the collagen fibrils show a central condensed material of 4 nm in diameter which remains preserved even when fibrils have been swollen and partly disorganized by the ethyl glycol dehydration. A cytochemical characterization using PTA and TCH treatments strongly suggests the glycoprotein nature of this central compact material and reveals the heterogeneous structure of each fibril. The first morphological evidence that the construction of the collagen fibrils must take place around a condensed material which serves as a scaffolding is obtained. These observations are in agreement with the now well established existence of heterotypic collagen fibrils in many extracellular matrices.