The morphogenesis of avian tendon

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Abstract
Observations by electron microscopy on thin sections of the metatarsal tendon of embryonic fowls show that in the 8-day embryo the earliest definable collagen fibrils of 80 angstrom in diameter are intimately associated with the cytoplasm of the compact, apparently syncytial, cells of which the tendon rudiment is composed. As development proceeds, some intracytoplasmic groups of fibrils are distinguishable, but intercellular spaces also develop and these gradually become filled with fibrils; finally, bundles are formed and lie packed between the adjacent cells. Soon the extracellular organization predominates until at 20 days the average diameter of the fibrils is 400 angstrom and the normal 640 angstrom periodicity of collagen has been achieved. The morphological features demonstrated have been correlated with histochemical data, and the possible function of the various cellular components in the formation of the intercellular substance has been discussed. By the use of sections in which fibrils have been cut exactly transverse to the bundle axis it has been shown that each fibril is invested by interfibrillar material. As the diameter of the fibrils increases with age the relative volume of interfibrillar material within a bundle diminishes; it is therefore concluded that this material must contain either collagen or the necessary precursors in order to account for the enlargement of the fibrils. Thus the interfibrillar material is of fundamental importance to the formation and growth of the collagen fibrils.