Ultrastructure of the collagen fibril. I. Some features of the structure of the collagen fibril.

In the human ovary, thyroid gland and in the rat tail tendon the plasma membrane of fibroblasts may "disappear" or "dissolve", leading to a direct contact of the cytoplasm with extracellular space. Extracellular filaments and collagen fibrils may grow directly out of the cytoplasm and from extracellular vesicles and cellular remnants. The filaments are complex structures composed of 3 to 5 subfilaments. Adjacent collagen fibrils are connected with interfibrillar bridges. In the rat tail tendon 2 different types and opposite orientated fibrils are found. After enzymatic treatment it becomes clear that the collagen fibril has a tridimensional organization and is composed of filaments and amorphous cementing matrix. The matrix is easily affected by various enzymes and washed away, revealing thus the inner spiral structure of the fibril.