The cartilage at the distal end of the femur in newborn infants is divided into two structurally different units.
The matrix at the articular surface contains bundles of fibrils which are 250 to 500 Angstrom units in diameter and have the periodic structure of collagen. They end abruptly on the joint surface and do not appear to be covered by a membrane of acellular or cellular matter.
In the matrix of the remainder of the cartilage the fibrils are much less densely packed and show slight tendency to form fiber bundles. In this loose network almost all of the fibrils are less than 250 Angstrom units in diameter. In these fibrils no periodic structure has been demonstrated regardless of the staining method employed with both phosphotungstic and osmic acids.
Occasional large fibril-like structures without evident periodic banding found in the matrix of the proliferative zone, circular structures in the zone of degenerating cartilage cells, and the stellate prolongations of the periphery of some cartilage cells out to the matrix only suggest the possibility of protoplasmic extensions of the cells into the matrix.