Postnatal development of structure and arrangement of tendon cells. A scanning and transmission electron microscope study in the rat calcaneal tendon.

The postnatal development of the three-dimensional structure and arrangement of the tendon cells in the calcaneal tendon of rats from 5 to 90 days of age were examined under the scanning and the transmission electron microscope (SEM and TEM). Exposed tendon cells were seen as winged bricks with a stellate profile in a cross section. Their slender perikarya were stacked successively in rows along the long axis of the tendon. The plate-like cytoplasmic processes, which were oriented along the perikaryal long axis, extended radially and joined those of the neighboring cells in rows. Therefore, the tubular channels for collagen fascicles were lined with the chained perikarya and their processes of the tendon cells. The cell rows were usually composed of many cells in developing stages, while short rows of one or two cells were observed in the fully developed stage. The cytoplasmic processes were the primary processes in the lateral extension, and their fine branches formed the secondary processes. The secondary processes were numerous at younger stages, showing a fine meshwork due to their mutual joining in the cross sections. In advanced stages, the meshes were coarse and the secondary processes were also perforated either with grouped fenestrations or large pores. In the fully developed stage, the secondary processes were fragmented on the perikarya, while the primary processes extended in the thin delicate sheets with large perforations. The findings in the present study suggest that the tendon cells are arranged in a three-dimensional network by their mutual joining. The tendon cells of the rat calcaneal tendon may not proliferate very much after birth, but do expand their nursing area in line with normal growth by an elongation of the main primary processes and a reduction of the secondary processes and perikaryal mass. The interfascicular clefts, which were caused by an intervention of either the processes at any developmental stages or the fragmented processes at a certain level of the tendon, may also play a role in the passage of tissue fluid.