An electron microscope study of the distal segment of the os penis of the rat

The d-segment of the rat penile bone at 14 weeks is composed of an outer zone of atrophy, a middle zone of hypertrophy and an inner zone of ossification. Hypertrophic chondrocytes in the middle zone generally present a few secretory vesicles and a poorly developed Golgi apparatus and rough endoplasmic reticulum, suggesting a serious decrease in the secretion of territorial matrix substances. Instead, most of the cells contain prominent glycogen lakes, lipid droplets and/or fine cytoplasmic filaments. These and other findings indicate that the chondrocytes are in a resting state. In the calcified layer of the zone, and in addition to the hypertrophic or degenerating chondrocytes, another type of cell is recognizable. These sometimes remain viable through the process of cartilage degeneration and may be liberated from the besieging calcified interterritorial matrix and from their own lacuna by chondroclasts. These surviving cells are more similar in ultrastructure to bone cells than to the adjacent chondrocytes, surrounded by the lamina limitans characteristic of the resting osteocytes. However, they are directly covered by a typical territorial matrix inside the lamina limitans and do not extend slender cell processes into the calcified matrix beyond the lamina limitans, and the territorial matrix is never calcified even after calcification of the interterritorial matrix. The cells, therefore, are regarded as belonging to the chondrocytes.