Electron microscopic observations on the fate of hypertrophic chondrocytes in condylar cartilage of rat mandible

The present study focused on the hypertrophic cell zone and the adjacent region of primary spongiosa in the mandibular condylar cartilage in growing rats (3 to 7 weeks old). In this cartilage, chondrocytes were not arranged in columns, and there was no clear distinction between longitudinal and transverse septum. The hypertrophic chondrocytes were not surrounded entirely by calcified matrix, and capillaries were in close contact with cartilage cells. The staining intensity of the pericellular matrix decreased in the lower hypertrophic cell zone in comparison with that in the upper part of the hypertrophic cell zone. Electron microscopic examinations indicated that the lowest hypertrophic cells contained lysosomes and pinocytotic vesicles. Some hypertrophic chondrocytes appeared to have been released from their lacunae and were observed in the region of the primary spongiosa. Hence it is suggested that the lowest hypertrophic chondrocytes in the rat mandibular condyle do not die but are released from their lacunae into the bone marrow. Further study is needed to determine whether or not these cells do indeed become osteoblasts and/or chondroclasts.