Ultrastructural and cytochemical studies on the remodelling of the tracheal cartilage

Ultrastructural and cytochemical studies on the remodelling of the rat tracheal cartilage have been carried out. The thickness of the tracheal cartilage was constant, during the observation periods (1 to 54 days after birth). The external perichondrium of the tracheal cartilage consisted of active fibroblasts and intercellular fibrils. The inner part of this perichondrium was a chondrogenic layer, where appositional growth was taking place. On the other hand, the internal perichondrium contained fibroblast-like cells, which were nearly twice as large as the external perichondrial fibroblasts in size and were arranged in three or four layers. The cells had well developed organella and large vacuoles which contained numerous fragments of fibrils and/or glycosaminoglycan. Many cytoplasmic processes protruded to the cartilage matrix, where the intercellular fibrils were particularly irregular in arrangement. Some vacuoles included collagen fibrils. Based on an intense acid phosphatase activity in these vacuoles and other findings, the fibrils were thought to be phagocytosed collagen of the cartilage matrix. An extensive alkaline phosphatase activity was demonstrated on the plasma membrane of fibroblasts and chondroblasts in the external perichondrium. The present investigation revealed distinct functional difference between the external and internal perichondrium of the tracheal cartilage. It is resorbed at the internal perichondrium, while it appositionally grows at the external perichondrium. The fibroblast-like cells of the internal perichondrium play an essential role in resorption of the matrix in cartilage remodelling.