EVIDENCE FOR TRANSFORMATION OF CHONDROCYTES AND SITE-SPECIFIC RESORPTION DURING THE DEGRADATION OF MECKELS CARTILAGE

It is unknown whether cells in the midportion of Meckel's cartilage un dergo transformation into other kinds of cell or whether resorption of cells occurs during development. Therefore, the midportion of Meckel' s cartilage from the mouse and the rat was subdivided into anterior an d posterior portions. The ultimate fates of these tissues were analyze d with a focus on resorption-related cells, death of chondrocytes by a poptosis, and transformation of the chondrocytes themselves. Cellular and extracellular features of mouse Meckel's cartilage were observed a fter von Kossa's staining and staining for acid phosphatase (APase) ac tivity, as well as by light and electron microscopy. To identify resor bing cells, immunostaining specific for macrophages and staining for t artrate-resistant acid phosphatase (TRAP) were performed. The DNA nick end-labeling (TUNEL) method was used for the detection of death of ch ondrocytes by apoptosis. The replacement of the extracellular matrix o f rat Meckel's cartilage was examined with double immunofluorescence s taining for type I and type II collagens. When the anterior midportion from embryonic mice on day 18 was examined after von Kossa's staining , it was clear that the extracellular matrix had already calcified and vascularization had been initiated that reflected the calcified matri x. TRAP staining and immunostaining for macrophages revealed two types of osteoclast and macrophages that were involved in resorption of the matrix. In the posterior midportion, no vascular invasion was evident , and chondrocytes were transformed directly into fibroblastic cells b y phenotypic conversion. In such cells we found reaction products spec ific for APase activity, suggestive of the intracellular degradation o f fine collagenous fibrils. Double immunofluorescence staining showed that cartilage-specific type II collagen was re placed by type I colla gen with the phenotypic transformation to fibroblastic cells. There we re no significant changes in the number of TUNEL-positive apoptotic ce lls from day 17 of gestation to day 6 after parturition. Death of chon drocytes by apoptosis was not, therefore, involved directly in the dis appearance of Meckel's cartilage. These results in the posterior midpo rtion served as an instance of phenotypic switches in differentiated c ells from chondrocytes to fibroblast-like cells. The present study ind icates that there is a difference between the ultimate fate of cells i n the posterior part and that of cells in the anterior part in the mid portion of Meckel's cartilage in the mouse and rat.