The fine structure of developing cartilage in the chick embryo

The fine structure of developing cartilage is described in the area of the future vertebral bodies in the cervical region of chick embryos. Incubation ages range from 36 hours, when the sclerotome of the somite begins to break up, to the thirteenth day. In the 36‐ to 72‐hour interval cells of the sclerotome become free and assume stellate form. Polyribosomes nearly fill the largely undifferentiated cytoplasm. Microfibrils and amorphous material appear in the connective tissue space The extent of the connective tissue space is determined by boundary (basement) membranes; externally by those of epithelium facing the geometric exterior of the organism and internally by those of endothelium, mesothelium, muscle, nerve and fat. The true contents of the connective tissue space are the connective tissues. As used in this paper “connective tissue space” refers to the extracellular portion, in which the fibrous and amorphous components exist. by the end of the third day. From the fourth through the sixth day both cellular contours and contents suggest fibroblasts. Microfibrillar diameter increases from 50 to 150 Å. Periodicity does not occur. Matrix granules appear and establish contact with microfibrils. From the eighth through the eleventh day the predominating cell types are chondroblasts and chondrocytes whose fine structure is typical. Microfibrils become more numerous and matrix granules larger. During days 11 through 13, chondrocytes show degenerative changes including lucid areas of nucleoplasm and cytoplasm with fragmentation of endoplasmic reticulum. Microfibrils and matrix granules are heavily concentrated in the connective tissue space and lucent osteoid appears.