Electron microscopic studies on guinea pig rib cartilage

Summary The guinea-pig rib cartilage consists of chondrocytes dispersed in an intercellular substance composed of collagen fibrils, often characteristically cross-striated, and polygonal granules. Electron-dense membrane-bounded matrix vesicles are also observed intercellularly, especially in the central, partly calcified zone of the cartilage. With respect to their location in a cross-section of the rib, the chondrocytes differ in size, shape and intracellular fine structure. Thus, three separate types of cells are recognized. Peripheral chondrocytes have a flattened shape and are largely occupied by the nucleus. In the cytoplasm, the granular endoplasmic reticulum is the most extensive organelle. Intermediate chondrocytes are oval or round in shape. The endoplasmic reticulum and the Golgi complex are both prominent. Mitochondria and membrane-bounded cytoplasmic dense bodies are more numerous than in the peripheral cells. The ground cytoplasm often contains a few lipid droplets. In the central chondrocytes, such droplets sometimes fill the entire cytoplasm. Concomitantly, the nucleus is usually completely heterochromatic and the cells are therefore regarded as being metabolically inert.After preparations including ruthenium red staining en bloc, the general stainability of the chondrocytes is decreased. Intracellularly, positive ruthenium red staining of granular material within the Golgi vacuoles are to be observed. Extracellularly, the matrix granules are stained with this polyvalent, cationic dye. Extraction of the cartilage with 4 M guanidine-HCl removes all matrix granules and about 70% of the proteoglycans, measured as hexosamine, from the tissue. It is concluded that the matrix granules contain proteoglycan complexes.Financial support was received from the Swedish Medical Research Council (proj. no. 12X-3355), the Swedish Cancer Society (proj. no. 100-K71-05XK), the King Gustaf V 80th Birthday Fund, the Harald and Greta Jeansson Foundation, the C. B. Nathhorst Foundation, and from the funds of Karolinska Institutet.The skilled technical assistance of Mrs. Eva Lundberg and the secretarial assistance of Mrs. Inger Åhrén are gratefully acknowledged. The authors are indebted to Dr. S. Lohmander for helpful suggestions during the progress of the work.