The cast of clasts: catabolism and vascular invasion during bone growth, repair, and disease by osteoclasts, chondroclasts, and septoclasts
Three named cell types degrade and remove skeletal tissues during growth, repair, or disease: osteoclasts, chondroclasts, and septoclasts. A fourth type, unnamed and less understood, removes nonmineralized cartilage during development of secondary ossification centers. "Osteoclasts," best known and studied, are polykaryons formed by fusion of monocyte precursors under the influence of colony stimulating factor 1 (CSF)-1 (M-CSF) and RANKL. They resorb bone during growth, remodeling, repair, and disease. "Chondroclasts," originally described as highly similar in cytological detail to osteoclasts, reside on and degrade mineralized cartilage. They may be identical to osteoclasts since to date there are no distinguishing markers for them. Because osteoclasts also consume cartilage cores along with bone during growth, the term "chondroclast" might best be reserved for cells attached only to cartilage. "Septoclasts" are less studied and appreciated. They are mononuclear perivascular cells rich in cathepsin B. They extend a cytoplasmic projection with a ruffled membrane and degrade the last transverse septum of hypertrophic cartilage in the growth plate, permitting capillaries to bud into it. To do this, antiangiogenic signals in cartilage must give way to vascular trophic factors, mainly vascular endothelial growth factor (VEGF). The final cell type excavates cartilage canals for vascular invasion of articular cartilage during development of secondary ossification centers. The "clasts" are considered in the context of fracture repair and diseases such as arthritis and tumor metastasis. Many observations support an essential role for hypertrophic chondrocytes in recruiting septoclasts and osteoclasts/chondroclasts by supplying VEGF and RANKL. The intimate relationship between blood vessels and skeletal turnover and repair is also examined.