Figure 1. I) Osteoblasts and chondroblasts are derived from a common precursor, the mesenchymal stem cell.

Figure 1

I) Osteoblasts and chondroblasts are derived from a common precursor, the mesenchymal stem cell. Osteoblasts differentiate to a committed preosteoblast and then on to a matrix synthesizing osteoblast. When bone formation switches off, osteoblasts can revert to a bone lining cell, a state in which they do not synthesize matrix. This is a reversible process and bone lining cells can rapidly be activated to become osteoblasts again. During bone formation a small number of osteoblasts become irreversibly embedded within the bone matrix. These cells do not die, but live on as osteocytes, connected with each other and with osteoblast on the bone surface. Chondroblasts rapidly proliferate during bone growth. When they start to generate cartilage matrix they are called chondrocytes. II) During embryogenesis two different types of bone formation take place. In the flat bones of the skull, pelvis and scapula intramembranous ossification occurs. Here, mesenchymal cells (MS) condense to form a fibrous plate which directly mineralises. Osteoblasts form from the mesenchymal stem cells and osteoclasts (OC) form from precursors delivered via the blood supply. Together they form and model the bone during growth. III) Long bones, such as femur, humerus and tibia, are formed by endochondral ossification. This means that first a cartilaginous model of the future bone is created. Cartilage cells form an unmineralised “bone anlage”, which is subsequently mineralised by hypertrophic chondrocytes (HC). This mineralized cartilage attracts blood vessels and they bring hemopoietic precursors and mesenchymal stem cells into the primitive bone marrow cavity (BM). Osteoclasts form and begin to resorb the mineralised cartilage, which acts as a template for osteoblasts, formed from the mesenchymal stem cells, that start to lay down bone matrix. With time a secondary ossification center forms above the growth plate. Articular cartilage (C) remains around the ends of the bone for smooth movement in joints. IV: Mature bone is constantly “remodelled” (= renewed) to repair microfractures and also to release calcium during normal homeostasis. To illustrate the different processes, a piece of bone in which pathological remodelling is taking place is shown. Under normal conditions it would be rare to encounter three osteoclasts in one section of adult bone. The image on the right illustrates the histological appearance of the bone and on the left the different cell types are shown in their relative position to each other. Bone formation follows bone resorption and inactive surfaces (i.e., where neither formation nor resorption takes place) are covered by bone lining cells. Osteoclasts and osteoblast are constantly differentiating from precursors present in the bone marrow (BM). The cell types are depicted as in the lineage diagram in I.

From: Integrins and Development: Integrins in Skeletal Cell Function and Development

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