Collagen from osteoblasts

Figure 48-11. Schematic illustration of the major cells present in membranous bone. Osteoblasts (lighter color) are synthesizing type I collagen, which forms a matrix that traps cells. As this occurs, osteoblasts gradually differentiate to become osteo-cytes. (Reproduced, with permission, from Junqueira LC, Carneiro J Basic Histology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

Fig. 1.6 (A and B) Scanning electron micro- implantation in the bone marrow showing for-graphs of the porous hydroxyapatite-collagen mation of new bone (white asterisk) attached nanocomposite scaffolds at different magnifi- directly to the nanocomposite (asterisk). Arrows cations. Arrowheads in B indicate the hydroxy- indicate cuboidal osteoblasts on the surface of apatite nanocrystals on the collagen fibrils. new bone. Adapted from [94], reproduced by Histology at (C) 1 week and (D) 4 weeks after permission of Wiley-VCH.

The principal cells in bone are the osteoclasts and osteoblasts. Osteoclasts, the cells responsible for resorption of bone, are derived from hematopoietic stem cells. Osteoblasts are derived from local mesenchymal cells. They are the pivotal bone cell, responsible for bone formation. Skeletal tissues are remodelled throughout a lifetime, alternating resorption phases by osteoclasts with periods of intense collagen synthesis. This balance is under the control of mechanical and hormonal stimuli, which ensure the appropriate performance of the bone. Skeletal tissues have three... 

The various cell types are distributed differently within various types of connective tissue fibroblasts often found lying close to collagen fibres, whilst macrophages and mast cells usually appear in clusters around blood vessels. Additionally, whereas fibroblasts are widespread, macrophages are absent from some types of connective tissue, tendons for example. Specialized cells also occur within particular locations for example osteoblasts and osteoclasts (bone), chondrocytes (cartilage), adipocytes (fat tissue) and haemopoietic cells (bone marrow). 

Osteoclasts are multinucleated cells found on the endosteal surface of bone, in Haversian systems and periosteal surfaces. PTH activates osteoclasts (indirectly via osteoblasts that possess PTH receptors). Calcitonin is a potent inhibitor of osteoclast activity. Local cytokine factors, including interleukin-1 (IL-1), tumour-necrosis factor (TNF), TGF- 0 and interferon-y (INF-y), are important regulators. Osteoclast resorption of bone releases collagen peptides, pyridinoline cross-links and calcium from the bone matrix, through the action of lysosomal enzymes (collagenases and cathepsins). The collagen breakdown products in serum and urine (e.g. hydroxyproline) can be used as biochemical markers. 

Bone tissue also contains osteoclasts, which are multinucleated cells that are derived from the hematopoietic (granulocyte-monocyte) cell line located in bone marrow. Osteoclasts are primarily responsible for bone resorption and they secrete enzymes and hydrochloric acid that break down collagen matrix and help dissolve the bone. The area where osteoclast cell membrane lies adjacent to bony tissue is known as a Howship s lacunae. The osteoclast cell membrane that lies in close proximity to bone can contain numerous villous extensions and form a ruffled border. These areas are also known as resorption pits. The plasmalemma border of the osteoclast cell in this region forms a specialized seal with the underlying bone to prevent the release of enzymes and hydrochloric acid. This process also results in the release of growth factors previously deposited in bone by osteoblasts, which are responsible for maintaining the process of regenerating new bone. 

Bone is synthesized by osteoblasts which transport calcium ions from blood into a secreted, uncalcified osteoid matrix composed mostly of type I collagen. During calcification, monocyte-like cells are attracted out of the adjacent blood capillaries and adhere to irregularities in the calcifying bone surface and eventually become osteoclasts. These cells, which resorb the bone, develop according to genetic and environmental stimuli that determine bone shape and response to stress. 

Skeletal tissue mineralization (bone formation) is initiated by osteoblasts, which secrete the osteoid matrix (Fig. 9.4). They express type I procollagen in secretory vesicles together with matrix vesicles that pinch off from the membrane. The matrix vesicles are pushed away from the cell surface, possibly by the flow of fluid containing calcium and phosphate ions that are also transported through the cell from the extracellular fluid on the outer surface. Collagen fibers develop further away from the cell surface than from fibroblasts. 

---