A cell is basic building unit of any tissue or organ. Like other organs bone is also made of these smallest units which are arranged to form the structure we see outwardly.
Following types of cells are found in the bone. Different types of cells are programmed for different kinds of functions.
The Osteoblast
Osteoblast are cuboidal mononuclear cells 15-30 µm in length, which form a contiguous monolayer over the surface of bone. They are derived from marrow stromal cells by differentiation of pre-osteoblasts and not themselves undergo mitosis.
The single nucleus is eccentrically placed and the abundant rough endoplasmic reticulum is characteristic of a cell engaged in protein synthesis. There is also a high level of alkaline phosphatase. Vesicles containing amorphous calcium phosphate may also be present.
The cells are connected together and to the processes of subjacent osteocytes by gap junctions. Inactive bone surfaces are lined by a monolayer of flattened cells which histological studies suggest are resting osteoblasts.
The osteoblast is responsible for the synthesis of the major proteins of bone including type I collagen and the non-collagenous proteins of bone such as osteocalcin (bone Gla protein) and osteonectin. The cell is also involved in the mineralization of bone and produces the enzyme alkaline phosphatase, which may be important for this.
Recent evidence suggests that the oteoblast plays a central role in controlling osteoclastic function. Thus osteoblasts and not osteoclasts have specific surface receptors for agents which stimulate bone resorption, such as 1,25-dihydroxy vitamin D3 and parathyroid hormone. Furthermore, osteoclasts in culture, which are therefore not in contact with osteoblasts, do not respond to these agents.
The mechanism by which the osteoblast signals the osteoclast to resorb is unclear. There is evidence that the osteoblast secrets soluble substances which include polypeptides and postaglandins which activate or inhibit osteoclastic activity. The osteoblasts may also cause a conformational change in the lining cells which cover the whole bone surface which makes the bone physically available to the osteoclast for resorption.
Collagenase secreted from the osteoblast may also play a role either by physically preparing the bone surface for attachment of the osteoclast or by allowing releases of an osteoclastic activating or chemotactic factor which is secreted within the bone, such as transforming growth factor beta. Since the coupling of osteoblastic and osteoclastic activity is of fundamental importance to this survival of the organism, it would not be surprising if it was mediated by more than one mechanism.
The Lining Cell And The Osteocyte
Some active osteoblasts are incarcerated in the matrix which they secrete and become buried deep within the bone. These osteocytes long processes through which they maintain contact with each other and with the superficial bone-lining cells.
This layer of flattened, inactive, cells, which are probably also derived from osteoblasts, covers the whole of the bone surface, functionally isolating it from the exterior. It is thought that this cell system may mediate a rapid calcium flux between bnoe and extracellular fluid, although the precise role of this in bone mineral homeostasis is unclear.
The Osteoclast
The osteoclast is a very large multinucleate giant cell attached to the bone surface. It has 15-20 nuclei and may measure several hundreds of micrometers across. It is the cell responsible for the resorption of living bone and it is formed by fusion of mononuclear cells.
Osteoclasts are derived from haemopoietic precursor cells, probably a circulating monocyte of the macrophage type. Evidence for this lineage has come from a variety of sources including studies on osteopetrosis, which is characterized by defective osteoclastic bone resorption.
Morphologically the nuclei are frequently indented with prominent nucleoli. Mitochondria are numerous, there is little rough endoplasmic reticulum and many lysosomes.
An important feature is the area of infolded plasma membrane, termed the ruffled border, which is surrounded by an organelle-free clear zone through which the osteoclast attaches to bone and which is the site of bone resorption. Osteoclasts contain the characteristic enzymes tartrate-resistant acid phosphatase (TRAP) and carbonic anhydrase.
The biochemical mechanisms involved in osteoclastic bone resorption have not been fully elucidated but it occurs in the enclosed space beneath the ruffled border enclosed by the clear zone. In order to create this enclosed space, the osteoclast attaches to bone firmly through special attachment proteins called ‘integrins’.
Carbonic Anhydrase catalyses the hydration of dissolved carbon dioxide and the carbonic acid so formed dissociates into bicarbonate and hydrogen ions, which are pumped through the ruffled border, so lowering the pH of the extracellular region within the clear zone.
The bone mineral is probably dissolved in the acid environment. Osteoclast do not produce collagenase (which is, in fact, synthesized by osteoblasts). They probably break down bone matrix by release of lysosomal proteases.
Other Cells
Other cells are described, including reticular cell which are found within the mesh-like storma of the bone marrow, and possess both osteogenic and haemopoietic potencies; endosteal cell which may be connected tissue cells or resting osteoblasts and fibroblasts with a basophilic cytoplasm and round nucleus.
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