Bone Remodelling Sample Clauses

Bone Remodelling. To maintain the structure-function relationship, bone is constantly broken down and rebuilt in a process known as bone remodelling and this process leads to the replacement of between 4 and 10% of bone annually in the adult human [1]. Bone remodelling is a coordinated process known as coupling in which there is localised removal of old bone, (resorption), and the replacement with newly formed bone (formation). There are four types of cell involved in bone remodelling which are responsible for the production, maintenance and resorption of bone: 1. Osteoblasts are mononucleate cells that descend from mesenchymal stem cells in bone marrow [4] and are responsible for the production of the protein matrix of bone which is made up of type 1 collagen and several non-collagenous proteins. This protein matrix is known as osteoid which creates the template for mineralisation and the production of mature bone [1]. 2. Lining cells are flat inactive osteoblasts that form a membrane at the surface of the bone tissue. 3. Osteoclasts are multinucleated cells that originate from monocytes and are responsible for the resorption of calcified bone and cartilage. During the process of bone resorption osteoclasts lie in small pits on the bone surface that are created by the digestion of underling bone. The pits (or Howship’s Lacunae) are formed in a sealing zone from the attachment of the cells plasma membrane to the underlying bone. A ruffled border secretes acid onto the lacunae thus acidifying and aiding dissolution of the mineralised bone matrix. 4. Osteocytes are also from the mesenchymal cell lineage and are the most abundant cellular component of bone. They are mature bone cells and their primary role is the maintenance of cellular activity and the regulation of bone turnover. Osteocytes are thought to be cells that control osteoblastic and osteoclastic activity within a basic multicellular unit (BMU), where bone remodelling occurs [5]. Osteocytes generate an inhibitory signal that is passed through their cell processes to osteoblasts for recruitment to enable bone formation [6]. 1. Activation begins as a result of mechanical or biochemical stimuli from quiescence. Under the influence of parathyroid hormone (PTH), calcitriol, or other bone marrow derived hormonal or cytokine signals osteocytes start to withdraw and swell. This action exposes the thin layer of unmineralised bone matrix or osteoid that encases the surface of bone. Newly activated epithelial or bone lining c...