Bone Introduction Course Materials

Bone Introduction   Main functions of bone include: o Mineral homeostasis e.g. calcium/phosphate o Haematopoiesis o Mechanical support for movement o Protection of viscera The regions of the bone include: o Diaphysis  The main or mid-section (shaft) of a long bone, it is made up of the cortical bone and usually contains the bone marrow and some adipose tissue (fat) o Metaphysis  The wider portion of a long bone adjacent to the epiphyseal plate / the junction between the shaft and the end – epiphyseal plates ("growth plates") are zones of cartilage located in the metaphysis + responsible for growth in the length of the bone (endochondral ossification) o Epiphysis  the rounded end of a long bone, at its joint with the adjacent bones  Bones are comprised of periosteum (outer fibrous layer), cortex (a thick outer layer of compact bone) and medulla (inner core – cancellous (spongy) bone: trabeculae separated by marrow of fat / haemopoietic tissue) o Compact (cortical) bone is very dense and solid, and it is always found on the surface of the bone. It forms the diaphysis of long bone and outer shell of flat bone, and is orderly and layered. It has mainly mechanical (strength) and protective functions (resisting forces) The basic functional unit of compact bone is the osteon , or Haversion System, containing osteocytes arranged in concentric layers (lamellae) around a central canal, containing one or more blood vessels and running parallel to the bone surface. o Spongy (cancellous) bone has a relatively porous structure, and it is found in the interior of bones. It is located at end of long bone (epiphysis) and inside cortex of flat bone. It consists of a network of interconnecting trabecular plates and rods, and is the major site of bone remodelling and mineral homeostasis o Note: the relative amounts of compact and spongy bone are related to the function of the bone. Compact bone is thickest where stresses arrive from a limited range of directions. Spongy bone is located where bones are not heavily stressed or where stresses arrive from many directions o Note: the endosteum lines the marrow cavity, trabeculae of the spongy bone, and the inner surfaces of the central canal.   Bone composition is made up of two parts – bone matrix and bone cells o Bone matrix is a specialised type of connective tissue; it is an organic matrix of collagen (osteoid), in which the inorganic component calcium salt (hydroxyapatite) is deposited. Overall it brings strength and rigidity to the structure. o Bone cells  Osteoblasts (immature cells of bone) cover the surface of the bone and are the most predominant cells. They produce collagen and other elements of bone matrix and are thus involved in mineralisation/calcification. They have receptors for hormones (PTH, vit D) and cytokines & GFs  Osteocytes (mature cells of bone) are derived from osteoblasts entrapped within newly formed bone matrix. They communicate via cell cytoplasmic processes. Osteocytes help to control calcium and phosphate levels in the microenvironment, and detect mechanical forces and translate them into biologic activity – a process called mechanotransduction  Osteoclasts are multinucleated giant cells that are derived from the bone marrow. They have a limited life span and essentially reabsorb bone matrix in depressions on bone surface (responsible for the removal/maintenance of bone) The proteins of bone include type 1 collagen and many noncollagenous proteins derived mainly from osteoblasts. Osteoblasts deposit collagen either in a random weave known as woven bone or in an orderly layered manner designated lamellar bone. Normally, woven bone is seen in sites of rapid bone formation such as the fetal skeleton and the base of growth plates. It is produced quickly and resists forces equally from all directions. The  presence of woven bone in the adult is always abnormal; however, it is not diagnostic of a particular disease. We will see examples of this later in this chapter. Lamellar bone, which gradually replaces woven bone during growth, is deposited much more slowly and is stronger than woven bone. Bone resorption/formation = balance between RANKL/OPG activity o The diagram below effectively shows how RANKL & M-CSF favour osteoclast differentiation & bone reabsorption, while OPG (osteoprotegrin) blocks RANKL/ RANK interaction