BIO241 Lecture 3 (2) - Bone Structure & Function PDF

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1 Bones can be classified according to the shape into four groups: 1- Long bones: are longe...

Dr. Ahmed Deghidy PhD Molecular Biology Alex U, Alexandria, Egypt Field of Basic Sciences ‫الدولية‬ ‫جامعة العلمين‬ 1 Bones can be classified according to the shape into four groups: 1- Long bones: are longer in one dimension than other bones 2- Short bones: are nearly equal in length and diameter 3- Flat bones: the bones of the skull and the sternum. 4- Irregular bones: vertebra  Histological structure of bone Mature bone is largely composed of cylindrical units called osteons or Haversian systems. The osteons consist of concentric lamellae of bone matrix surrounding a central canal "Haversian Canal"  Histological structure of bone The Bone covering The Bone Cells The Bone Matrix The Bone Tissue  3- The bone cells (There are four types of bone cells) I-Osteoproginator cells II-Osteoblasts III-Osteocytes IV-Osteoclasts  I-Osteoproginator cells Derived from mesenchymal stem cells Give rise to osteoblasts. proliferate by repeated cell divisions giving rise to the more differentiated osteoblasts. They are active during growth and during healing of bone fracture. Sites: The periosteum ,the endosteum and lining the haversian canals.  II- The Osteoblasts (the bone forming cell) Secrete the extracellular matrix of bone Once the cell is surrounded with its secreted matrix, it is referred to as an osteocyte. Function: 1- Secretion of protein components of the bone matrix. 2- Secretion of alkaline phosphatase enzyme ,which is essential for the perception of calcium salts from the blood into the bone matrix. Sites: periosteum-endosteum-lining the Haversian canals  III-Osteocyte: Within the bone matrix are spaces called lacunae (contains osteocyte). The osteocyte extends numerous processes into small tunnels called canaliculi. Canaliculi course through the mineralized matrix, connecting adjacent lacunae and allowing contact between the cell processes of neighboring osteocytes. Function: The osteocyte is responsible for the maintenance of the bone structure as a whole. Sites: in a well –organized bone units (Haversian system).  IV-Osteoclast Osteoclasts are giant ,irregular ,motile cells ,multinucleated. Osteoclasts are bone-resorbing cells. Osteoclasts are phagocytic cells, are concerned with removal (eating up) of old tissue (bone resorption) by secreting hydrolytic lysosomal enzymes, allowing the deposition of new bone elements. Sites : present on bone surfaces where bone is being removed or remodeled (reorganized) or where bone has been damaged “bone resorption”. Osteoclast  Bone remodeling occurs continuously throughout life. In compact bone, remodeling resorbs parts of old osteons and produces new ones. Osteoclasts remove old bone and form small, tunnel-like cavities. Such tunnels are quickly invaded by osteoprogenitor cells from the endosteum or periosteum and sprouting loops of capillaries. Osteoblasts develop, line the wall of the tunnels, and begin to secrete osteoid in a cyclic manner, forming a new osteon with concentric lamellae of bone and trapped osteocytes. In healthy adults 5%-10% of the bone turns over annually.