Bones….a dynamic tissue
Current knowledge of microstructure and physiological functions of bones are far from complete….more often controversy
Major functions
• Provision of a structural support for body tissues and an attachment for muscles, tendons and ligaments • Formation of lever system that permits body movement • A major role in mineral homeostasis by providing a reservoir for body calcium, phosphorus and magnesium salts • Formation and protection of hemopoietic tissue
Architecture of Bone
Two forms of architecture – Woven bone and Lamellar bone • Woven bone – Transitional and relatively fragile form most commonly seen during the
– – – – rapid phases of bone formation in embryonic life or at zones of endochondral ossification during fracture repair In adults, near sites of tendon insertion or ligament attachment
• Lamellar bone – greater durability by collagen fibres in different orientation
– Two kinds of Lamellar bone: Compact and Trabecular bone
• Compact bone
– makes up the shaft of long bones and the outside surfaces of other bones.
• osteons (Haversian systems). • concentric lamellae • Smaller canals, or canaliculi, • fine cellular extensions • Perforating canals (Volkmann's canals)
• Spongy bone
– Trabeculae are similar to osteons. – canaliculi
• However, no central canal is necessary.
Types of Bones
• Long bones are longer than they are wide. The length of the bone, or shaft, widens at the extremities (ends). • Short bones are cube like, about as long as they are wide. • Flat bones, such as ribs or skull bones, are thin or flattened. • Irregular bones, such as vertebrae, facial bones, and hip bones, have specific shapes unlike the other types of bones. The following two bone types are usually classified separately: • Sesamoid, or round bones, such as the kneecap, are found embedded within certain tendons. • Sutural, or Wormian bones, occur between the sutures (joints) of the cranial bones of the skull.
Main features of a long bone
• The diaphysis, or shaft, is the long tubular portion of long bones. It is composed of compact bone tissue. • The epiphysis (plural, epiphyses) is the expanded end of a long bone. • The metaphysis is the area where the diaphysis meets the epiphysis. It includes the epiphyseal line, a remnant of cartilage from growing bones. • The medullary cavity, or marrow cavity, is the open area within the diaphysis. The adipose tissue inside the cavity stores lipids and forms the yellow marrow. • Articular cartilage covers the epiphysis where joints occur. • The periosteum is the membrane covering the outside of the diaphysis (and epiphyses where articular cartilage is absent). It contains osteoblasts (bone-forming cells), osteoclasts (bonedestroying cells), nerve fibers, and blood and lymphatic vessels. Ligaments and tendons attach to the periosteum. • The endosteum is the membrane that lines the marrow cavity.
Main features of short, flat, and irregular bones
• In short and irregular bones, spongy bone tissue is encircled by a thin layer of compact bone tissue. • In flat bones, the spongy bone tissue is sandwiched between two layers of compact bone tissue. The spongy bone tissue is called the diploe. • Periosteum covers the outside layer of compact bone tissue. • Endosteum covers the trabeculae that fill the inside of the bone. • In certain bones (ribs, vertebrae, hip bones, sternum), the spaces between the trabeculae contain red marrow, which is active in hematopoiesis.
Bone cells
• Controversial Origin – but….Bone marrow origin
– Osteogenic cells (osteoprogenitor cells) - stem cells, which lie in the connective tissue between the trabeculae; differentiate from Mesenchymal cells – CELL MODULATION: Osteogenic cells Osteoblasts Osteocytes
• cell differentiation, a term applied to irreversible changes in structure and function.
– Osteoclasts – Mono-nuclear phagocytes
Osteoblasts
• Bone-forming cells • Single layer; contains Alkaline phosphatase • Shape: Inactive Osteoblasts - Squamous Active Osteoblasts - Cuboidal or pyramidal • Adjacent to osteoid in newly forming bone • Functions:
– Secrete the organic components of the bone matrix (collagenous fibers and ground substance) called osteoid – Controls rapid phase(1-4 days) of mineralization
Osteocytes
• Blasts trapped in matrix Osteocytes • Micro canaliculi attach to adjacent Osteocytes • Functions:
– Initiate bone remodelling by detection of micro damage, signal transmission to Blasts and clasts – Bone Pump – Osteocytic Osteolysis – Collagen synthesis – Control of Matrix mineralization – Long term Calcium exchange – Plasma protein uptake
Osteoclasts
• Resorption of bone • Variable size and highly mobile • lie in shallow depressions on the bone surface called the Howship’s lacunae. • Acid Phosphatase
Bone tissue fluid
• Present in Lacunae and Canaliculi • High Potassium content but low Calcium, magnesium, sodium and albumin concentration • Separated from blood by Osteocytes and Osteoblasts layer – through which metabolites and ions get transferred.