Bone

Bone

1. A highly vascular connective tissue made of specialised cells in a matrix composed of minerals, water and protein fibres.

2. 2 types

   • Compact (dense)
     • Forms the surface layers of mature bones
     • found mainly in shafts of long bones (where strong, tubular structure is required)
     • consists of a no. of cylindrical structures called haversian systems (or osteons) which contain:
       • a central haversian canal
       • lamellae
       • lacunae
       • canaliculi
       • interstitial lamellae
       • circumferential lamellae
       • Volkmann's canals
       • periosteum
   • Cancellous (spongy)
     • found in parts of bone where lightness, strength and area are required
     • has trabeculae instead of osteons
       • plates and bars of bones arranged in a haphazard manner
       • organised to provide max strength
       • follow lines of stress and can realign if direction of stress changes
       • contain: lamellae, osteocytes, lacunae and canaliculi
         • canaliculi connect to adjacent cavities instead of a central canal
       • BONE MARROW found between trabeculae
         • at birth it is red and produces cells in the blood e.g. RBCs, WBCs.
         • in adults, red found in: upper femora, vertebrae, scapulae, sternum, clavicles, diploe of skull bones and hip bones.
           • elsewhere, red becomes inactive yellow marrow

3. OSSIFICATION - Formation of bone from connective tissue and requires: adequate calcium and phosphate in blood and vitamins A, C and D.

   • influenced by hormones: parathormone, growth hormone, thyroxine and testosterone
   • intramembranous - takes place in a membrane
     • osteogenic fibres and bone cells appear in the connective tissue and calcium salts deposited to form osteoid tissue
     • ossification spreads from the centre outwards
   • intracartilaginous
     • process by which bone formation takes place e.g. in a long bone
       • Primary centre of ossification - middle of diaphysis, osteoblast appear and calcium is laid down, osteoclasts remodel bone into required shape, bone is also being built up on outside of shaft to form periosteum
         • Secondary centre of ossification - appears at end of bone to form epiphyses (which are separated from diaphysis by epiphyseal plate
           • Growth - occurs during childhood by production of bone at epiphyseal plate nearest shaft - metaphysis.
             • Fusion- of epiphysis with diaphysis occurs when bone reaches desired size (approx. 20 years)

4. Functions

   • Shape
   • Protection
   • Movement
   • Sound transduction
   • Production of RBCs- haematopoiesis
   • Mineral storage
   • Growth factor storage
   • Fat storage

5. Types of cells within bone

   • Osteogenic cell - found in inner layer of periosteum - develops into osteoblasts
   • Osteoblast - forms bone matrix
   • Osteocyte - mature bone cell which maintains bone tissue
   • Osteoclast - functions in bone resorption and remodelling
     • Bone tissue is maintained by a balance between the activity of osteoblasts that form bone and osteoclasts that break it down - remodelling

6. BLOOD SUPPLY

   • Periosteum has arteries which supply it and also enter the diaphysis through many perforating Volkmann's canals - this supplies the outer part of compact bone
     • A hole in compact bone (nutrient foramen) allows passage of nutrient artery into medullary cavity
       • Divisions supply inner part of compact bone, spongy bone and red marrow
       • nutrient arteries divide into ascending and descending branches in medullary cavity
       • some bones have 1 nutrient artery (tibia) and some have several (femur)
       • nerves also pass through same nutrient foramen
     • broken bones are very painful as periosteum is richly supplied with nerves

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