Bone is a specialized connective tissue that makes up , together with cartilage , the skeletal system
- Flat bones and long bones
- Cancellous bone and cortical bone
- Periosteal surface ( external ) and endosteal surface ( internal )
HISTOLOGY
- Matrix and mineral
- Collagen fibers with preferential orientation and crystals of hydroxyapatite
- Proteoglycans and Glycoproteins
- Noncollagen proteins : ex. osteopontin
- Cells
- Osteoblasts
- Features :
- They arise from osteoprogenitor cells of mesenchymal origin and are the source of the terminally differentiated osteocyte
- They never appear or function individually , but are always found in clusters of cuboidal calls along the bone surface .
- Their plasma membrane is characteristically rich in alkaline phosphatase and has been shown to have receptors for parathyroid hormone and not for calcitonin ; osteoblasts also express receptors for estrogen and Vit D in their nuclei .
- They form bone :
- Synthesis and intracellular processing of type I collagen
- Secretion and extracellular processing of the collagen
- Formation of microfribrils , fibrils and ultimately fibers from the collagen
- Maturation of the collagen matrix with subsequent nucleation and growth of hydroxyapatite crystals
- They are autocrine regulatory cells ; they can synthesize and deposit growth factors in bone matrix which , when released by bone resorption process , restimulate further osteoblastic activity
- They mediate the systemic signals ( IL-6,TNF-alfa,M-CSF,RANK-L ...) for the recruitment and activity of osteoclasts
- Osteoclasts :
- Features :
- Differentiation
- It is a giant multinucleated cell usually found in contact with a calcified bone surface and within a lacuna ; there usually are only one or two per site .
- They are motile cells and they move across the bone surface
- They resorb bone by the production of proteolytic enzymes and hydrogen ions in the localized environment under the "ruffled border" of the cell formed by deep foldings of the plasma membrane in the area facing the bone matrix ; it's surrounded by a ring of contractile proteins serving to attach the cell to the bone surface and sealing off the subosteoclastic bone resorbing comparment .
FUNCTIONS
- Shape = Bones provide a frame to keep the body supported.
- Blood production by marrow
- Detoxification = Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion
- Sound transduction = Bones are important in the mechanical aspect of hearing.
- Protective = for vital organs and bone marrow
