Bone: Structure & Function

Introduction

·     Bone is a specialized connective tissue organized in formation of endoskeleton.

·     It arises by intramembranous or endo­chon­dral ossification.

Functions

1.  Structural support of the body

2.  Protection of vital organs

3.  Formation of series of mechanical levers through which attached muscles and ligaments can move the body

4.  Store house of calcium and phosphorus.

Macroscopic Structure

·     Bone is composed of hard inorganic and resilient orga­nic component.

·     Bone is resilient in compression and tension.

·     The tensile strength of bone resembles cast iron, but with only one-third of its weight.

·     Tubular structure of long bone is the strongest, lightest and hence most economical arrangement of material.

·     Cut surface of a long bone shows outer corti­cal or compact bone and inner cancellous or spongy bone (Fig. 2).

·     Dense cylindrical diaphysis of long bone surrounds marrow cavity.

·     Metaphysis of long bone contains mainly cancellous bone, can­cel­lous and cortical bone modify their structure in response to loading, hormonal and other influences.

·     Cortical bone is dense, compact and provides maximum strength for given weight.

·     Cancellous bone is trabecular bone and its arrangement follows pressure or stress lines.

Arrangement of Bony Lamellae

Mature bone is composed of layers, hence, it is called lamellar bone.

Haversian System in Compact Bone

·     The bone consists of numerous cylindrical units known as haversian systems (Fig).

·   Each system consists of a central haversian canal surrounded by concentric lamellae of bony tissue.

·     Numerous lacunae intervene between these lamellae.

·  The lacu­nae com­muni­cate with each other and with central canal by numerous radiating canaliculi.

·    The central canal contains small artery and vein.

· The lacunae are filled with osteocytes, and canali­culi contain protoplasmic processes of osteocytes and convey outwards the nutritive materials by diffusion.

·   The haversian canals run longitudinally and com­mu­ni­cate with the medullary cavity and with the surface of bone by numerous oblique channels known as Volkmann’s canals which con­tain blood vessels and nerves.

·  Interstitial lamellae with lacunae and canaliculi occupy the intervals between haversian systems.

·     Circumferential lamellae encircle inner and outer surfaces of bone. These are held together by the perfo­rating fibers of Sharpey.

·     Each haversian system is demarcated from other by cement line, which is strongly basophilic and devoid of collagen fibers and is rich in mineral salts.

In spongy bone, haversian system is not usually found as osteocytes get their nutrition from blood vessels of tissues around them.

Periosteum

·     Periosteum is a tough thin connective tissue mem­brane which covers the bone except for articular carti­lage surface and attachments of tendons, ligaments and joint capsule.

Structure of Periosteum

·     Periosteum consists of outer fibrous layer and inner cellular layer which is more vascular.

·     Outer fibrous layer consists of dense collagen fibrous tissue and fibroblast-like cells.

·     Inner vascu­lar layer contains osteogenic cells having poten­tiality to form new bone.

·     The periosteum changes with age.

üThe thick cellu­lar vascular periosteum of infant and children readily forms new bone.

üWith increasing age, periosteum becomes thinner and less vascular.

üIts ability to form new bone also decreases.

Blood supply of periosteum 

·     Periosteal blood vessels lie on the outer fibrous layer of periosteum and at intervals anastomose with the blood vessels of adja­cent muscles.

·     Branches of the vessels pene­trate the fibrous layer and supply the deeper layer of perio­steum.

Nerve supply of the periosteum 

·     Nerve fibers accom­pany the periosteal blood vessels, they are mainly vasomotor.

Blood Supply of Long Bone

·       Blood supply of long bone accounts for 5 to 10 percent of the cardiac output.

·       Long bone receives blood supply from various sourcesà

1.      Nutrient arteries
2.      Epiphy­seal arteries
3.      Metaphyseal arteries
4.      Periosteal arteries

Marrow

·     Red marrow is present throughout in the medullary cavity in embryo which changes to white by 12 years of age except at metaphysis, cancellous tissue of the vertebra, ribs, skull and innominate bone.

·     Marrow stroma is made up of network of reti­cular cells and their fibers with endothelial cells lining the sinusoidal blood vessel walls.

·     Hemo­poietic cells are loosely held within the network.

·     Macrophages are of different histogenic cell type.

·     The reticular or endo­thelial cells are capable of form­ing extracellular con­nective tissue fibers, where­as the macrophages, mono­cytes and their pre­cur­sors are a part of macrophage monocyte system.

·     Stromal components of marrow are continuous with osteogenic connective tissue cells of the perio­steal and endosteal surfaces and haversian canals of the bone.

·     Marrow stromal cells and osteogenic canal tissue can form a bone.

Bone Cells

·     Formation and maintenance of bone is due to the actions of various types of the bone cells.

·     Four types of bone cells are recognized, viz. osteo­pro­genitor cells, osteoblasts, osteocytes, and osteo­clasts.

a) Osteoprogenitor Cells

·       Osteoprogenitor cells are undifferentiated mesen­chy­mal stem cells.

·       Some of them are committed to form bone tissue, others are inducible seen in other connective tissue.

·       These cells give rise to hetero­trophic bone formation.

·       Committed osteopro­genitor cells differen­tiate into osteoblasts in more vascular tissue with sufficient oxygen tension.

B) Osteoblasts

·     Osteoblasts are precursor cells, which give rise to osteocytes.

·     They are oval cells with eccentric nuc­leus and many cytoplasmic processes.

·     The cyto­plasm is strongly basophilic due to ribosomes attached to the endoplasmic reticulum which secrete bone matrix.

·     In addition to rough endo­plasmic reticulum, cytoplasm also contains golgi apparatus, mitochondria and other cell organelles.

·     They lie on the bone surfaces where, when stimu­lated form new organic matrix and participate in matrix mineralization.

·     Mineralization is controlled by alkaline phosphatase secreted by osteo­blasts.

·     They assume flattened form after decreasing synthetic activity and remain on bone surface or can get surrounded by none matrix and become osteocytes.

·     This cell is involved in

1)    Synthesis of major bone proteins including collagen I, noncollagen proteins as osteocalcin and osteonectin

2)    In mineralization of bone and

3)    Control osteoclastic function through specific sur­face receptors for agents which stimulate bone resorption as vit 1-25 D3, parathyroid hor­mone.

·     The mechanism by which osteoblasts signals osteoclast to resorb bone in unclear but is thought to be mediated by more than one mechanism.

C) Osteocytes

·     Osteocytes are flattened cells with central nucleus and numerous cytoplasmic processes.

·     They occupy spaces in the matrix called as lacunae.

·     Canaliculi radiate from each lacuna and permit diffusion of nutritive material.

·     Protoplasmic processes occupy the canaliculi.

·     The osteocytes remain alive in calci­fied matrix and secrete alkaline phosphatase to maintain calcification.

·     This cell system may mediate a rapid calcium flux between bone and extracellular fluid.

D) Osteoclasts

·     Osteoclasts help in resorption of bone.

·     They are large irregular and multinucleated cells.

·     They are found in direct contact with bone after eroding it in Howship’s lacuna.

·     The cells are provided with brush border for absorption of the bone.

·     Osteoclasts are large cells with 15 to 20 nuclei indented with prominent nucleoli, numerous mitochondria, little rough endoplasmic reticulum, pos­sess eosinophilic cytoplasm and have no processes.

·     Cyto­plasm contains numerous lysosomes filled with acid phosphatase and other hydrolytic enzymes.

·     Osteo­clasts are derived from the fusion of the mononuclear phagocyte system of bone marrow.

·     High levels of parathyroid hormone sti­mu­late osteoclast activity.

·     They remove both mineral and organic component of the bone matrix.

·     Osteoclasts have a ruffled border through which osteocytes are attached to the bone at the site of resorption by special protein “integrin”.

·     Osteoclasts contain enzymes, tartarase-resistant acid phospha­tase (TRAP) and carbonic anhydrase.

·     Carbonic anhydrase  cata­lyzes the hydration of dissolved carbon dioxide and carbonic acid thus formed dissociates into bicar­bo­nates and hydrogen which is pumped through ruffled border, thus lowering pH in the ruffled zone. The bone mineral is probably dis­solved into acid environment.

E) Other Cells

Reticular cells, endosteal cells, fibroblasts, etc. are also present.