#13327 - Bone - Organisation of the Body

Bone

Function: strong, rigid endoskeleton to which skeletal muscles are attached to permit movement

-for protection: skull and the ribcage, vertebrae-spinal cord, pelvic bones-reproductive organs

-Storage of minerals: Calcium ions (98% of body’s calcium ions are in the bones-calcium homeostasis, phosphate ions)

-Blood cell production- bone marrow produces erythrocytes, leukocytes, platelets

Ultra structure –osseous tissue

cells embedded in collagenous extracellular matrix- type 1 collagen- osteoid- mineralised by deposition of calcium hydroxyapatite- bone strength and rigidity

-osteocytes-inactive osteoblasts trapped within the osteoid, mature bon cells- assist in the nutrition of the bone-maintain protein and mineral content of the matrix

-osteoblasts: immature bone cells, found on the inner and outersurface of bone, synthesise the osteoid and mediate the mineralisation- cells lined up at the bone surfaces

Osteoblasts and osteocytes are formed from the differentiation of mesenchymal stem cells- ‘osteoprogenitor cells’ found on inner and outer surfaces of bone

-osteoclasts- phagocytic cells that erode bone- important in the constant turnover and refashioning of bone

Osteoclasts are multinucleate phagocytic cells derived from the macrophage monocyte cell line

OSTEOBLASTS

-vitamins D and K are involved in osteoblast gene expression- deficiency of vitamin D leads to Scurvy as there is little osteoblast formation so there is a failure to absorb calcium into the cell- thin bones, not resistant to compressibility

--active osteoblasts- large broad spindle shaped or cuboidal cells with abdundant basophilic cytoplasm- RER, golgi A- high rate of type 1 collagen synthesis

-osteoblasts secrete alkaline phosphatase- mineral deposition, also secrete osteonectin (glycoprotein) binds the calcium and the hydroxyapaptite and collagen- osteomalacia, disease which results from failure of normal mineralisation of newly formed osteoid-defieciency of calcium/deficiency of phosphate due to kidney failure- bone is soft and prone to fracture

-inactive osteoblasts- narrow attenuated spindle shaped cells lying on the bone surface

OSTEOCLASTS

-Large multinucleate phagocytic cells formed from blood borne monocyte precursors- reabsorption of bone

-attach via integrins in the osteoclast membrane to arg, gly,asp (RGD) sequences in matrix protein osteopontin (glycoprotein) secreted by osteoblasts

-lies on the surface of HOWSHIP’S LACUNAE- small cavity formed from the underlying digestion of bone

-at the contact zone- Osteoclasts- fine microvilli- ruffled border- Electron microscope- ruffled border – produces and pumps hydrogen ions via a proton pump.- the hydrogen ions dissolve the minerlased bone matrix and releases calcium ions, phosphate ions Carbonic anhydrase produces more protons via the production of carbonic acid- chloride is exchanged for the bicarbonate produced by a membrane eexchanger .Chloride passes into the lacuna to form HCL- breaks down the salt matrix- proteases are secreted to break down the collagen and other proteins within the bone tissue – calcium and signalling peptides are released from the degraded bone by the proteases- signalling peptides can stimulate osteoblasts to fill in the holes created

-osteoblasts secrete osteoclast differentiation factor- rank ligand- when this ligand binds to the activating receptors on the osteoclasts it leads to the differentiation of osteoclast progenitors and when it binds to the osteoclasts it stimulates the activity- bone resorption

-osteoblasts also secrete osteoprotegerin- which inhibits the action of the rank ligand by acting as a decoy receptor and inhibits bone breakdown

-the balance of RANK l and OPG determines bone turnover

-osteoclastic activity- bone remodelling- in response to growth or changing mechanical stresses upon the skeleton- stresses are detected by osteocytes and periosteal osteoblasts- osteoclasts degrade the unwanted bone and then osteoblasts are recruited where they lay down new bone and then become inactive bone lining cells

-osteoclats- maintainence of blood calcium homeostasis by their response to parathyroid hormone

BONE MATRIX

-Matrix- 90% collagen-mostly type 1, there are hole zones where mineral deposition occurs-mineral content of bone- calcium and phosphate in the form of hydroxyapatite crystals- crystals are attached to the collagen fibres

-the collagen give tensile strength, calcium salts give compressive strength

-ground substance- rest proteoglycans contribute a smaller proportion of matrix than cartilage- mostly consist of chondroitin sulphate and hyaluronic acid- proteoglycans aggregates-proteoglycans control water content of bones

-Glycoproteins: ostecalcin-binding calcium during mineralisation, osteonectin, sialoproteins

Structure:

PERIOSTEUM-the outersurface of most bones-isolates and protects the osseous tissue which has 2 layers- fibrous is covered by a layer of condensed fibrous tissue and the cambium layer has osteoprogenitor cells and osteoblasts. When no new bone is being formed on bone surface, cells are flattened with spindle shaped nuclei The osteoblasts are responsible for increasing the width of the bone. When there is a fracture the osteogenitor precursor cells are responsible for healing. When no new bone is formed-cells are flat, with spindle shaped nuclei but when there is new bone formation at the perosteal surface- cells proliferate and increase in size to become osteoblasts

-bones have a dense rigid outer shell of compact bone (the cortex)- CORTICAL BONE (beneath the periosteum)-Function: Strength and protection-conducts stress from one area of the body to another area of body, generates tremendous strength, weak strength when applied from the side, reservoir for calcium and phosphate, keep concentration of these ions constant in the blood-very heavy 80% of body weight

-compact bone is made up of parallel bony columns- made up of concentric bony layers/ Lamellae- central channel containing blood vessels, lympathics, nerves- canals of Havers/Haversian canal

-each harvesian system (osteon) develops by osteoclastic tunnelling of a mass of compact bone to form a broad channel into which blood vessels and nerves grow after which becomes lined internally by active osteoblasts lay down concentric laemella

-deposition of successive lamellae- diameter of the Haversian canal decreasing and osteoblasts are trapped as osteocytes in spaces called lacunae- osteocytes maintain the structural integrity of mineralised matrix and mediate short term release or depositin of calcium for calcium homeostasis-the activity of osteocytes in calcium regulation is controlled by plasma calcium concentration and hormones parathyroid and calcitonin-secreted by parathryoi and thyroid glands- the osteocytes don’t completely fill lacuna, narrow space is filled with extracellular bone fluid

-there are interconnecting canals between lacunae and central canal known as canaliculi- contain fine cytoplasmic extension of osteocytes that are linked together by gap junctions- Canaliculi provide passage for circulation of Extracellular fluid and diffusion of metabolites between lacunae and vessels of the Haversian canals

Outer circumferential lamella: outermost aspect of compact bone, concentric lamellae of dense cortical bone laid by osteoblasts of periosteium

Inner circumferential lamellae: concentric lamellae of dense cortical bone, abuts the marrow cavity- innermost layer of inner circumferential lamellae has a layer of inactive flat osteoblasts on its surface-when activated these cells enlarge to become active cuboidal osteoblasts and synthesise new lamellar osteoid which on mineralisation forms another layer of inner circumferential lamella

-This occurs regularly as part of constant dynamic refashioning of the bone- dominant during bone growth and in response to increased or altered stress on cortical bone- for example on long bones during increased physical training for running

-The ends of interconnecting network of trabecular/cancellous bone-attach to the circuferential lamellae of cortical bone

-The inactive osteoblasts of the endosteum also extend onto the surface of trabecular bone and deposit new osteod when required for strengthening or remodelling

-central medullary/ cancellous zone of thin, interconnecting narrow bone trabeculae- the number, orientation, thickness of these bone trabeculae depend on the stresses to which the bone is exposed –creates strength when force is applied to the side

-trabecular bone- network of interconnecting struts- orientated in position to provide maximum strength for minimum mass. Made of lamellar bone with osteocytes which exchange metabolites via the canaliculi which communicate with each other and with blood sinusoids in the haemopoietic marrow.

-marrow space- mixture of fatty marrow made of adipose tissue and haemopoietic marrow composed of RBC, WBC precursors with numerous thin walled vessels

-the spaces in the medullary bone between trabeculae is filled with haemopoietic bone marrow

TYPES OF BONES

-woven bone- randomly arranged collagen fibres in the osteoid- osteoblasts produce the osteoid rapidly- fetal developemtn or in adults where there is a pathological rapid new formation

-lamellar bone- regular parallel bands of collagen arranged in sheets

INTRAMEMBRANOUS OSSIFICATION- embryonic development of flat bones from an embryonic tissue, mesenchyme (embryonic connective tissue)

-VAULT OF THE SKULL, MAXILLA, MOST OF MANDIBLE

-Intramembranous bone occurs from condensed mesenchymal tissue

-mesenchymal stem cells...