Mkgrady8 Bone

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function of bone

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physical support and storage of calcium and phosphate for release into blood

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osteoprogenitor cells

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give rise to osteoblasts

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osteoblasts shape and function

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cuboidal to oval and make most of the matrix; located in the periosteum

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osteocytes location and function

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have long processes in channels called canaliculi; involved in sensing mechanical stress and signaling osteoblasts to make bone; also transport soluble calcium between blood and bone

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osteoclasts origin, function, and location

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monocytes fuse to form multinucleate osteoclasts (so much larger than the osteoblasts or cytes); they create cavities in bone (howship's lacunae) by secreting acid and enzymes that erode bone thereby liberating calcium and phosphate to the blood; osteoporin binds to interns on the surface of the osteoclast thereby creating a "sealing zone" where the enzymes an the acidic environment is sequestered within howship's lacunae; binding of osteopontin and the interns activates the osteoclast the secrete (so it forms a sealed circle to secrete acid into); locate on top of the bone matrix (so in the periosteum next to the osteoblasts)

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organic component of the bone matrix

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35%; GAGs, glycoproteins (osteocalcin and others), and type I collagen (high tensile strength)

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inorganic component of the bone matrix

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65%; calcium+phosphorus= hydroxyapatite crystals, Ca5(PO4)3(OH), Ca5(PO4)3F is even stronger

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histological types of bone- mature bone

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composed of layer of bone; can be compact bone aka lamellar bone or spongy bone aka cancellous or trabecular

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compact bone

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aka lamellar; found in long and flat bones where the layers are concentrically organized into Haversian systems (layers organized around blood vessels)

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spongy bone

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aka cancellous or trabecular; found primarily in marrow cavity where the layers are organized more like sheets of paper in a book

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histological types of bones- immature bone

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aka woven bone; found in developing bones; bone cells and matrix are loosely organized around blood vessels without obvious layers

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the 2 surface layers of compact bone

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periosteum and endosteum; they are on either side of the matrix of the bone

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periosteum composition in bones

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fibroblasts and osteoprogenitor cells, osteoblasts, and collagen; on the other side of this layer is the outside of the bone

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endosteum composition in bones

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osteoblasts; on the other side of this layer is the marrow cavity of the bone

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the osteocytes within compact bone are organized into

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haversian systems which are made of sleeves of bone around a canal where the nutrient artery feeds vessels in the canals

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make up of a Haversian system

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each canal at the center contains a fenestrated capillary, small nerves, and connective tissue; each layer of bone around the canal is called a lamella (sleeve of bone); there can be many sleeves around each vessel; osteocyte cell bodies form the boundaries of the layers; osteocytes are functionally connected through gap junctions allowing compounds to flow between bone and blood (especially calcium and phosphate)

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what about the large molecules that need to travel from the blood vessel in the haversian canal to the osteocytes?

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they diffuse in the canlicular fluid outside of the osteocyte processes; this is a necessary mechanism because proteins can't pass through a gap junction (form cell to cell); bone minerals can also move in this fluid; so they skirt the edges of the osteocytes as they move outward through canaliculi to osteocytes in farther out lamella

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canaliculi

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channels in bone containing long processes of osteocytes; so these connect the different lamellar layers

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a different lamellar arrangement that can be found in compact bone

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volkman's canal runs out from osteocytes and runs at right angles to the Haversian systems and lack lamellae; so basically this is a haverian cannel that decided to run perpendicular to all of the other haversian canals

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osteoclasts erode

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a channel through existing Haversian systems; blood vessels invade the canal and bring osteoblasts with them; the osteoblasts attach to the wall of the channel and lay down new concentric layers of bone making a new Haversian system; the part of the old Haversian system that is left is the interstitial lamellae

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interstitial lamellae

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cutting cones of osteoclasts create interstitial lamella; blood vessels grow in between osteoclasts benign in osteoblasts and progenitor cells attached to its surface; so think of them this way- you have Haversian canals but they are all circular so there must be something filling in the spaces that aren't filled with the circles so that area is the interstitial lamellae which are remnants of old haversian canals

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the 2 types of bone growth

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intramembranous and endochondral

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intramembranous bone growth

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bone forms form a sheet of embryonic connective tissue called mesenchyme; this forms some of the fact bones of the skull and part of the clavicle

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endochondral bone growth

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bone forms from an existing piece of hyaline cartilage; all of the long bones and many of the flat bones form by this process

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intramembranous bone formation step 1

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mesenchymal cells (embryonic fibroblasts) differentiate into osteoblasts; osteoblasts begin to make matrix and get trapped within it becoming osteocytes

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intramembranous bone formation step 2

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with continued production of matrix, spicules of bone are formed; then the osteoblasts get completely trapped in the matrix they become osteocytes; osteoclasts remove bone helping to shape the matrix; new osteoblasts make bone on the surface of the spincules

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intramembranous bone formation step 3

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spicules of bone eventually grow together and blood vessels are now trapped in long channels; this stage the bone is called immature or woven bone

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intramembranous bone formation step 4

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finally the spaces around the vessels is filled in with bone layer by layer

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endochondral bone formation step 1

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bone formed from a hyaline cartilage model

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endochondral bone formation step 2

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perichondrium differentiates into a periosteum and osteoblasts start to make a collar of bone around the cartilage

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endochondral bone formation step 3

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chondrocytes enlarge and produce factors that initiate calcification of surface of the lacunae; this cuts off nutrients to the chondrocytes and they die

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endochondral bone formation step 4

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osteoclasts cut a channel into the bone and cartilage; blood vessels enter the channel bringing stem cells that will give rise to bone cells

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endochondral bone formation step 5

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osteoblasts differentiate from stem cells and attach to the calcified surfaces that previously containing the chondrocytes; the osteoblasts lay down bone matrix

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endochondral bone formation step 6

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chondrocytes at both ends of the cartilage model begin to divide; the dividing cells push the resting cartilage cells outward elongating the model; the epiphyseal plates are now formed at each end of the bone; the bony collar grows thicker and elongates to keep up with the epiphyseal plate growth

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zones of the epiphyseal plate (from just one end of a long bone)

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starting from the osteocytes- ossification, calcification, hypertrophy, proliferation, and resting

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regulation of bone formation- growth hormone

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stimulates hepatocytes to make IGF-1 that stimulates bone formation

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reugulation of bone formation- estrogen

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prevents loss of bone; also shuts off growth plate activity in cartilage at puberty in women

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regulation of bone formation- testosterone

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stimulates bone growth

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regulation of bone formation- mechanical stress

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stimulates bone formation; osteopontin binds osteocytes to matrix and this allows them to sense when bone is stressed so they are stress sensors; they then release unknown factors that influence osteoblasts and osteoclasts

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regulation of bone formation- PTH

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stimulate osteoclast production

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regulation of bone formation- calcitonin

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inhibits osteoclast function

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bone repair after a fracture

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chondrogenic cells (stem cells) invade the break and differentiate into chondroblasts and make hyaline cartilage; cartilage is then replaced by bone (endochondral ossification)