2Bone Histology Bone cells OsteoblastsOsteocytesOsteoclastsStem cells or osteochondral progenitor cellsWoven bone: collagen fibers randomly orientedLamellar bone: mature bone in sheetsCancellous bone: trabeculaeCompact bone: dense
3Bone StructureBones are composed of connective tissue, chemicals, and fatsSolid outer layer - compact boneComposed of osteonsAn inner layer of spongy bonea honeycomb of flat, needle-like projections called trabeculae.Above: Note the relationship btwn the compact and spongy bone.Below: Close up of spongy bone.Bones are organs. Thus, they’re composed of multiple tissue types.Consists of multiple cylindrical structural units known as osteons or haversian systems.Imagine these osteons as weight-bearing pillars that are arranged parallel to one another along the long axis of a compact bone.Trabeculae: interconnecting rods or plates of bone. Like scaffolding.Spaces filled with marrow.Covered with endosteum.Oriented along stress lines
4Compact Bone Volkmann’s canals Haversian canals Perpendicular to the haversian canals.Connect the blood and nerve supply in the periosteum to those in the haversian canals and the medullary cavity.Haversian canalsallow the passage of blood vessels, lymphatic vessels, and nerve fibers.Surrounded by layers of bone called a lamella.osteonEach osteon consists of a single central canal, known as a haversian canal, surrounded by concentric layers of calcified bone matrix.Haversian canals allow the passage of blood vessels, lymphatic vessels, and nerve fibers.Each of the concentric matrix “tubes” that surrounds a haversian canal is known as a lamella.All the collagen fibers in a particular lamella run in a single direction, while collagen fibers in adjacent lamellae will run in the opposite direction. This allows bone to better withstand twisting forces.Lying in between intact osteons are incomplete lamellae called interstitial lamellae. These fill the gaps between osteons or are remnants of bone remodeling.Volkmann’s canal: perpendicular to long axis. Both perforating and central canals contain blood vessels. Direct flow of nutrients from vessels through cell processes of osteoblasts and from one cell to the next
5Note the gross differences between the spongy bone and the compact bone in the above photo. Do you see the trabeculae?Compare compact and spongy bone as viewed with the light microscope
6Bone Cells Osteoblasts Ossification Bone building cells Synthesize and secrete collogen fibers and other organic components of the bone matrixInitiate calcificationFound in the periosteum and the endosteumOssificationFormation of bone by osteoblasts.Cells surround themselves by matrix.osteoblastsOsteoblastsBone-building cells.Synthesize and secrete collagen fibers and other organic components of bone matrix.Initiate the process of calcification.Found in both the periosteum and the endosteumCollagen produced by E.R. and golgi. Released by exocytosis. Precursors of hydroxyapetite stored in vesicles, then released by exocytosis.The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve just secreted.Bone tissue is a type of connective tissue, so it must consist of cells plus a significant amount of extracellular matrix.Bone matrix
7Bone Cells Osteocytes. Lacunae Canaliculi Mature bone cells. Osteoblasts that have become trapped by the secretion of matrix.Responsible for maintaining the bone tissueLacunaespaces occupied by osteocyte cell bodyCanaliculicanals that allow for nutrient filled liquid to fill the lacunaeOsteoblasts that have matured into osteocytes. Surrounded by matrix, but they can no longer longer secrete matrix.Nutrients diffuse through tiny amount of liquid surrounding cell and filling lacunae and canaliculi. Then can transfer nutrients from one cell to the next through gap junctions.
8Bone CellsOsteoclastsCells that ecretes digestive enzymes to digest bone matrixbone resorptionConcentrated in the endosteum.On the side of the cell that faces the bone surface,ruffled border.Pumps out hydrogen ionsCreate an acid environment that eats away at the matrix.Osteoclasts. Huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell).Resorption of bone which is part of normal bone growth, development, maintenance, and repair.Osteoclasts are found on a Ruffled border: where cell membrane borders bone and resorption is taking place.H ions pumped across membrane, acid forms, eats away bone.Release enzymes that digest the bone.Derived from monocytes (which are formed from stem cells in red bone marrow)Multinucleated and probably arise from fusion of a number of cells
9Bone Cells Why is there a depression underneath the osteoclast? What advantage might a ruffled border confer?What is the name of the third cell type shown here?What do you think the tan material represents?Here, we see a cartoon showing all 3 cell types. Osteoblasts and osteoclasts are indicated.Note the size of the osteoclast (compare it to the osteoblast), and note the ruffled border.
10Long Bone Structure Diaphysis - Shaft Epiphysis - End of the bone Compact boneEpiphysis - End of the boneCancellous boneEpiphyseal plate - growth plateHyaline cartilage; present until growth stopsEpiphyseal line: bone stops growing in lengthMedullary cavity: contains marrowIn children medullary cavity is red marrow,In adults marrow is yellow in limb bones and skull (except for epiphyses of long bones).Red marrow is found in in the cavities of the spongy bone of flat bonesShaft plus 2 expanded ends.Shaft is known as the diaphysis.Consists of a thick collar of compact bone surrounding a central marrow cavityIn adults, the marrow cavity contains fat - yellow bone marrow.Expanded ends are epiphysesThin layer of compact bone covering an interior of spongy bone.Joint surface of each epiphysis is covered w/ a type of hyaline cartilage known as articular cartilage. It cushions the bone ends and reduces friction during movement.
11Long Bone Structure Periosteum Endosteum. Outer is fibrousInner is single layer of bone cells including osteoblasts, osteoclasts and osteochondral progenitor cellsconnected to bone matrix via Sharpey’s fibersEndosteum.Similar to inner layer of periosteum.Lines all internal spacesThe external surface of the entire bone except for the joint surfaces of the epiphyses is covered by a double-layered membrane known as the periosteum.Outer fibrous layer is dense irregular connective tissue.Inner cellular layer contains osteoprogenitor cells and osteoblasts.Periosteum is richly supplied with nerve fibers, lymphatic vessels and blood vessels.Periosteum is connected to the bone matrix via strong strands of collagen.Internal bone surfaces are covered with a delicate connective tissue membrane known as the endosteum.Covers the trabeculae of spongy bone in the marrow cavities and lines the canals that pass through compact bone.Contains both osteoblasts and osteoclasts.
12Structure of Bone Microscopic Macroscopic Osteocytes Diaphysis LamellaeHaversian canalsOsteonVolkmann’s canalsLacunaeDiaphysisPeriosteumSharpy’s fibersEpiphysesEpiphyseal plateEpiphyseal lineYellow marrowMedullary cavityHow do spongy and compact bone look different?What is the anatomical name for the shaft of a long bone? For its endsHow does the structure of compact bone differ from the structure of spongy bone when viewed with the naked eye?What is the passageway connecting neighboring osteocytes in an osteon?What is yellow marrow?
13Bone Matrix Consists of organic and inorganic components. Organic component are secreted by the osteoblasts:Collagen fibersElastinInorganic componentCalcium phosphateCalcium hydroxidemagnesium, fluoride,& sodium.Three-dimensional array of collagen molecules. The rod-shaped molecules lie in a staggered arrangement which acts as a template for bone mineralization. Bone mineral is laid down in the gaps.Consists mainly of 2 salts: calcium phosphate and calcium hydroxide.Bone matrix. Like reinforced concrete. Rebar is collagen fibers, cement is hydroxyapetite
15Bone MatrixAs we age…What if the Calcium phosphate, Calcium hydroxide and the other minerals were removed from this bone?Who knows what osteoperosis is? – loss of calcium in the bones. When does this happen? When we are older. Why does it happen? – we are no longer adding calcium to our bones and we need the calcium in our blood to maintain homeostasis. It has to com from some where so if we don’t eat it it comes from our bonesCLICK What happens if we remove the calcium and other minerals from a bone?What did we learn happens to collegen and elastin in our skin as we age? – it breaks downCLICK What would happen if we removed the elastin and collegin from a bone?What if the collagen and elastin were removed from this bone.
19Bone Development Prenatal: cartilage model Fetus: some conversion to boneChildhood: primary and secondary ossification sites formedAdolescence: cartilage growth plate elongates
20Mature Bone Remodeling and Repair Changes in shape, size, strength:Dependent on diet, exercise, ageBone cells regulated by hormones:Parathyroid hormone (PTH): removes calcium from boneCalcitonin: adds calcium to boneRepair: hematoma and callus formation
21Functions of the Skeletal System Protection: encases most internal organsSupport: allows body positionsPermit movement: muscle attachments for movementMineral reservoir: calcium, phosphorus