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Bone

Structure of Bone11--PeriosteumPeriosteum22-- Bone cellsBone cells33-- Bone matrixBone matrix44-- EndosteumEndosteum

-PERIOSTEUM• vascular CT membrane

covering bone from outside• Two layers

• Outer fibrous• Inner osteogenic

-Bone Cells

Bone CellsOsteogenic cells

(osteoprogenitor cells)Origin pericytesL.M.: They are flat cells with

pale basophilic cytoplasmandcentral flat nuclei

E.M.: Rich in ribsomes and rER.•Site 1-BM cavities,

•2-Endosteum &•3-inner layer of Periosteum

Function Bone GrowthHealing of bone Fractures

OsteogenicOsteogenic cells (cells (osteoprogenitorosteoprogenitor cells)cells)

Bone CellsOsteoblast cellsOrigin osteogenic cellsL.M. : They are small, oval branched cells

with few cytoplasmic processes • They have oval, eccentric, open face

nuclei.• They have deep basophilic cytoplasm

with negative Golgi image close to the nucleus.

E.M.: They show plenty of ribosomes, rER, well developed Golgi apparatus and mitochondria.

•.periosteumlayer of osteogenicActivated inner -1Site •2- Endosteum.

•3- Walls of bone marrow spaces.•Function

))Bone Forming CellsBone Forming Cellscells (cells (OsteoblastOsteoblast

))Bone Forming CellsBone Forming Cellscells (cells (OsteoblastOsteoblast•:It is rich in these enzymes

•Which facilitates deposition of :enzymephosphataseAlkaline calcium by creation of alkaline medium.

•which inhibit the action of :enzymePyrophosphatasepyrophosphate substances “these substances retard the process of calcification” i.e. indirect calcification effect.

•Function :

•1-They are responsible for bone formation by synthesis and secretion of bone matrix (Osteocalcin protein).

•2- They are concerned with calcification of bone by secretion of:•Alkaline Phosphatase and pyrophosphatase enzymes.

•Matrix vesicles, budding from the cell membrane which store Ca+.

•They change into osteocytes when they are•surrounded by lacunae and calcified matrix.

•

Bone CellsOsteocyte cellsOrigin OsteoblastL.M. - They are oval, branched cells.• They have oval, central nuclei.• The cytoplasm is slightly basophilic rich is

alkaline phosphatase enz.• Osteocytes can’t divide so present singly in

each lacuna..E.M.: - They contain rER, ribosomes, Golgi

apparatus, and many cytoplasmicmicrotubules.

• The cells are present inside lacunae between bone lamellae

• They are connected with one another by processes passing through bone canaliculiwhich connect adjacent lacunae together.

•These cell processes intercommunicatewith one another by gap junctions.

•Which allow the electrolytes to pass freely from one cell to the other.

Bone CellsOsteocyte cellsSites

Function:• They form bone matrix and

vesicles rich in enzymes which preserve the integrity of the bone matrix and maintain its inorganic components.

• They are related to mobilization of Ca+ from the bone to the blood in times of need.

Bone CellsOsteoclastOrigin Monocytes

Bone CellsOsteoclastSite

•1- On the inner surface of bone where resorptiontakes place:

•2- Bone marrow spaces.•3- Medullary cavities.

•4- Endosteum. •Each cell lies in a shallow

cavity called “HowshipsLacuna”.

Bone CellsOsteoclastOsteoclastL.M.: • Size: Oteoclast is large cell “20-

30 um”.• Nucleus: It is multinuclated cell

“4-50” nuclei• Shape: It is irregular in shape.• Cytoplasm: is foamy acidophilic.• Border: The cells have striated

or brush border facing the bone surface.

Bone CellsOsteoclastE.M.:

•The osteoclast shows the following 4-zones:-•(1) Ruffled or striated zone.

•It is formed of:•Numerous finger like processes projecting from the cell

membrane.•Collagenous fibrils from the resorped bone matrix.

•(2) Clear Zone:•It is a ring shaped region under the ruffled part of the cell

membrane.•It is rich in actin filaments.

•It is called clear because it is devoid of cell organelles.•(3) Vesicular Zone:

•It is a zone of lysosomal vesicles of variable shape and size.

•The vesicles are rich in acid phosphatase enzyme.•(4) Basal Zone:

•It contains nuclei (4-50) & cell organelles •(Golgi apparatus & mitochondria) and cell inclusions.

Bone CellsOsteoclast

•Function:••11-- They are concerned with bone They are concerned with bone

resorptionresorption during ossification causing during ossification causing remodeling of the bone by the following remodeling of the bone by the following mechanisms.mechanisms.

••AA-- Secretion of acids Secretion of acids ““lacticlactic and and carbonic acidscarbonic acids”” in the tissue fluids in the tissue fluids creates an acidic medium causing creates an acidic medium causing decalcification of bone matrix.decalcification of bone matrix.

••BB-- Secretion of Secretion of collagenasecollagenase enzymeenzymewhich dissolves bone matrix during which dissolves bone matrix during ossification.ossification.

••22--They remove bone debris during They remove bone debris during ossification and after healing of bone ossification and after healing of bone fracture.fracture.

BONE MATRIXGeneral charcters: It is hard in consistency.It is rich in blood supply.Structure of the bone matrix:

Organic components 50%– Collagen (typeI) It constitutes

about 90% of the organic component.,

– sulphatedglycosaminoglycans,

– osteonectin glycoproteinWhich anchor cells with bone matrix ,

– osteocalcin protein It helps bone calcification.

• Inorganic components 50%Ca++They are called hydroxy apitate crystals

which responsible for Bone hardening

P+ phosphorus saltsNA+ CO3-citrate Fe+ Mg+

The hardening of the matrix is due to1- Hydroxy apitate Crystals2- Collagen fibrils

ENDOSTEUMDef. vascular CT membrane that lines

inner surface of bone, BM cavities &

Stru. Contain Osteogenic, Osteoblast & Osteoclast

Function1. Provide nourishment &bl.supply

to bone2. Its osteogenic cells &

osteoblasts & osteoclasts are concerned with bone formation and resorptionduring growth & healing after fracture.

Types of Bone