Bone l cell biology and histology revision for dental students
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Bone
►definition:
- Highly specialized variety of c.t. that forms the skeleton & protect soft organs & serves for attachment of muscle
►consist
of :
- Bone cells (osteogenic , osteoblast , osteocyte , osteoclast) , bone matrix , periosteum and endosteum .
Bone cells
Osteogenic
Origin :
- Pericytes less differentiated cells which present around blood capillaries.
Sites
- Periosteum
- Inner osteogenic layer
- Endosteum
- Bone marrow cavities .
Light microscope :
- Shape : flat cells
- Nuclei: flat & central
- Cytoplasm: pale basophilic .
Electron microscope :
- Rich in ribosomes & rough endoplasmic reticulum
Function
- Capable to divide and
give osteoblasts (bone
forming cell) during:
- Growth of bone.
- Healing of fracture bone
Osteoblast
Origin
- Activated osteogenic .cells
differentiated cells & can’t divide
Sites
- Periosteum ( activated inner osteogenic layer )
- Endosteum.
- Bone marrow spaces walls
Light microscope :
- Shape : small
, oval , branched cells
- nuclei :oval, eccentric , open face
- cytoplasm : deep basophilic cytoplasm negative golgi image
Electron microscope :
- Rich in ribosomes & rough endoplasmic reticulum
- Well developed golgi
Function
- responsible
for bone formation by
synthesis & secretion of bone matrix
- calcification
of bone by secretion of: phosphatase enzyme
Osteocyte
Origin :
- Mature osteoblasts
Sites
- Can’t divide so , present singly in each lacuna
Light microscope :
- shape :oval , branched : cells
- nuclei central oval,
- cytoplasm : is slightly basophilic rich is alkaline phosphatase enzyme
Electron microscope :
- Rough endoplasmic reticulum ribosomes, golgi apparatus, and many cytoplasmic microtubules .
- Cells are present inside lacunae between bone lamellae
- Inter connected by processes passing through bone canaliculi which connect adjacent lacunae together.
- Cell processes communicate with one another by gap junctions that allow the electrolytes to pass freely from one cell to the other
Function
- form bone matrix and vesicles rich in enzymes which preserve the integrity of the bone matrix
Osteoclast
Origin :
- monocytes
Sites
- On inner surface of bone where resorption occur :
- Endosteum
- Bone marrow spaces
- Medullary cavities
- N.B: Each cell lies in a shallow cavity called “Howships Lacuna”
Light microscope :
- Shape: irregular , large cell with striated or brush border facing bone surface
- nuclei: multinuclated cell “4-50”
- Cytoplasm : foamy acidophilic
Electron microscope :
- Striated (ruffled) zone.: Numerous finger
like processes projecting from
- the cytoplasm is rich in mitochondria &
lysosomes vesicles
Function
- bone resorption during ossification causing remodeling of the bone
- remove bone debris after fracture healing
Bone Matrix
General character
- consistency is hard or solid
- blood supply is rich
General structure
Organic components:
- Represents about 50 %of the dry weight of the bone matrix
- Include :
- Sulphated GLUCOSAMINOGLYCAN (GAGs)
- Glycoprotein = Osteonectin Which anchor cells with bone matrix
- Protein = Osteocalcin : It helps bone calcification
- collagen “Type I” 90% of the organic component
Inorganic component
- Represent about 50% of the dry weight of bone matrix.
- include :-
- salts that form insoluble crystals called “Hydroxy apitate crystals”
- mainly of calcium and phosphorus salts.
- In small amounts Sodium, carbonate, citrate, iron. Mg.
Types of bones
1- compact bone
- It is solid ivory like with no apparent holes
- It is formed of bone lamellae in Regular pattern
- Outer “external” circumferential lamellae
- Inner “Internal” circumferential! Lamellae
- Haversian system = Osteon Interstitial “Inter- Haversian” Lamella
Sites :
- skull Outer & inner tables of the flat bones
- long bones : Shaft
- vertebrae & ribs: Outer covering
2- cancellous bone
- It is like - sponge with many holes
- irregularly arranged bars or trabeculae that branch & anastmose
- trabeculae; are separated by bone marrow spaces of irregular shape and size that are filled with activated red bone marrow
Sites :
- skull: Central part of flat bones
- long bones :Epiphysis
- vertebrae and sternum : Bodies
- Young embryonic bone
Compact bone structure
1- Periostium
Definition
- It is a vascular C.T. bone the covering membrane from outside
Structure
Outer fibrous layer
- Rich in collagenous fibers , blood vassels &fibroblast
Inner osteogenic layer
- Osteogenic of formed spindle-shaped cells change into when activated osteoblast
Function :
- Provide attachment for muscles, ligaments & tendons.
- Provide bone with blood supply and nourishment
- Inner osteogenic layer is important for formation of bone during its growth and after its fracture
2- endostium
Definition
- It is a vascular C.T. membrane that lines the inner surface of the bone, bone marrow cavities and Haversian canals
Structure
- Rich in
- Bone cells : osteogenic cells, osteoblasts, osteoclasts
- Blood vessel
Function :
- Provide bone with blood supply & nourishment
- Bone cells bone formation and resorption during growth & healing after fracture
3-bone matrix & bone cells
- Bone lamellae are deposited in
regular pattern as follow
- Outer “external” circumferential lamellae: present
under the periosteum and parallel to it.
- Inner “Internal” circumferential Lamellae :present under the
endosteum and parallel to the medullary bone cavity, which contains the bone
marrow
- Haversian system = Osteon : the
structural unit of the bones It , It is
formed of:
A- Haversian canal
- It runs parallel to \ longitudinal axis of the bone
- It contains loose C.T., rich in blood Vessels, osteogenic cells
B- Concentric bone lamella
- 4-20 cylinders of concentric bone lamellae arranged around Haversian canal
C- Osteocytes :
- Found inside their lacuna in between the bone lamellae
Inter Haversian” Interstitial Lamellae “:
- Formed of calcified osteoid tissue present between Haversian systems
- Osteocytes are irregularly arranged between these lamellae
Volkman’s canals
- Transverse or oblique canals
- Connect Haversian canals together and with periosteal or endosteal blood vessels
- Lined with osteogenic cells and contain blood vessel
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