BONE
Bone matrix BM: calcified extracellular material
Connective tissue containing osteogenic cells
surround bones:
1. Endosteum: on internal surface surrounding BM
cavity
2. Periosteum: surrounds external surface
3. Bone matrix is usually softened by immersion in a
decalcifying solution before paraffin embedding,
or embedded
Bone cells are of 3 major types:
1. Osteoblasts:
Synthesize and secrete the organic components of bone matrix which include type 1 collagen
fibers, proteoglycans, and several glycoproteins (osteonectin)
Located on surfaces of bone matrix
Have a cuboidal to columnar shape and basophilic cytoplasm
When their synthesizing activity declines, they become more flat and basophilic characteristic
declines too. They are the bone lining cells when not activated on both surfaces endosteum and
periosteum.
, Produce osteoid: not calcified layer of new material on bone matrix between osteoblast layer and
preexisting bone surface.
This process of bone appositional growth is completed by subsequent deposition of calcium salts
into the newly formed matrix.
Osteocalcin: vitamin-K dependent polypeptide secreted by osteoblasts that with various other
glycoproteins binds Ca2+ ions calcification.
Osteoblasts also release membrane-enclosed matrix vesicles rich in alkaline phosphatase PO4-
with Ca2+ form hydroxyapatite crystals which start the first step of calcification.
2. Osteocytes:
Osteoblasts differentiate further to osteocytes in the lacunae.
OSL which are found in cavities ( lacunae ) between bone
matrix layers ( lamellae ), with cytoplasmic processes
extending into small canaliculi btw lamellae ( retain Ca++
homeostasis)
Almond-shaped
Maintain bony matrix.
They communicate with one another through gap junctions on
dendritic processes in the canaliculi and on osteoblasts.
helps diffusion of metabolites with blood
Have less RER, smaller Golgi complex, and more condensed nuclear
chromatin than osteoblasts.
Their secretions help in bone remodeling.
Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of
extracellular fluid between the bone matrix and the osteocytes and their processes.
3. Osteoclasts:
, Very large, motile cells with multiple nuclei
Play a major role in matrix resorption during bone growth and remodeling.
They originate from the fusion of bone marrow-derived cells the reason they have multiple nuclei
Their development requires 2 polypeptides produced by osteoblasts:
1. Macrophage-colony-stimulating factor
2. Receptor activator of nuclear factor-kB ligand (RANKL)
They lie in depressions called resorption cavities or Howship lacunae.
It secretes collagenase, cathepsin K and other enzymes and pumps H + to produce an acidic environment
locally for dissolving hydroxyapatite and promoting the localized digestion of matrix proteins.
Its activity is regulated by signaling factors and hormones.
They have receptors for calcitonin, a thyroid hormone.
OB activated by (PTH) M-CSF, RANKL, and factors that regulate the formation and activity of OCL.
Bone Matrix:
Bone matrix BM: calcified extracellular material
Connective tissue containing osteogenic cells
surround bones:
1. Endosteum: on internal surface surrounding BM
cavity
2. Periosteum: surrounds external surface
3. Bone matrix is usually softened by immersion in a
decalcifying solution before paraffin embedding,
or embedded
Bone cells are of 3 major types:
1. Osteoblasts:
Synthesize and secrete the organic components of bone matrix which include type 1 collagen
fibers, proteoglycans, and several glycoproteins (osteonectin)
Located on surfaces of bone matrix
Have a cuboidal to columnar shape and basophilic cytoplasm
When their synthesizing activity declines, they become more flat and basophilic characteristic
declines too. They are the bone lining cells when not activated on both surfaces endosteum and
periosteum.
, Produce osteoid: not calcified layer of new material on bone matrix between osteoblast layer and
preexisting bone surface.
This process of bone appositional growth is completed by subsequent deposition of calcium salts
into the newly formed matrix.
Osteocalcin: vitamin-K dependent polypeptide secreted by osteoblasts that with various other
glycoproteins binds Ca2+ ions calcification.
Osteoblasts also release membrane-enclosed matrix vesicles rich in alkaline phosphatase PO4-
with Ca2+ form hydroxyapatite crystals which start the first step of calcification.
2. Osteocytes:
Osteoblasts differentiate further to osteocytes in the lacunae.
OSL which are found in cavities ( lacunae ) between bone
matrix layers ( lamellae ), with cytoplasmic processes
extending into small canaliculi btw lamellae ( retain Ca++
homeostasis)
Almond-shaped
Maintain bony matrix.
They communicate with one another through gap junctions on
dendritic processes in the canaliculi and on osteoblasts.
helps diffusion of metabolites with blood
Have less RER, smaller Golgi complex, and more condensed nuclear
chromatin than osteoblasts.
Their secretions help in bone remodeling.
Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of
extracellular fluid between the bone matrix and the osteocytes and their processes.
3. Osteoclasts:
, Very large, motile cells with multiple nuclei
Play a major role in matrix resorption during bone growth and remodeling.
They originate from the fusion of bone marrow-derived cells the reason they have multiple nuclei
Their development requires 2 polypeptides produced by osteoblasts:
1. Macrophage-colony-stimulating factor
2. Receptor activator of nuclear factor-kB ligand (RANKL)
They lie in depressions called resorption cavities or Howship lacunae.
It secretes collagenase, cathepsin K and other enzymes and pumps H + to produce an acidic environment
locally for dissolving hydroxyapatite and promoting the localized digestion of matrix proteins.
Its activity is regulated by signaling factors and hormones.
They have receptors for calcitonin, a thyroid hormone.
OB activated by (PTH) M-CSF, RANKL, and factors that regulate the formation and activity of OCL.
Bone Matrix:
