Bone is the main constituent of the adult skeleton.
• It confers mechanical and metabolic functions to the
skeleton • Histological examination of bone
• It is classified as a specialized connective tissue required decalcification before paraffin
composed of:
or plastic fixation followed by a
◦ Calcified Extracellular Material: Bone Matrix
◦ Three Major Cells Comprising Bone: specialized microtome sectioning
‣ Osteocytes: Found in Lacunar Cavities between
Lamellar Bones with Cytoplasmic Processes extending through canaliculi to
make contact with vessels
‣ Osteoblasts: synthesize organic components of the bone matrix
‣ Osteoclasts: multinucleate giants cells involved in bone resorption and
remodeling
• Bone are lined with layer of connective tissue containing osteogenic cells:
◦ Endosteum on the inner surface surrounding marrow cavity
◦ Periosteum on the external surface
OSTEOBLAST
• Located on the Surface of the Bone Matrix
◦ Secretes the Organic Components of not yet calcified Osteoid:
‣ Type I Collagen
‣ Proteoglycans
‣ Glycoproteins
◦ In addition they produce:
‣ Macrophage Colony Stimulate Factor(M-CSF)
‣ Receptor for activation of Nuclear Factor Kappa B(RANKL)
‣ Osteoprotegerin
‣ Osteocalcin(for bone mineralization)
‣ Osteopontin(sealant)
‣ Sialoprotein(binding of osteoblasts to extracellular matrix)
• They Resemble Simple Cuboidal Epithelium.
◦ Cuboidal/Columnar: Active, Intensely Basophilic
‣ Active Cells have prominent RER and Golgi Complex with secretory vesicles
◦ Flat: Inactive, less Basophilic(Mainly Bone Lining Cells of Endosteum and
Periosteum)
• Osteoblasts extend cytoplasmic processes through the osteoid which connect them to
neighboring osteoblasts via gap junctions.
• Osteoblasts contain receptors for PTH, Estrogen and Progesterone.
• Osteoid Calcification:
◦ Osteoblasts release:
‣ Vit-K Dependent Osteocalcin which binds Ca+
‣ Matrix Vesicles rich in Phosphate PO4- and Alkaline Phosphatases which
hydrolyze the alkali phosphate
• These ions form as a foci for the formation of Hydroxyapatite
[Ca10(PO4)6(OH)2] which serves are the bases of calcification by
accretion of more ionic minerals.
• Osteoblasts surrounded by osteoid are called Osteocytes.
OSTEOCYTE
• Flat Almond Shaped Cells with few RER, Golgi, and Clumped up Chromatin
• High Nucleus to Cytoplasm ration.
• They are:
◦ Located Between Lamella
◦ Enclosed in Lacunae and bathed in extracellular fluid.
◦ Have Cytoplasmic Processes that extend through canaliculi and make contact with
blood vessels and other osteocyte processes via gap junctions.
• Their Function:
◦ Maintenance of Bone Matrix
‣ Production of proteins such as Sclerostin
‣ Production of Cytokines that regulate bone remodeling
‣ Calcium Homeostasis via exchange with extracellular fluid through Osteocytic
osteolysis.
‣ Sensors for detection of bone stress
OSTEOCLAST
• Osteoblasts under stimulation of Para-thyroid Hormone(PTH) produce two polypeptides,
Monocyte/Macrophage Colony Stimulating Factor(M-CSF) and Nuclear Factor κB Ligand
(RANKL) which stimulate the differentiation and activation of Osteoclasts
• Osteoblasts can also prevent the activation of Osteoclasts by secreting
Osteoprotegerin which acts as a decoy ligand for RANK.
, • Calcitonin can also prevent osteoclast activation.
• Osteoclasts are large multinucleated(~50), motile cells that perform the function of
matrix resorption and remodeling and are derived from monocytes.
◦ Osteoclasts are eosinophilic
◦ They lie within resorption to Howship Lacunae/cavities
◦ They form sealed junctions via Osteopontin with Ruffled Borders(allows for
adhesion with matrix) secrete
‣ Collagenase
‣ Cathepsin K
‣ Proton Pump--> Make environment acidic
in order to break down the Hydroxyapatite and promote digestion of matrix
proteins.
BONE MATRIX
• Inorganic Components:
◦ Hydroxyapatite Crystals [Ca₁₀(PO₄)₆(OH)₂] composed of:
‣ Calcium Phosphate
‣ Magnesium
‣ Bicarbonate
‣ Potassium
‣ Citrate
‣ Sodium Ion
• Organic Components:
◦ Type I Collagen(95%)
◦ Very small amount of Type IV Collagen
◦ Proteoglycans:
‣ Chondroitin Sulfate
‣ Keratin Sulfate
‣ Hyaluronic Acid
◦ Glycoproteins
‣ Osteonectin- influences the calcification of bone by
binding calcium and Type I collagen in the bone matrix
‣ Osteocalcin
‣ Osteopontin
◦ Sialoprotein- adheres bone cells to bone matrix
PERIOSTEUM and ENDOSTEUM
• Periosteum is a connective tissue layer that covers the bone
on its surface with the exception of articulating synovial
surfaces and where muscles attach.
◦ Composition:
‣ Outer Dense Connective Tissue layer with Type I
Collagen Perforating/Sharpey Fibers attach periosteum
to the bone matrix as well as blood vessels, nerves
and lymphatics
‣ Inner Osteogenic layer(Cambium) which contains Osteoprogenitor Cells for
Appositional Growth
• Endosteum is a specialized connective tissue layer which lines the farrow cavities
and supplies osteoprogenitor cells and osteoblasts for bone growth and repair.
TYPES OF BONES
• It confers mechanical and metabolic functions to the
skeleton • Histological examination of bone
• It is classified as a specialized connective tissue required decalcification before paraffin
composed of:
or plastic fixation followed by a
◦ Calcified Extracellular Material: Bone Matrix
◦ Three Major Cells Comprising Bone: specialized microtome sectioning
‣ Osteocytes: Found in Lacunar Cavities between
Lamellar Bones with Cytoplasmic Processes extending through canaliculi to
make contact with vessels
‣ Osteoblasts: synthesize organic components of the bone matrix
‣ Osteoclasts: multinucleate giants cells involved in bone resorption and
remodeling
• Bone are lined with layer of connective tissue containing osteogenic cells:
◦ Endosteum on the inner surface surrounding marrow cavity
◦ Periosteum on the external surface
OSTEOBLAST
• Located on the Surface of the Bone Matrix
◦ Secretes the Organic Components of not yet calcified Osteoid:
‣ Type I Collagen
‣ Proteoglycans
‣ Glycoproteins
◦ In addition they produce:
‣ Macrophage Colony Stimulate Factor(M-CSF)
‣ Receptor for activation of Nuclear Factor Kappa B(RANKL)
‣ Osteoprotegerin
‣ Osteocalcin(for bone mineralization)
‣ Osteopontin(sealant)
‣ Sialoprotein(binding of osteoblasts to extracellular matrix)
• They Resemble Simple Cuboidal Epithelium.
◦ Cuboidal/Columnar: Active, Intensely Basophilic
‣ Active Cells have prominent RER and Golgi Complex with secretory vesicles
◦ Flat: Inactive, less Basophilic(Mainly Bone Lining Cells of Endosteum and
Periosteum)
• Osteoblasts extend cytoplasmic processes through the osteoid which connect them to
neighboring osteoblasts via gap junctions.
• Osteoblasts contain receptors for PTH, Estrogen and Progesterone.
• Osteoid Calcification:
◦ Osteoblasts release:
‣ Vit-K Dependent Osteocalcin which binds Ca+
‣ Matrix Vesicles rich in Phosphate PO4- and Alkaline Phosphatases which
hydrolyze the alkali phosphate
• These ions form as a foci for the formation of Hydroxyapatite
[Ca10(PO4)6(OH)2] which serves are the bases of calcification by
accretion of more ionic minerals.
• Osteoblasts surrounded by osteoid are called Osteocytes.
OSTEOCYTE
• Flat Almond Shaped Cells with few RER, Golgi, and Clumped up Chromatin
• High Nucleus to Cytoplasm ration.
• They are:
◦ Located Between Lamella
◦ Enclosed in Lacunae and bathed in extracellular fluid.
◦ Have Cytoplasmic Processes that extend through canaliculi and make contact with
blood vessels and other osteocyte processes via gap junctions.
• Their Function:
◦ Maintenance of Bone Matrix
‣ Production of proteins such as Sclerostin
‣ Production of Cytokines that regulate bone remodeling
‣ Calcium Homeostasis via exchange with extracellular fluid through Osteocytic
osteolysis.
‣ Sensors for detection of bone stress
OSTEOCLAST
• Osteoblasts under stimulation of Para-thyroid Hormone(PTH) produce two polypeptides,
Monocyte/Macrophage Colony Stimulating Factor(M-CSF) and Nuclear Factor κB Ligand
(RANKL) which stimulate the differentiation and activation of Osteoclasts
• Osteoblasts can also prevent the activation of Osteoclasts by secreting
Osteoprotegerin which acts as a decoy ligand for RANK.
, • Calcitonin can also prevent osteoclast activation.
• Osteoclasts are large multinucleated(~50), motile cells that perform the function of
matrix resorption and remodeling and are derived from monocytes.
◦ Osteoclasts are eosinophilic
◦ They lie within resorption to Howship Lacunae/cavities
◦ They form sealed junctions via Osteopontin with Ruffled Borders(allows for
adhesion with matrix) secrete
‣ Collagenase
‣ Cathepsin K
‣ Proton Pump--> Make environment acidic
in order to break down the Hydroxyapatite and promote digestion of matrix
proteins.
BONE MATRIX
• Inorganic Components:
◦ Hydroxyapatite Crystals [Ca₁₀(PO₄)₆(OH)₂] composed of:
‣ Calcium Phosphate
‣ Magnesium
‣ Bicarbonate
‣ Potassium
‣ Citrate
‣ Sodium Ion
• Organic Components:
◦ Type I Collagen(95%)
◦ Very small amount of Type IV Collagen
◦ Proteoglycans:
‣ Chondroitin Sulfate
‣ Keratin Sulfate
‣ Hyaluronic Acid
◦ Glycoproteins
‣ Osteonectin- influences the calcification of bone by
binding calcium and Type I collagen in the bone matrix
‣ Osteocalcin
‣ Osteopontin
◦ Sialoprotein- adheres bone cells to bone matrix
PERIOSTEUM and ENDOSTEUM
• Periosteum is a connective tissue layer that covers the bone
on its surface with the exception of articulating synovial
surfaces and where muscles attach.
◦ Composition:
‣ Outer Dense Connective Tissue layer with Type I
Collagen Perforating/Sharpey Fibers attach periosteum
to the bone matrix as well as blood vessels, nerves
and lymphatics
‣ Inner Osteogenic layer(Cambium) which contains Osteoprogenitor Cells for
Appositional Growth
• Endosteum is a specialized connective tissue layer which lines the farrow cavities
and supplies osteoprogenitor cells and osteoblasts for bone growth and repair.
TYPES OF BONES
