Bone and Bone Precursors

The skeleton

• Mineralized connective tissue

• Ligaments– connecting bone to bone– holds some organs in place

• Tendons– Extension of muscle and bone

Flow chart for histogenesis of skeletal tissues

Mesenchymal cells

Fibroblastcollagen Myoblast Muscle cellsScleroblast

OsteoblastChondroblast Odontoblast

Ameloblast

BoneCartilage Dentin

Enamel

Osteoprogenitors

An Example: (chondrification)

• Mesenchymal cells retract processes• Congregate

• These cells become “Chondroblasts”– Secrete matrix about themselves

– Form lacunae

– Become “Chondrocytes”

• Chondroblasts in middle– 2-4 cells within a lacunae – still “chondroblasts”

– Result from a single cell mitotically dividing.

– Secrete matrix – push away from each other – “chondrocytes”

– “interstitial growth”

Chondrification Center

An Example: (chondrification)

An Example: (chondrification)

• Mesenchyme cells on periphery– Differentiate to form fibroblasts that produce:

• Dense irregular collagenous connective tissue (perichondrium)

• Responsible for growth and maintenance of cart.

• Perichondrium - two layers– Outer fibrous layer– Inner cellular layer – mainly chondrogenic cells

» Mitosis to form chondroblasts –produce matrix» “appositional growth”

• Picture of cart.

An Example: (chondrification)

Bone• Specialized connective tissue of:

– calcified bone matrix– 3 cell types

• Osteocytes, in lacunae• osteoblasts make new matrix and maintain old matrix• osteoclasts: multinucleated giant cells phagocytose

bone matrix in remodeling bone

• Viewed as ground bone sections or decalcified and embedded paraffin sections

• Endosteum and periosteum

Bone Functions

• Support soft tissues

• Locomotion

• Protect vital organs (skull, ribs, vertebrae)

• Bone marrow (hematopoiesis)– Process of differentiation of various cells from

stem cells

• Reservoir of calcium and phosphate

Bone Matrix

• Synthesized and maintained by osteoblasts and osteocytes

• 50% of dry wgt is inorganic: calcium and phosphate

• Hydroxyapatite crystals

• Organic part is 95% type I collagen with amorphous ground substance (glycoproteins)

Membrane BoneCompact lamellar bone

Compact lamellarbone

Lamellar bone

Membrane bone