10/13/26 Objectives: Bone Development & Clinical Application

Osteoblasts

matrix-synthesizing cell, responsible for bone growth

Osteocytes

mature bone cell, monitors and maintains the mineralized bone matrix

Osteoclasts

none-resorbing cell

Two types of bone ossification

• Intramembranous (flat bones of the skull + parts of clavicle and mandible)

• Endochondral

Intramembranous ossification step 1

Mesenchymal cells cluster around a vessel and become osteoblasts, osteoblasts secrete osteoid and an ossification center forms in fibrous connective tissue

Intramembranous ossification step 2

Osteoblasts grow and continue to secrete osteoid, trapped osteoblasts now become osteocytes

Intramembranous ossification step 3

Osteoid continues to form around vessels forming an immature spongy bone, mesenchyme adheres to the external surface and becomes the periosteum

Intramembranous ossification step 4

Immature spongy bone near the periosteum is remodeled and replaced with compact bone, central immature spongy bone matures

Endochondral ossification step 1

Mesenchymal cells specialize into osteoblasts, perichondrium becomes periosteum, osteoblasts secrete osteoid creating a bone collar, chondrocytes left within hypertrophy and become primary ossification center

Endochondral ossification step 2

Chondrocytes from the primary ossification center ossify surrounding cartilage, chondrocytes die and matrix begins to dissolve, cavities are now formed within the model, remaining cartilage (ends) are healthy and growing rapidly to elongate the model

Endochondral ossification step 3

Cavities are invaded by the periosteal bud, osteoclasts partially erode the cartilage matrix, osteoprogenitor cells become osteoblasts (spongy bone formation!!)

Endochondral ossification step 4

Osteoclasts break down the newly formed spongy bone and open the cavity, the epiphyses stay as cartilage ends through birth (growing rapidly), after birth secondary ossification centers develop at epiphyses

Endochondral ossification step 5

Secondary ossification mimics that of primary except the spongy bone remains and no cavity is formed, the space between the diaphysis and epiphysis is the epiphyseal plate and is hyaline cartilage

Postnatal growth - Length step 1

Proliferation zone “grove zone”: Chondrocytes rapidly divide and older chondrocytes get pushed down

Postnatal growth - Length step 2

Hypertrophic zone: Older chondrocytes enlarge lacunal implode leaving empty spaces

Postnatal growth - Length step 3

Clarification zone: Surrounding matrix calcifies, chondrocytes die

Postnatal growth - Length step 4

Ossification: Bone marrow elements invade, blasts rebuild new bone, clasts breakdown cartilage

Postnatal growth - Width step 1

Osteoblasts in the periosteum secrete bone matrix to the external bone surface (outer)

Postnatal growth - Width step 2

Osteoclasts on the endosteal surface (inner) remove bone

Parathyroid hormone

Indirectly activates osteoclasts, which resorb bone and raise blood Ca2+ concentration; inhibits urinary Ca2+ excretion; promotes calcitriol synthesis

Calcitriol

Promotes intestinal absorption of Ca2+ and phosphate; reduces urinary excretion of both; promotes both resorption and mineralization; stimulates osteoclast activity

Calcitonin

Promotes mineralization and lowers blood Ca2+ concentration in children, but usually has little effect in adults; may prevent bone loss in pregnant and lactating women

Estrogen & testosterone

Increase osteoblast activity → build bone

Growth Hormone

Stimulates bone elongation and cartilage proliferation at epiphyseal plate; increases urinary excretion of Ca2+ but also increases intestinal

Mechanical stress

Wolff’s law → The body adapts to the forces placed upon it

Fracture repair step 1

Hematoma formation (24-48 hrs): The hematoma is converted to granulation tissue by invasion of cells and blood capillaries

Fracture repair step 2

Soft callus formation: Deposition of collagen and fibrocartilage converts granulation tissue to a soft callus → fibroblasts lay down collagen

Fracture repair step 3

Hard callus formation: Osteoblasts deposit a temporary bony collar around the fracture to unite the broken pieces while ossification occurs

Fracture repair step 4

Bone remodeling: Small bone fragments are removed by osteoclasts, while osteoblasts deposit spongy bone and the convert it to compact bone

Fracture repair length: steps 1-3

3-8 weeks

Fracture repair length: step 4

3-4 months

Non-Displaced fracture

touching

Displaced fracture

not touching

Incomplete fracture

part of the way through

Complete fracture

all the way through

Closed fracture

still inside the skin

Open fracture

through the skin

Bone disorders - Osteoporosis

Group of diseases in which bone resorption outpaces bone deposit (clast>blast),

• 1 in 3 women over 50 will have a fracture due to osteoporosis

• Risk factors; low calcium & vit D, sedentary lifestyle, parathyroid disorder, menopause (low estrogen), genetic

Bone disorders - Osteomalacia

Group of diseases in which bones are poorly mineralized

• Results in weak and soft bones (calcium salts not sufficient)

• Chickhood version is Rickets: bowed legs, deformities to the skull, pelvis, and ribs

• Scurvy in adults

Bone disorders - Osteitis deformans (Paget's disease)

Haphazard bone deposit and resorption

• Abnormally high ratio of spongy bone

• Commonly at the spine, pelvis, femur, and skull

• 3% if elderly North Americans, rare before 40