Ossification Process in Bone Formation

Ossification

• There are two ways of ossification: intramembranous and endochondral ossification
• Intramembranous ossification is a direct bone formation without a cartilage step, seen mainly in bone growth in width (appositional bone growth)
• It includes the cranial vault, facial bones, clavicles, and cortical bone

Intramembranous Ossification

• Occurs when bone needs to grow in width
• Involves the formation of cortical bone

Endochondral Ossification

• Starts with a hyaline cartilage bone model
• Chondrocytes become hypertrophic and die, creating space for blood vessels to develop
• Stem cells for osteoblasts and osteoclasts are formed, leading to the primary ossification center
• Osteoclasts remove cartilage, replaced by true bone via osteoblasts
• Creates the medullary cavity and secondary ossification centers
• Found in axial and appendicular skeleton, responsible for longitudinal bone growth

Regulation of Longitudinal Growth

• Regulated in two ways: paracrine regulation and systemic regulation
• Will be discussed later

Endochondral Bone Formation

• Found in joint cartilage and fracture healing
• Responsible for longitudinal bone growth

Differences between Articular and Growth Plate Cartilage

Articular Cartilage

• Found at the distal ends of bones
• Responsible for joint formation and motility, weight bearing, and resistant to resorption
• Diseases: osteoarthritis (arthrosis)

Growth Plate Cartilage

• Entrapped between epiphyseal and metaphyseal bone
• Responsible for longitudinal bone growth
• Disappears at the end of puberty
• Diseases: growth disorders

Non-neoplastic Pathology of Bone

Bone Fracture

• Loss of bone integrity due to mechanical injury and/or diminished bone strength
• Types: normal fracture (due to acute trauma), stress or fatigue fracture (due to repetitive mechanical stress), and pathological fracture (in weakened bone due to pre-existing lesion/tumor)

Fracture Healing

• Involves the formation of a callus, a hard mass of skeletal repair tissue that unites the fractured bone ends
• Stages: inflammatory stage, formation of granulation tissue, formation of soft callus, and remodeling to form lamellar bone

Myositis Ossificans

• Formation of bone within muscle
• Can occur after hemorrhage or tissue injury, due to metaplasia (formation of tissue where normally not seen)
• Result of reprogramming of stem cells, similar to fracture healing with proliferation of fibroblasts, infiltration of immune cells, and zonal architecture