Human Bone Structure and Types

Bone Structure

• The skeleton consists of two large parts: Axial and Appendicular.
• Axial part: composed of bones of the skull, vertebral column, ribs, and sternum, forming the central line of the body.
• Appendicular part: consists of the axial part and bones of the limbs.

Axial and Appendicular Skeleton

• The union between axial and appendicular skeletons is formed by the pectoral girdle and pelvic girdle.

Types of Bones

• Long bones: examples include femur and humerus.
• Flat bones: generally used for protection or muscle attachment (e.g. sternum or parietal).
• Short bones: small bones found in some joints (e.g. carpal bones in the wrist and tarsal bones in the ankles).
• Sesamoid bones: bones embedded within tendons or muscle, used to transfer force (e.g. patella).
• Irregular bones: bones that do not occupy any of the other categories (e.g. vertebrae).

Limb Development

• Limb development begins during the 4th week of gestation, with limb buds appearing from the lateral plate mesoderm.

Bone Development

• Endochondral ossification: the process of converting a cartilage template into a long bone.
• Intra-membranous ossification: the production of flat bone from stem cells without a cartilage template.

Endochondral Ossification Process

• 5 weeks: complete cartilage template for long bones.
• 8 weeks: collar of bone starts forming around the middle of the bone.
• 12 weeks: primary ossification centre develops, causing bone development.
• At birth: secondary ossification centres form at the ends of the bone.
• During childhood: epiphyseal growth plates develop, and secondary ossification centres develop into bone.
• Adulthood: full bone length is achieved, epiphyseal growth plates fuse, and the ends of the bones remain capped with cartilage.

Descriptive Regions of Bones

• Epiphysis: ends of long bones, develop from secondary ossification centres.
• Epiphyseal plate: growth site in length, contains cartilage in actively growing bones.
• Metaphysis: junction where the diaphysis meets the epiphysis.
• Diaphysis: shaft of a long bone, represents the primary ossification centre.

Bone Cells

• Osteoblasts: cells responsible for bone formation, secrete collagen and other components for building mineralized bone matrix.
• Osteocytes: mature bone cells from osteoblasts, regulate osteoblasts and osteoclasts in response to mechanical signals.
• Osteoclasts: large cells responsible for bone resorption, essential for bone remodeling, repairing microdamage, and regulating calcium levels.

Osseous Tissues

• Compact bone: bony lamellae are strongly packed together with no space in between, more dense and rigid.
• Cancellous/Spongy/Trabecular bone: irregular bone trabeculae are loosely distributed, leaving space for bone marrow.

Haversian Systems (Osteons)

• Rings around a central canal with blood vessels and nerves for osteocytes, strengthen bones, enhance resistance and durability.

Bone Extracellular Matrix

• Organic substance: collagen provides flexibility and strength to bone, glycoproteins and proteoglycans aid in organization and stress resistance.
• Inorganic substance: primarily hydroxyapatite crystals, a calcium phosphate compound, gives bone hardness and resistance to compression.

Bone Remodelling

• Process that involves the removal of old bone tissue (resorption) and the formation of new bone tissue.

Stages of Fracture Repair

• Inflammatory phase (hematoma): blood clotting and inflammatory response begins, specialized cells rush to the injury site.
• Soft/fibrocartilaginous callus formation: fibroblasts and chondrocytes create a soft callus composed of cartilage and collagen, stabilizing the fracture site.
• Hard/bony callus formation: osteoblasts produce new bone tissue, replacing the cartilaginous callus, forming a hard bony callus made of woven bone.
• Remodeling phase: the final stage of fracture repair, where the bone is fully restored to its original shape and function.