Bone Classification and Structure

Bone Classification and Structure

• The adult human skeleton comprises 206 bones.
• There are two types of bone tissue: compact bone and spongy bone.
• Compact bone is dense and appears smooth and homogeneous.
• Calcium ions and salts are deposited in compact bone, contributing to its hardness.
• Spongy bone has a spiky, open structure resembling a sponge.
• Even though it appears porous, spongy bone is not soft.
• The holes in spongy bone are called Haversian canals and contain blood vessels, allowing them to pass through.
• Children's bones contain red marrow until the age of 6 or 7 when it is gradually replaced with yellow marrow.
• In adult bones, red marrow is confined to cavities in the spongy bone of the axial skeleton, hip bones, and the ends of long bones
• Bone is lightweight and resists tension and other forces.
• Organic components (collagen fibers) provide flexibility and tensile strength to bone.
• Calcium salts deposited in the bone matrix make it hard and resist compression.

Microscopic Anatomy of Bone

• Under a microscope, spongy bone is composed of small needlelike pieces of bone called trabeculae. These trabeculae enclose spaces which contain marrow, blood vessels, and nerves.
• In compact bone, mature bone cells (osteocytes) are located in tiny cavities within the bone matrix called lacunae.
• Lacunae are arranged in concentric circles called lamellae, surrounding central canals (also known as Haversian canals).
• An osteon or Haversian System is a complex consisting of a central canal, lamellae, and lacunae. It is the structural and functional unit of compact bone.
• Central canals run lengthwise through the bone matrix, carrying blood vessels and nerves to all parts of the bone.
• Tiny canals called canaliculi radiate outward from the central canals to all lacunae, forming a transportation system connecting bone cells for nutrient supply and waste removal.
• Perforating canals (Volkmann's canals) run perpendicular to the central canals, completing the communication pathway from the bone's exterior to its interior.

Bone Formation, Growth, and Remodeling

• Ossification is the process of bone formation.
• It occurs on hyaline cartilage models or fibrous membranes.
• Long bone growth has two main phases.
• During embryonic development, the skeleton is primarily made of hyaline cartilage.
• In young children, most cartilage is replaced by bone, remaining in isolated areas, including the bridge of the nose, parts of the ribs, and joints.

Phases of Long Bone Growth

• Osteoblasts (bone-forming cells) cover the hyaline cartilage model with bone matrix.
• Hyaline cartilage is completely covered by bone matrix, forming a bone "collar."
• During fetal development, cartilage is replaced by bone, leaving two regions in long bones: articular cartilages and epiphyseal plates.
• Articular cartilages cover bone ends.
• Epiphyseal plates are responsible for the elongation of long bones.

Bone Growth and Remodeling

• New cartilage forms continuously on the external face of articular cartilages and epiphyseal plates.
• Old cartilage is then broken down and replaced with bone matrix.
• Osteoblasts in the periosteum add bone matrix to the outside of the diaphysis, causing the bone to widen.
• Osteoclasts in the endosteum remove bone from the inner surface of the diaphysis, enlarging the medullary cavity.
• These processes happen at a similar rate, leading to an increase in long bone circumference.
• Bone growth is controlled by hormones like growth hormone and sex hormones.
• Bone growth ends during adolescence when epiphyseal plates are fully converted to bone.
• Parathyroid hormone ensures sufficient calcium in the blood.
• Bone is a dynamic tissue constantly undergoing remodeling.
• Bone remodeling is a response to changes in calcium ion levels in the blood and the migration of osteoblasts and osteoclasts.

Fracture Repair

• Repair of bone fractures involves four major events: hematoma formation, fibrocartilage callus formation, bony callus formation, and bone remodeling.
• When a bone breaks, blood vessels are ruptured, leading to a blood-filled swelling (hematoma).
• The hematoma deprives bone cells of nutrients, causing their death.
• A fibrocartilage callus forms consisting of cartilage matrix and bone matrix, acting as a splint.
• New capillaries and phagocytes replace dead tissue.
• The fibrocartilage callus is gradually replaced by a bony callus made of spongy bone.
• Bone remodeling occurs in response to mechanical stresses, forming a strong, permanent patch at the fracture site.

The Axial Skeleton

• The axial skeleton is divided into three parts: the skull, the vertebral column, and the thoracic cage.