Bones and Bone Structure

Introduction to the Structure and Growth of Bones

The skeletal system is a dynamic organ system essential for support, movement, protection, and mineral storage. It is divided into two main divisions and consists of various bone types, each with unique markings and internal structures.

• Axial skeleton: Composed of 80 bones, including the skull, thorax, and vertebral column. It forms the longitudinal axis of the body.

• Appendicular skeleton: Composed of 126 bones, including the limbs and girdles that attach them to the axial skeleton.

• Functions of the skeletal system:

  • Support

  • Mineral and lipid storage

  • Blood cell production (hematopoiesis)

  • Protection of internal organs

  • Leverage for movement

Classification of Bones and Bone Markings

Bones are classified by shape and surface features, which relate to their function and anatomical location.

• Flat bones: Thin, parallel surfaces (e.g., cranial bones, sternum, ribs, scapulae). Provide protection and muscle attachment.

• Sutural bones: Irregular bones between cranial bones; variable in size and number.

• Long bones: Long and slender (e.g., limb bones).

• Irregular bones: Complex shapes (e.g., vertebrae, pelvis, facial bones).

• Sesamoid bones: Small, flat, develop in tendons (e.g., patella).

• Short bones: Small, boxy (e.g., carpals, tarsals).

Bone markings are surface features that serve as attachment sites or passageways for nerves and blood vessels.

• Elevations/Projections: Process, tubercle, tuberosity, trochlea, condyle, trochanter, facet, crest, line, spine, ramus.

• Depressions/Grooves/Tunnels: Canal (meatus), sinus, foramen, fissure, sulcus, fossa.

Structure of a Typical Long Bone

Long bones are specialized for transmitting forces and have a complex internal structure.

• Epiphysis: Expanded ends, mostly spongy bone covered by compact bone; articular cartilage covers joint surfaces.

• Metaphysis: Connects epiphysis to diaphysis.

• Diaphysis: Shaft containing the medullary (marrow) cavity, which holds red marrow (hematopoiesis) and yellow marrow (fat storage).

• Blood supply: Nutrient arteries/veins, metaphyseal arteries/veins, and periosteal vessels supply bone tissue.

• Innervation: Sensory nerves supply the periosteum, diaphysis, and epiphyses.

Bone Cells and Bone Matrix

Bone tissue is maintained by four main cell types, each with distinct roles in bone formation, maintenance, and remodeling.

• Osteogenic cells: Stem cells that differentiate into osteoblasts; important for growth and repair.

• Osteoblasts: Produce new bone matrix (osteoid) and initiate mineralization; become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells that resorb bone matrix, releasing minerals (osteolysis).

Bone matrix: Composed of collagen fibers (flexibility) and hydroxyapatite crystals (strength).

Compact Bone vs. Spongy Bone

Bone tissue is organized into two main types, each with unique structural and functional properties.

• Compact bone: Dense, organized into osteons (Haversian systems) with concentric lamellae around a central canal. Strong along its length.

• Spongy bone: Network of trabeculae; spaces filled with red marrow; no osteons; nutrients diffuse through canaliculi.

Appositional Bone Growth

Appositional growth increases the diameter of bones by adding new layers under the periosteum.

• Osteogenic cells in the periosteum differentiate into osteoblasts, adding circumferential lamellae.

• Osteoclasts enlarge the medullary cavity by resorbing bone from the inner surface.

• Periosteum: Outer fibrous and inner cellular layers; isolates bone, provides route for vessels/nerves, and participates in growth/repair.

• Endosteum: Incomplete cellular layer lining the medullary cavity; active in growth and remodeling.

Endochondral Ossification

Most bones form by replacing a hyaline cartilage model through endochondral ossification.

1. Cartilage model enlarges; chondrocytes die, leaving cavities.

2. Blood vessels invade perichondrium; osteoblasts form superficial bone.

3. Blood vessels penetrate cartilage; primary ossification center forms spongy bone.

4. Medullary cavity forms; bone grows in length and diameter.

5. Secondary ossification centers form in epiphyses.

6. Epiphyses fill with spongy bone; articular cartilage and epiphyseal plate remain.

7. At puberty, epiphyseal plate closes, leaving an epiphyseal line.

Intramembranous Ossification

Some bones (e.g., skull, clavicle) form directly from mesenchymal tissue without a cartilage model.

1. Mesenchymal cells differentiate into osteoblasts, forming ossification centers.

2. Osteoid matrix is secreted and mineralized; osteoblasts become osteocytes.

3. Bone grows as spicules; blood vessels invade and become trapped.

4. Spongy bone forms; remodeling produces compact bone and periosteum.

Abnormalities of Bone Growth

Disorders of bone growth can result in characteristic physical signs and are often related to hormonal or genetic factors.

• Pituitary growth failure: Inadequate growth hormone; short bones.

• Achondroplasia: Slow epiphyseal cartilage growth; short limbs, normal trunk.

• Marfan syndrome: Excessive cartilage formation; tall, slender limbs; cardiovascular risks.

• Gigantism: Excess growth hormone before puberty; extreme height.

• Acromegaly: Excess growth hormone after epiphyseal closure; thickened bones, especially in face, jaw, hands.

• Fibrodysplasia ossificans progressiva (FOP): Bone forms in muscles and connective tissue.

• Congenital talipes equinovarus (clubfoot): Abnormal muscle development distorts bones of the feet.

Physiology of Bones

Mineral Storage and Calcium Homeostasis

Bones act as reservoirs for minerals, especially calcium and phosphate, which are vital for physiological processes.

• Calcium: Most abundant mineral; essential for muscle contraction, blood clotting, and nerve function.

• Calcium levels are regulated by the intestines (absorption), bones (storage/release), and kidneys (excretion).

Hormonal Regulation of Calcium Metabolism

Calcium homeostasis is maintained by three primary hormones:

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, enhancing intestinal absorption (via calcitriol), and reducing renal excretion.

• Calcitriol: Active form of vitamin D; increases intestinal absorption of calcium.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts, decreasing intestinal absorption, and increasing renal excretion.

The skeleton serves as a calcium reserve, and the balance between deposition and resorption affects bone strength.

Bone Fractures and Repair

Fractures are breaks in bone due to mechanical stress. Healing involves a sequence of events:

1. Fracture hematoma formation: Blood clot forms at the site.

2. Callus formation: Internal (spongy bone) and external (cartilage and bone) calluses stabilize the fracture.

3. Spongy bone formation: Cartilage is replaced by spongy bone; dead bone is removed.

4. Compact bone formation: Spongy bone is remodeled into compact bone; original shape is restored.

Types of fractures:

• Closed (simple): Internal, no skin break.

• Open (compound): Bone projects through skin; risk of infection.

• Transverse: Break across long axis.

• Spiral: Produced by twisting.

• Displaced/Nondisplaced: Abnormal/normal alignment.

• Compression: Vertebral collapse, often in osteoporosis.

• Greenstick: Incomplete break, common in children.

• Comminuted: Bone shatters into fragments.

• Epiphyseal: At growth plate; may affect growth.

• Pott’s fracture: At ankle, both malleoli.

• Colles fracture: Distal radius break.

Key Equation:

Hydroxyapatite formation in bone matrix:

Example: A child with pituitary growth failure will have shorter bones due to reduced growth hormone, while a person with Marfan syndrome will have abnormally long limbs due to excessive cartilage formation at growth plates.