Bones and Bone Structure

Functions of the Skeletal System

The skeletal system is a complex framework that provides support, protection, and movement for the human body. It consists of bones, cartilages, ligaments, and other connective tissues that stabilize and interconnect the bones.

• Structural Support: The skeleton forms the internal framework that supports the body and maintains its shape.

• Mineral and Lipid Storage: Bones store essential minerals such as calcium and phosphate, as well as lipids in yellow bone marrow.

• Blood Cell Production: Red bone marrow produces red blood cells, white blood cells, and platelets.

• Protection: Bones protect vital organs (e.g., skull protects the brain, ribs protect the heart and lungs).

• Leverage for Movement: Bones act as levers that muscles pull on to produce movement.

Classification of Bones

Bones are classified according to their shapes and structures, each adapted for specific functions in the body.

• Sutural Bones (Wormian Bones): Small, flat, irregular bones found between the flat bones of the skull. Their number varies among individuals.

• Irregular Bones: Complex shapes, such as vertebrae and pelvic bones.

• Short Bones: Boxy in shape, found in the carpal (wrist) and tarsal (ankle) bones.

• Flat Bones: Thin with parallel surfaces, such as the skull roof, sternum, ribs, and scapulae.

• Long Bones: Long and slender, found in the arms, legs, palms, soles, fingers, and toes (e.g., humerus, femur).

• Sesamoid Bones: Small, round, and flat bones found within tendons near joints (e.g., patella). Their number and location vary among individuals.

Bone Markings (Surface Features)

Bone markings are structural features adapted for specific functions, such as muscle attachment or passage of nerves and blood vessels.

• Projections: Sites for muscle, tendon, and ligament attachment or articulation with other bones.

• Openings and Depressions: Allow passage of blood vessels and nerves.

Structure of Bones

Long Bone Structure

Long bones have a characteristic structure that supports their function in movement and weight-bearing.

• Diaphysis: The tubular shaft, composed of compact bone surrounding a medullary (marrow) cavity.

• Epiphysis: The expanded ends, primarily made of spongy bone (trabecular bone).

• Metaphysis: The narrow region connecting the diaphysis and epiphysis.

Flat Bone Structure

Flat bones consist of a core of spongy bone (diploë) sandwiched between two layers of compact bone (cortex).

Bone Tissue (Osseous Tissue)

Bone tissue is a specialized connective tissue with a dense matrix and specialized cells.

• Matrix: Two-thirds is calcium phosphate (as hydroxyapatite crystals), providing hardness and resistance to compression. One-third is collagen fibers, providing flexibility and tensile strength.

• Cells: Osteocytes reside in lacunae, connected by canaliculi for nutrient and waste exchange. The periosteum covers bone surfaces (except at joints) and consists of outer fibrous and inner cellular layers.

Types of Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells that divide to produce osteoblasts; important for fracture repair. Located in periosteum and endosteum.

• Osteoblasts: Immature bone cells responsible for osteogenesis (production of new bone matrix). They secrete osteoid and promote calcification. Once surrounded by matrix, they become osteocytes.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and help repair damaged bone. They do not divide.

• Osteoclasts: Large, multinucleate cells that resorb bone matrix (osteolysis) by secreting acids and enzymes. Not related to other bone cell lineages.

Compact Bone and Spongy Bone

Compact Bone

Compact bone is dense and forms the outer layer of bones, providing strength for weight-bearing.

• Osteon: The basic functional unit, consisting of concentric lamellae around a central canal containing blood vessels.

• Lamellae: Layers of bone matrix; can be concentric (osteons), interstitial (between osteons), or circumferential (outer/inner surfaces).

• Perforating (Volkmann's) Canals: Perpendicular channels for blood vessels and nerves.

Spongy Bone

Spongy bone consists of a network of trabeculae, making bones lighter and able to withstand stress from multiple directions. It supports and protects red bone marrow and, in some bones, contains yellow bone marrow for fat storage.

Functional Integration

In long bones like the femur, spongy bone in the epiphyses transfers weight to the compact bone shaft, with the medial shaft under compression and the lateral shaft under tension.

Surface Coverings

• Periosteum: Outer membrane with fibrous and cellular layers; isolates bone, provides a route for blood vessels/nerves, and participates in growth and repair. Perforating (Sharpey's) fibers strengthen attachment to tendons and ligaments.

• Endosteum: Incomplete cellular layer lining inner surfaces (medullary cavity, trabeculae, central canals); contains osteogenic cells, osteoblasts, and osteoclasts; active in growth, repair, and remodeling.

Bone Formation and Growth

Bone formation (ossification) is the process of replacing other tissues with bone. Calcification is the deposition of calcium salts, necessary for ossification. Two main mechanisms exist:

• Endochondral Ossification: Bone replaces a cartilage model. Most bones form this way.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue, forming dermal bones (e.g., flat bones of the skull).

Endochondral Ossification Steps

1. Chondrocytes enlarge and the matrix calcifies; chondrocytes die.

2. Blood vessels grow around cartilage; perichondrium cells become osteoblasts, forming a bone collar.

3. Blood vessels penetrate cartilage; fibroblasts become osteoblasts, forming spongy bone at the primary ossification center.

4. Remodeling creates a marrow cavity; bone thickens, cartilage at epiphyses replaced by bone.

5. Secondary ossification centers form in epiphyses.

6. Epiphyses fill with spongy bone; epiphyseal plate separates diaphysis and epiphysis.

7. At puberty, ossification outpaces cartilage growth, leading to epiphyseal closure and formation of the epiphyseal line.

Bone Growth

• Interstitial Growth: Growth in length at the epiphyseal plate; new cartilage forms on the epiphyseal side and is replaced by bone on the diaphyseal side. Growth ends with epiphyseal closure (visible as epiphyseal lines on X-rays).

• Appositional Growth: Growth in width; osteoblasts add circumferential lamellae, while osteoclasts enlarge the medullary cavity.

Intramembranous Ossification Steps

1. Mesenchymal cells differentiate into osteoblasts, secrete osteoid, which calcifies to form bone matrix.

2. Osteoblasts become osteocytes; bone grows in spicules.

3. Blood vessels grow between spicules; bone growth accelerates and vessels become trapped.

4. Osteoblasts near blood vessels continue depositing bone, forming spongy bone.

5. Remodeling produces compact bone; periosteum forms from outer connective tissue.

Blood Supply to Bones

• Nutrient artery and vein: Supply the diaphysis.

• Metaphyseal vessels: Supply epiphyseal cartilages.

• Periosteal vessels: Supply superficial osteons and secondary ossification centers.

• Lymphatic vessels and sensory nerves: Present in periosteum and bone.

Bone Remodeling

Bone remodeling is the continuous recycling and renewal of bone matrix, involving osteocytes, osteoblasts, and osteoclasts. It allows bones to adapt to stress and maintain mineral homeostasis. Balance between osteoblast and osteoclast activity is essential for bone strength.

Exercise, Nutrition, and Hormones

Bone development and maintenance depend on physical activity, nutrition, and hormones.

• Exercise: Weight-bearing activities stimulate bone strength; inactivity leads to bone loss.

• Nutrition: Adequate intake of calcium, phosphorus, magnesium, fluoride, iron, and manganese is essential. Vitamin D (as calcitriol) is required for calcium absorption; vitamin C for collagen synthesis; vitamins A, K, and B12 for osteoblast activity.

• Hormones: Growth hormone, thyroxine, sex hormones, parathyroid hormone, and calcitonin regulate bone growth and calcium homeostasis.

Abnormalities include pituitary growth failure, gigantism, acromegaly, and Marfan syndrome.

Calcium Homeostasis

Bones store 99% of the body's calcium, which is vital for physiological processes such as nerve and muscle function. Calcium levels are regulated by hormones:

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

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

Disorders include osteomalacia (poor mineralization) and rickets (vitamin D deficiency).

Fractures and Bone Repair

Fractures are breaks in bones due to physical stress. They are classified by their characteristics:

• Open (Compound) Fracture: Bone projects through the skin.

• Closed (Simple) Fracture: Bone does not penetrate the skin.

• Other Types: Transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, and Pott’s fractures.

Fracture Repair Steps

1. Fracture Hematoma Formation: Blood clot forms, bone cells die.

2. Callus Formation: Internal callus of spongy bone and external callus of cartilage/bone stabilize the fracture.

3. Spongy Bone Formation: Osteoblasts replace cartilage with spongy bone.

4. Compact Bone Formation: Remodeling restores bone structure; repaired bone may be thicker/stronger.

Effects of Aging on Bones

Aging leads to decreased bone mass and increased risk of fractures.

• Osteopenia: Inadequate ossification, beginning between ages 30 and 40; more pronounced in women.

• Osteoporosis: Severe bone loss compromising function; common in postmenopausal women and older men. Can be accelerated by hormonal changes or certain cancers.