Bones and Bone Structure: Study Notes for Anatomy & Physiology

Bones and Bone Structure

Functions of the Skeletal System

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

• Structural support: Provides the rigid framework that supports the body and maintains its shape.

• Storage of minerals and lipids: Bones store minerals such as calcium and phosphate, and lipids in yellow bone marrow.

• Blood cell production: Red bone marrow produces red blood cells, white blood cells, and platelets (hematopoiesis).

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

• Leverage for movement: Bones act as levers for muscles, enabling movement.

Classification of Bones

Bone Classification by Shape

Bones are classified according to their shapes, which relate to their functions and locations 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 (e.g., vertebrae, pelvic bones).

• Short bones: Boxy in appearance (e.g., carpal and tarsal bones).

• Flat bones: Thin with parallel surfaces (e.g., skull roof, sternum, ribs, scapulae).

• Long bones: Long and slender (e.g., humerus, femur, bones of arms, legs, palms, soles, fingers, toes).

• 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 anatomical features on the surface of bones that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels.

• Projections: Sites for attachment or articulation (e.g., process, ramus).

• Openings and depressions: Allow passage of blood vessels and nerves (e.g., foramen, canal, fissure, sinus, meatus, fossa, sulcus).

Structure of a Long Bone

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, made mostly of spongy bone (trabecular bone).

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

Structure of a Flat Bone

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

Bone Tissue (Osseous Tissue)

Composition and Characteristics

Bone tissue is a dense connective tissue composed of specialized cells embedded in a solid extracellular matrix.

• 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.

• Osteocytes: Mature bone cells located in lacunae, connected by canaliculi for nutrient and waste exchange.

• Periosteum: A membrane covering the outer surface of bones (except at joints), consisting of outer fibrous and inner cellular layers.

Types of Bone Cells

Bone contains four main types of cells, each with distinct functions:

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

• 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 that maintain the 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

Structure of Compact Bone

Compact bone is dense and forms the outer layer of bones. Its basic unit is the osteon (Haversian system).

• Osteon: Contains a central canal with blood vessels, surrounded by concentric lamellae of bone matrix.

• Perforating (Volkmann's) canals: Perpendicular to the surface, connect blood vessels of osteons.

• Lamellae: Concentric, interstitial, and circumferential types.

• Osteocytes: Located in lacunae between lamellae.

Structure of Spongy Bone

Spongy bone (cancellous bone) is found at the ends of long bones and inside flat bones. It is lighter and less dense than compact bone.

• Trabeculae: Network of struts and plates; no osteons present.

• Red bone marrow: Fills spaces between trabeculae; site of blood cell production.

• Yellow bone marrow: Stores fat in some spongy bone.

Functional Integration

Compact and spongy bone work together to support weight and resist forces. For example, in the femur, spongy bone in the epiphysis transfers weight to the compact bone shaft, which resists compression and tension.

Surface Coverings of Bone

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

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

Bone Formation and Growth

Ossification and Calcification

Ossification (osteogenesis) is the process of bone formation. 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 (e.g., flat bones of the skull).

Endochondral Ossification

Endochondral ossification involves several steps, beginning with a cartilage model and ending with mature bone.

1. Chondrocytes enlarge and die as the matrix calcifies.

2. Blood vessels invade the perichondrium; osteoblasts form a bone collar.

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

4. Medullary cavity forms as bone remodeling occurs.

5. Secondary ossification centers form in the epiphyses.

6. Epiphyses fill with spongy bone; epiphyseal plate remains for growth.

7. At puberty, epiphyseal closure occurs, leaving an 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 stops with epiphyseal closure (epiphyseal line remains).

• Appositional growth: Growth in width; osteoblasts add layers to the outer surface, while osteoclasts enlarge the medullary cavity.

Intramembranous Ossification

Intramembranous ossification forms dermal bones (e.g., flat bones of the skull, mandible, clavicles) directly from mesenchymal tissue.

1. Mesenchymal cells differentiate into osteoblasts and secrete osteoid, which calcifies.

2. Osteoblasts become osteocytes; bone grows in spicules.

3. Blood vessels grow into the area; spicules interconnect, trapping vessels.

4. Spongy bone forms; compact bone develops on the surface.

5. Periosteum forms from connective tissue on the surface.

Bone Remodeling and Homeostasis

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.

• Balanced activity maintains bone strength.

• Excess osteoclast activity weakens bones; excess osteoblast activity strengthens bones.

Exercise, Nutrition, and Hormones

Effects on Bone Development

• Exercise: Stimulates bone remodeling; weight-bearing exercise increases bone strength.

• Nutrition: Adequate intake of calcium, phosphorus, magnesium, fluoride, iron, and manganese is essential.

• Vitamins: Vitamin D (calcitriol) for calcium absorption; vitamin C for collagen synthesis; vitamins A, K, and B12 for osteoblast activity.

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

Abnormal bone development: Includes pituitary growth failure, gigantism, acromegaly, and Marfan syndrome.

Calcium Homeostasis

The Role of Calcium in the Skeletal System

Bones store 99% of the body's calcium, which is vital for physiological processes such as nerve impulse transmission and muscle contraction.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and reducing kidney excretion.

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

Osteomalacia: Weak, flexible bones due to poor mineralization (rickets is a form caused by vitamin D deficiency).

Fractures and Bone Repair

Types of Fractures

• 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, Pott’s fracture.

Steps in Fracture Repair

1. Fracture hematoma formation: Blood clot forms at the site; bone cells die.

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

3. Spongy bone formation: Osteoblasts replace cartilage with spongy bone.

4. Compact bone formation: Remodeling restores bone to near-original shape; repaired bone may be slightly thicker.

The Effects of Aging on Bones

Osteopenia and Osteoporosis

• Osteopenia: Inadequate ossification (bone mass reduction) begins between ages 30–40; more pronounced in women.

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