Bones and Bone Structure

Functions of the Skeletal System

The skeletal system is a complex framework that supports and protects the body, facilitates movement, and serves as a reservoir for minerals and lipids. It is composed of bones, cartilages, ligaments, and other connective tissues that stabilize and interconnect the bones.

• Structural support: Provides the rigid framework for the body and supports soft tissues.

• Mineral and lipid storage: Stores calcium, phosphorus, and lipids in yellow bone marrow.

• Blood cell production: Hematopoiesis occurs in red bone marrow, producing red and white blood cells and platelets.

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

• Leverage: Acts as levers for muscles to produce movement.

Classification of Bones

Bones are classified by 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. 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 and legs).

• Sesamoid bones: Small, round, and flat; found within tendons near joints (e.g., patella).

Bone Markings (Surface Features)

Bones have characteristic surface features that serve as attachment points for muscles, tendons, and ligaments, or as passages for nerves and blood vessels.

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

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

Structure of a Long Bone

Long bones have a specialized 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 zone connecting the diaphysis and epiphysis.

Structure of a Flat Bone

Flat bones consist of a core of spongy bone sandwiched between two layers of compact bone. In the cranium, the spongy core is called diploë.

Bone Tissue (Osseous Tissue)

Composition and Properties

Bone tissue is a dense connective tissue composed of specialized cells and 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 in bone formation, maintenance, and remodeling.

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

• Osteoblasts: Immature bone cells responsible for osteogenesis (production of new bone matrix); secrete osteoid and promote mineralization; become osteocytes when surrounded by matrix.

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

• Osteoclasts: Large, multinucleate cells that resorb bone matrix (osteolysis); secrete acids and enzymes to dissolve bone and release minerals; not related to other bone cell lineages.

Compact Bone and Spongy Bone

Compact Bone Structure

Compact bone is organized into osteons, the basic functional units, which provide strength and support.

• Osteon: Contains a central canal with blood vessels; surrounded by concentric lamellae.

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

• Lamellae: Layers of bone matrix; can be concentric, interstitial, or circumferential.

• Lacunae: Spaces housing osteocytes between lamellae.

Spongy Bone Structure

Spongy bone lacks osteons and is composed of a network of trabeculae, making bones lighter and able to withstand stress from multiple directions.

• Trabeculae: Meshwork of supporting bundles of bone matrix.

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

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

Coordinated Functions of Compact and Spongy Bone

Compact and spongy bone work together to support weight and resist forces. For example, in the femur, trabeculae transfer weight from the hip to the shaft, with the medial shaft under compression and the lateral shaft under 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 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

Ossification and Calcification

Bone formation (ossification or osteogenesis) is the process by which new bone is produced. Calcification is the deposition of calcium salts, essential for ossification.

• Endochondral ossification: Bone replaces a cartilage model; primary method for forming long bones.

• Intramembranous ossification: Bone develops directly from mesenchymal tissue; forms flat bones of the skull, mandible, and clavicles.

Endochondral Ossification

Occurs in seven main steps, beginning with a cartilage model and ending with the formation of bone tissue and the closure of the epiphyseal plate.

Bone Growth

• Interstitial growth: Growth in length at the epiphyseal plate; new cartilage is replaced by bone until epiphyseal closure (formation of epiphyseal line).

• Appositional growth: Growth in width; osteoblasts add new bone at the surface, while osteoclasts enlarge the medullary cavity.

Intramembranous Ossification

Occurs in the deeper layers of the dermis, forming dermal bones. Mesenchymal cells differentiate into osteoblasts, which secrete matrix and form bone tissue.

Blood Supply and Innervation of Bones

• Nutrient artery and vein: Supply the diaphysis and ossification centers.

• Lymphatic vessels: Present in the periosteum and enter the bone.

• Sensory nerves: Innervate the periosteum, endosteum, medullary cavity, and epiphyses.

Bone Remodeling

Bone remodeling is the continuous recycling and renewal of bone matrix, involving osteocytes, osteoblasts, and osteoclasts. It maintains bone strength and mineral homeostasis.

• Balanced activity: Osteoblast and osteoclast activity is normally balanced; imbalance leads to bone weakening or strengthening.

Exercise, Nutrition, and Hormones

Effects of Exercise

• Physical activity stimulates bone remodeling, making bones thicker and stronger.

• Inactivity leads to rapid loss of bone mass.

Nutrients and Hormones

• Minerals: Calcium, phosphorus, magnesium, fluoride, iron, manganese.

• Calcitriol and vitamin D: Essential for calcium and phosphate absorption.

• Vitamin C: Required for collagen synthesis and osteoblast differentiation.

• Vitamins A, K, B12: Stimulate osteoblast activity and protein synthesis.

Abnormal Bone Development

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

• Gigantism: Excess growth hormone before puberty; tall stature.

• Acromegaly: Excess growth hormone after epiphyseal closure; thickened bones.

• Marfan syndrome: Excess cartilage formation; tall, slender limbs.

Calcium Homeostasis

The Skeleton as a Calcium Reserve

• Bones store 99% of the body's calcium, which is vital for physiological processes such as nerve and muscle function, blood clotting, and cellular mechanisms.

Hormonal Regulation of Calcium

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

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

Disorders of Calcium Homeostasis

• Osteomalacia: Weak, flexible bones due to poor mineralization.

• Rickets: A form of osteomalacia in children, often due to vitamin D deficiency.

Fractures

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.

Effects of Aging on Bones

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

• Osteoporosis: Severe loss of bone mass, leading to brittle bones; common in postmenopausal women and older men; can be accelerated by hormonal changes or certain cancers.

Additional info: Bone health is influenced by genetics, lifestyle, nutrition, and hormonal balance. Preventive measures include adequate calcium and vitamin D intake, regular weight-bearing exercise, and monitoring for risk factors in aging populations.