Chapter 6: Bones and Bone Structure

6-1 Functions of the Skeletal System

The skeletal system is a complex framework that provides structure, protection, and support for the human body. It is composed 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, primarily calcium and phosphorus, as well as lipids in yellow bone marrow.

• Blood Cell Production: Red bone marrow produces red blood cells, white blood cells, and platelets in a process called hematopoiesis.

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

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

6-2 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. Their number varies among individuals.

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

• Short Bones: Boxy in appearance, 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 can vary.

Bone Markings (Surface Features)

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

• 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

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

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

• Metaphysis: The narrow region 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).

6-3 Bone Tissue

Characteristics of Bone (Osseous) Tissue

• Dense Matrix: Composed of protein fibers and deposits of calcium salts.

• 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.

The bone matrix is two-thirds calcium phosphate (as hydroxyapatite crystals) and one-third collagen fibers. The crystals provide hardness and resistance to compression, while collagen fibers offer flexibility and tensile strength.

Types of Bone Cells

• 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 (bone formation). They secrete osteoid, the organic matrix, and promote mineralization. Once surrounded by matrix, they become osteocytes.

• Osteocytes: Mature bone cells in lacunae, maintaining the bone matrix and aiding in repair. They have cytoplasmic extensions through canaliculi.

• Osteoclasts: Large, multinucleate cells that break down bone matrix (osteolysis) by secreting acids and enzymes. They are not related to osteogenic cells.

6-4 Compact Bone and Spongy Bone

Compact Bone Structure

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

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

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

• Lacunae: Spaces housing osteocytes between lamellae.

Spongy Bone Structure

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

• Red Bone Marrow: Fills spaces between trabeculae, produces blood cells.

• Yellow Bone Marrow: Stores fat in some spongy bone.

• Lightweight and Stress-Resistant: Spongy bone reduces bone weight and withstands stress from multiple directions.

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 the medullary cavity, trabeculae, and central canals; contains osteogenic cells, osteoblasts, and osteoclasts; active in bone growth, repair, and remodeling.

6-5 Bone Formation and Growth

Ossification and Calcification

• Ossification (Osteogenesis): The process of bone formation.

• Calcification: Deposition of calcium salts, necessary for ossification.

There are two main forms of ossification:

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

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

Bone Growth

• Interstitial Growth: Growth in length at the epiphyseal (growth) plate; new cartilage forms on the epiphyseal side and is replaced by bone on the diaphyseal side. Growth stops with epiphyseal closure, leaving an epiphyseal line.

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

Intramembranous Ossification (Dermal Ossification)

• Occurs in the deeper layers of the dermis, forming dermal bones.

• Mesenchymal stem cells differentiate into osteoblasts, which secrete osteoid and promote mineralization.

• Osteoblasts become osteocytes as they are trapped in matrix; bone grows outward in small struts (spicules).

• Blood vessels grow into the area, supporting further bone growth and forming spongy bone.

Blood and Nerve Supply to 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.

6-6 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 throughout life.

• Balanced Activity: Normally, osteoblast and osteoclast activity is balanced. Imbalance can lead to weakened or excessively strong bones.

6-7 Exercise, Nutrition, and Hormones

Effects of Exercise

• Physical activity and weight-bearing exercise stimulate bone remodeling, making bones thicker and stronger.

• Inactivity leads to rapid loss of bone mass.

Nutrients and Hormones Essential for Bone Health

• 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, resulting in short bones.

• Gigantism: Excess growth hormone before puberty, resulting in tall stature.

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

• Marfan Syndrome: Excessive cartilage formation, resulting in tall, slender limbs.

6-8 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: Bones become weak and flexible due to poor mineralization.

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

6-9 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.

6-10 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 and increased fracture risk. Accelerates after menopause in women and can be a secondary effect of cancer.