Bones and Bone Structure: Study Notes for Anatomy & Physiology

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Chapter 6: Bones and Bone Structure

6.1 Functions of the Skeletal System

The skeletal system is essential for support, movement, and protection in the human body. It consists of bones, cartilages, ligaments, and other connective tissues.

Support: Provides structural framework for the body and supports soft tissues.
Storage of Minerals and Lipids: Stores calcium, phosphorus, and lipids (in yellow bone marrow).
Blood Cell Production: Red bone marrow produces red blood cells, 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.

6.2 Classification of Bones

Bones are classified by their shape and structure, each with functional significance. Bone markings are surface features that serve as attachment points or passageways.

Sutural Bones (Wormian Bones): Small, flat, irregular bones found between skull bones; number varies among individuals.
Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).
Short Bones: Boxy in shape (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., arms, legs, palms, soles, fingers, toes).
Sesamoid Bones: Small, round, and flat; develop within tendons (e.g., patella); number and location vary.

Bone Markings

Projections: Sites for muscle, tendon, and ligament attachment; also at articulations.
Openings and Depressions: Allow passage of blood vessels and nerves.

Structure of a Long Bone

Diaphysis: Shaft; composed of compact bone surrounding the medullary (marrow) cavity.
Epiphysis: Expanded ends; mostly spongy (trabecular) bone.
Metaphysis: Region where diaphysis and epiphysis meet.

Structure of Flat Bones

Consist of spongy bone (diploë) between two layers of compact bone (cortex).
Example: Parietal bones of the skull.

6.3 Bone Tissue

Bone tissue is a dense, supportive connective tissue with specialized cells and a solid extracellular matrix reinforced by collagen fibers.

Matrix: Dense due to calcium salt deposits; contains osteocytes in lacunae organized around blood vessels.
Canaliculi: Narrow passageways for exchange of nutrients, wastes, and gases.
Periosteum: Covers outer bone surfaces (except joints); has outer fibrous and inner cellular layers.

Bone Matrix Composition

Calcium Phosphate: About two-thirds of bone mass; interacts with calcium hydroxide to form hydroxyapatite crystals.
Other Salts: Includes calcium carbonate and ions like magnesium.
Collagen Fibers: About one-third of bone mass; provide flexibility and tensile strength.

Bone Cells

Osteogenic Cells: Stem cells that divide to produce osteoblasts; found in periosteum and endosteum; aid in fracture repair.
Osteoblasts: Immature cells that synthesize new bone matrix (osteoid); become osteocytes when surrounded by matrix.
Osteocytes: Mature bone cells in lacunae; maintain bone matrix and help repair damaged bone.
Osteoclasts: Large, multinucleate cells that resorb bone matrix; important for calcium homeostasis; derived from monocyte/macrophage lineage.

6.4 Compact Bone and Spongy Bone

Bone tissue is organized into compact and spongy forms, each with distinct structures and functions.

Compact Bone

Osteon: Functional unit; consists of concentric lamellae around a central canal with blood vessels.
Perforating Canals: Perpendicular to bone surface; carry blood vessels into deep bone and marrow.
Lamellae Types: Concentric (around central canal), interstitial (between osteons), circumferential (outer/inner surfaces).

Spongy Bone

Lacks osteons; matrix forms trabeculae (open network).
Spaces filled with red bone marrow (site of blood cell formation) or yellow bone marrow (fat storage).

Weight-Bearing Bones

Trabeculae in epiphyses transfer forces efficiently; medial shaft compresses, lateral shaft experiences tension.

Periosteum and Endosteum

Periosteum: Outer membrane; isolates bone, provides route for vessels/nerves, aids in growth and repair; fibers interwoven with tendons/ligaments.
Endosteum: Incomplete cellular layer lining medullary cavity; active in growth, repair, remodeling; contains osteoblasts, osteogenic cells, osteoclasts.

6.5 Bone Formation and Growth

Bone develops through ossification (osteogenesis) and calcification. Two main mechanisms are endochondral and intramembranous ossification.

Endochondral Ossification

Most bones form this way; primary ossification center develops in hyaline cartilage, which is gradually replaced by bone.
Seven main steps (details in lab).
Interstitial Growth: Growth in length; secondary ossification centers form; epiphyseal closure marks end of growth (epiphyseal line remains).
Appositional Growth: Growth in width; circumferential lamellae added; osteoclasts enlarge medullary cavity.

Intramembranous Ossification

Also called dermal ossification; occurs in the dermis.
Produces dermal bones (e.g., mandible, clavicles).
Five main steps (details in lab).

Blood Supply to Bones

Nutrient Artery and Vein: Enter diaphysis via nutrient foramina.
Metaphyseal Vessels: Supply epiphyseal cartilages (growth regions).
Periosteal Vessels: Supply superficial osteons and secondary ossification centers.
Periosteum also contains lymphatic vessels and sensory nerves.

6.6 Bone Remodeling

Bone remodeling is a continuous process involving osteocytes, osteoblasts, and osteoclasts. It maintains bone strength and mineral homeostasis.

Balanced activity keeps bones healthy.
Excessive removal weakens bones; excessive deposition strengthens bones.

6.7 Exercise, Nutrition, and Hormones

Bone health depends on physical activity, nutrition, and hormonal regulation.

Exercise

Stimulates osteoblasts; bones adapt to stress by becoming thicker and stronger.
Inactivity leads to rapid bone degeneration (up to one-third of mass lost in weeks).

Nutrition and Hormones

Minerals: Calcium, phosphorus, magnesium, fluoride, iron, manganese required.
Calcitriol: Made in kidneys from vitamin D (cholecalciferol); essential for calcium/phosphate absorption.
Vitamin C: Needed for collagen synthesis; stimulates osteoblast differentiation.
Vitamin A: Stimulates osteoblast activity.
Vitamins K and B: Required for synthesis of bone proteins.
Growth Hormone & Thyroxine: Stimulate bone growth.
Sex Hormones (Estrogen, Testosterone): Stimulate osteoblasts.
Parathyroid Hormone & Calcitonin: Regulate calcium ion homeostasis.

6.8 Calcium Homeostasis

Bones store 99% of the body's calcium, which is vital for many physiological processes. Calcium levels are tightly regulated by hormones.

Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption (via calcitriol), and reducing kidney excretion.
Calcitonin: Decreases blood calcium by inhibiting osteoclasts, increasing kidney excretion, and reducing intestinal absorption.

6.9 Fractures

Fractures are breaks in bones due to physical stress. They are classified by their characteristics and repaired in a series of steps.

Types of Fractures

Open (Compound): Bone breaks through the skin.
Closed (Simple): Bone does not break through the skin.
Other Types: Transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, Pott’s.

Steps in Fracture Repair

Fracture Hematoma Formation: Blood clot forms; bone cells die.
Callus Formation: Endosteum and periosteum cells form internal and external calluses to stabilize break.
Spongy Bone Formation: Osteoblasts replace cartilage with spongy bone.
Compact Bone Formation: Spongy bone is remodeled; repaired bone may be thicker/stronger.

6.10 Effects of Aging on the Skeletal System

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

Osteopenia: Inadequate ossification; begins between ages 30–40; more pronounced in women.
Osteoporosis: Severe bone mass loss; affects 29% of women and 18% of men over 45; accelerates after menopause.
Cancer and Bone Loss: Some cancers release osteoclast-activating factor, causing severe osteoporosis.

Table: Types of Bone Cells and Their Functions

Cell Type	Origin	Main Function
Osteogenic Cells	Mesenchymal stem cells	Divide to produce osteoblasts; aid in repair
Osteoblasts	From osteogenic cells	Produce new bone matrix (osteoid)
Osteocytes	From osteoblasts	Maintain bone matrix; repair bone
Osteoclasts	Monocyte/macrophage lineage	Resorb bone matrix; release minerals

Key Equations

Hydroxyapatite Formation:

Example: The patella is a sesamoid bone that develops within the tendon of the quadriceps femoris muscle, improving leverage and protecting the knee joint.

Additional info: Some details about the steps of ossification and fracture repair are summarized due to reference to lab coverage in the original material.