Introduction to the Skeletal System

The skeletal system is a complex framework that provides structure, support, and protection to the human body. It consists of bones, cartilages, ligaments, and other connective tissues that stabilize or interconnect the bones. The adult human skeleton contains 206 bones, though this number is higher in newborns due to the presence of unfused bones.

• Bones: Rigid organs forming the skeleton.

• Cartilages: Flexible connective tissue at joints and in some structures.

• Ligaments: Connect bones to other bones.

• Other connective tissues: Stabilize and support skeletal elements.

Question: Are there more or fewer bones in a newborn? Answer: More, because many bones fuse during development.

Functions of the Skeletal System

Primary Functions

• Support: Provides a structural framework for the body.

• Storage of Minerals and Lipids:

  • Minerals: Calcium and phosphorus are stored in bone matrix.

  • Lipids: Stored in yellow bone marrow as an energy reserve.

• Blood Cell Production:

  • Occurs in red bone marrow (hematopoiesis).

  • Produces red blood cells, white blood cells, and platelets.

• Protection:

  • Skull protects the brain.

  • Rib cage protects heart and lungs.

  • Vertebral canal protects spinal cord.

  • Pelvis supports abdominopelvic organs.

• Leverage: Bones act as levers with muscles to produce movement.

Classification of Bones

By Shape

• Long bones: Long and slender (e.g., femur, humerus).

• Short bones: Boxy, nearly equal in length and width (e.g., carpals).

• Flat bones: Thin, parallel surfaces (e.g., parietal bone, sternum).

• Irregular bones: Complex shapes (e.g., vertebrae).

• Sesamoid bones: Small, round, develop within tendons (e.g., patella).

• Sutural bones: Small bones between skull bones; number varies between individuals.

By Internal Tissue Organization

• Compact bone: Dense and solid.

• Spongy bone: Network of trabeculae with open spaces.

Bone Markings

Bone markings are surface features with functional significance, such as attachment points for tendons/ligaments or passageways for nerves and blood vessels.

• Depressions, grooves, tunnels, openings: Passage for blood vessels and nerves.

• Elevations or projections: Attachment sites or articulation points.

Bone Structure

Structure of a Long Bone

• Diaphysis: Shaft; thick wall of compact bone surrounding the medullary (marrow) cavity.

• Epiphysis: Expanded ends; mostly spongy bone, articulates with other bones.

• Metaphysis: Region where diaphysis and epiphysis meet.

Structure of a Flat Bone

• Two layers of compact bone with spongy bone (diploë) in between; resembles a sandwich.

• Example: Parietal bone of the skull.

Bone as a Connective Tissue

Bone is a specialized, dense connective tissue composed of cells and an extracellular matrix.

• Matrix:

  • Collagen fibers: Provide strength and flexibility (1/3 of bone weight).

  • Ground substance: Mainly hydroxyapatite crystals (), imparting hardness and density.

• Cells (only 2% of bone mass):

  • Osteocytes: Mature bone cells in lacunae; maintain matrix and help repair bone.

  • Osteoblasts: Immature cells that produce new bone matrix (osteoid); become osteocytes.

  • Osteogenic cells: Stem cells in periosteum/endosteum; produce osteoblasts, aid in repair.

  • Osteoclasts: Multinucleate cells that break down bone matrix (osteolysis), releasing calcium and phosphate.

Bone Homeostasis

• Bone density: Balance between osteoblast (building) and osteoclast (breakdown) activity.

• Calcium and phosphate regulation: Osteoclasts increase blood levels; osteoblasts decrease blood levels by depositing minerals in bone.

Compact Bone vs. Spongy Bone

Compact Bone

• Composed of osteons (Haversian systems), the functional unit.

• Lamellae: Layers of bone matrix.

  • Concentric lamellae: Circles around central canal.

  • Circumferential lamellae: Wrap around outer/inner surfaces.

  • Interstitial lamellae: Fill spaces between osteons.

Spongy Bone (Cancellous Bone)

• No osteons; matrix forms trabeculae (branching struts).

• Spaces filled with:

  • Red bone marrow: Site of hematopoiesis; found in epiphyses, diploë, sternum, ileum.

  • Yellow bone marrow: Fat storage; found in medullary cavity of long bones.

Functional Integration

• Compact and spongy bone work together to support body weight and transfer forces (e.g., in the femur).

Bone Membranes

• Periosteum: Outer covering; isolates bone, provides route for vessels/nerves, participates in growth/repair, anchors tendons/ligaments via Sharpey's fibers.

• Endosteum: Inner lining; lines medullary cavity, trabeculae, and central canals; contains bone cells; active in growth/remodeling.

Bone Formation and Growth

Cartilage Growth

• Interstitial growth: Expansion from within.

• Appositional growth: Addition at the surface.

Ossification (Osteogenesis)

Replacement of other tissues with bone. Two main types:

• Endochondral ossification:

  • Most bones form from hyaline cartilage models.

  • Seven steps from cartilage model to adult bone, including formation of primary and secondary ossification centers, growth plate activity, and eventual closure at puberty.

• Intramembranous ossification:

  • Occurs in flat bones of the skull, mandible, clavicle.

  • Mesenchymal cells differentiate into osteoblasts, form spicules, trap blood vessels, and remodel into compact bone.

Bone Growth

• Interstitial growth: Lengthening at epiphyseal plates.

• Appositional growth: Thickening by adding layers to the outside; osteoclasts enlarge medullary cavity.

Blood, Lymph, and Nerve Supply

• Nutrient artery/vein: Enter diaphysis via nutrient foramen.

• Metaphyseal vessels: Supply epiphyseal cartilage during growth.

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

• Lymphatic vessels and sensory nerves: Present in periosteum; responsible for pain sensation in fractures.

Bone Remodeling

Bone remodeling is a lifelong process involving the recycling and renewal of bone matrix, maintaining bone health and mineral balance.

• Osteocytes: Remove and replace calcium salts.

• Osteoblasts: Form new osteons.

• Osteoclasts: Break down bone matrix.

• Balance between deposition and resorption determines bone strength.

• Imbalance can lead to osteopenia (mild loss) or osteoporosis (severe loss).

Toxins in Bones

• Heavy metals (lead, strontium, cobalt, radioactive elements) can be incorporated into bone matrix and released slowly, causing health issues.

Effects of Exercise, Nutrition, and Hormones on Bones

Exercise

• Weight-bearing exercise increases osteoblast activity, making bones thicker and stronger.

• Inactivity leads to rapid bone loss (up to one-third in weeks).

Nutrition

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

• Vitamins:

  • Vitamin C: Collagen synthesis, osteoblast differentiation.

  • Vitamin A: Stimulates osteoblasts.

  • Vitamins K, B12: Synthesize bone proteins.

  • Vitamin D3: Needed for calcitriol synthesis.

Hormones

• Calcitriol: Promotes calcium and phosphate absorption in the gut.

• Growth hormone, thyroxine: Stimulate bone growth.

• Estrogens, androgens: Stimulate osteoblasts.

• Calcitonin, parathyroid hormone (PTH): Regulate blood calcium and phosphate.

The Role of Calcium in Bone Physiology

• Bones store 99% of body calcium; essential for bone structure and physiological processes (e.g., muscle contraction, nerve transmission).

• Calcium homeostasis is tightly regulated by hormones:

Fractures

Definition and Repair

• Fracture: Crack or break in bone due to physical stress.

• Repair occurs in four steps:

  1. Bleeding and formation of fracture hematoma.

  2. Callus formation (internal and external).

  3. Spongy bone formation replaces cartilage.

  4. Compact bone formation and remodeling.

Major Types of Fractures

Effects of Aging on Bones

• Osteopenia: Inadequate ossification, begins age 30-40; women lose 8% bone mass/decade, men 3%.

• Osteoporosis: Severe bone loss, affects function; 29% of women and 18% of men over 45 affected.

• Sex hormones (estrogens, androgens) help maintain bone mass; loss accelerates after menopause.

• Most affected: epiphyses, vertebrae, jaws (fragile limbs, height loss, tooth loss).

• Cancer: Some tumors release osteoclast-activating factor, causing rapid bone loss.