The Skeletal System

Overview and Divisions

The skeletal system is a fundamental component of human anatomy, providing structure, protection, and support for the body. It consists of bones, cartilages, ligaments, and other connective tissues. The human skeleton is divided into two main regions:

• Axial Skeleton: Composed of 80 bones, including the skull, thorax, and vertebral column. It forms the longitudinal axis of the body.

• Appendicular Skeleton: Composed of 126 bones, including the limbs and girdles that attach them to the axial skeleton.

Functions of the Skeletal System

The skeletal system performs several essential functions that are critical for maintaining homeostasis and overall health:

• Support: Provides structural support and a framework for the attachment of soft tissues and organs.

• Storage of Minerals and Lipids: Bones act as reservoirs for minerals such as calcium and phosphate, and store lipids in adipose tissue.

• Blood Cell Production: Red bone marrow produces all formed elements of blood, including red blood cells, white blood cells, and platelets.

• Protection: Protects delicate tissues and organs (e.g., ribs protect heart and lungs, skull protects brain).

• Leverage: Bones act as levers to change the magnitude and direction of muscle forces, enabling movement.

Bone Classification and Surface Markings

Categories of Bone Based on Shape

Bones are classified into six categories based on their shape, each with distinct structural and functional characteristics:

• Flat Bones: Thin, parallel surfaces; protect underlying tissues and provide surface area for muscle attachment. Examples: cranial bones, sternum, ribs, scapulae.

• Sutural Bones: Irregular bones found between cranial bones; size and shape vary.

• Long Bones: Long and slender; found in limbs. Example: femur (largest bone).

• Irregular Bones: Complex shapes with short, flat, notched, or ridged surfaces. Examples: vertebrae, pelvis, facial bones.

• Sesamoid Bones: Small, flat, shaped like a sesame seed; develop inside tendons. Example: patella.

• Short Bones: Small and boxy. Examples: carpals (wrist), tarsals (ankle).

Surface Markings of Bones

Surface markings, also known as bone features or bony landmarks, are structural features related to specific functions:

• Elevations/Projections: Sites for tendon and ligament attachment or articulation with adjacent bones (e.g., process, tubercle, tuberosity, condyle).

• Depressions/Grooves/Tunnels: Sites for blood vessels or nerves to lie alongside or penetrate bone (e.g., canal, sinus, foramen, fissure, sulcus, fossa).

Functional Anatomy of a Long Bone

Major Parts of a Long Bone

Long bones have specialized regions that contribute to their function and growth:

• Epiphysis: Expanded ends, mostly spongy bone, covered by compact bone and articular cartilage.

• Metaphysis: Connects epiphysis to diaphysis.

• Diaphysis: Shaft, composed of compact bone, contains the medullary cavity.

• Medullary Cavity: Contains red bone marrow (blood cell production) and yellow bone marrow (energy reserve).

Blood Supply and Innervation

Bone growth and maintenance require an extensive blood supply:

• Nutrient Artery and Vein: Supply blood to osteons and marrow cavity.

• Metaphyseal Artery and Vein: Supply blood to metaphysis and connect to epiphyseal vessels.

• Periosteum: Contains smaller blood vessels, lymphatic vessels, and sensory nerves.

Bone Tissue

Cell Types in Bone Tissue

Bone tissue contains four main cell types, each with distinct roles:

• Osteocytes: Mature bone cells, maintain matrix, occupy lacunae, interconnected by canaliculi.

• Osteoblasts: Produce new bone matrix (osteogenesis), become osteocytes when surrounded by matrix.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts, important in fracture repair.

• Osteoclasts: Remove and remodel bone matrix, release acids and enzymes for osteolysis.

Bone Matrix Composition

The bone matrix is a composite material providing both strength and flexibility:

• Collagen Fibers: ~1/3 of bone weight, provide flexibility.

• Calcium Phosphate: ~2/3 of bone weight, forms hydroxyapatite crystals for strength.

• Other Salts and Ions: Include calcium carbonate, sodium, magnesium, fluoride.

Formula for Hydroxyapatite:

Compact and Spongy Bone Structure

Compact Bone

Compact bone is organized into osteons, which are concentric lamellae surrounding a central canal:

• Osteon: Functional unit, contains osteocytes in lacunae, canaliculi for nutrient exchange.

• Central Canal: Contains blood vessels.

• Lamellae: Dense, strong, and resistant to longitudinal stress.

Spongy Bone

Spongy bone is found where bones are not heavily stressed or stress is multidirectional:

• Trabeculae: Struts and plates forming an open network.

• Red Bone Marrow: Located between trabeculae, site of blood cell production.

• No Blood Vessels in Matrix: Nutrients reach osteons via canaliculi.

Appositional Bone Growth

Mechanism of Appositional Growth

Appositional growth increases the diameter of existing bones:

• Osteogenic Cells: Differentiate into osteoblasts under periosteum.

• Successive Layers: Circumferential lamellae are added.

• Osteoclasts: Remove matrix at inner surface, enlarging medullary cavity.

Periosteum and Endosteum

The periosteum and endosteum are connective tissue layers involved in bone growth and repair:

• Periosteum: Two layers (fibrous and cellular), isolates bone, provides route for blood and nerves, participates in growth and repair.

• Perforating Fibers: Collagen fibers cemented into lamellae, provide strong attachment.

• Endosteum: Incomplete cellular layer lining medullary cavity, active during growth, repair, and remodeling.

Endochondral and Intramembranous Ossification

Endochondral Ossification

Most bones form by replacing hyaline cartilage models through endochondral ossification:

• Cartilage Model Enlarges: Chondrocytes die, leaving cavities.

• Blood Vessels Invade: Osteoblasts form superficial bone.

• Primary Ossification Center: Spongy bone forms in shaft.

• Secondary Ossification Centers: Form in epiphyses.

• Epiphyseal Plate: Site of bone lengthening until adulthood.

Intramembranous Ossification

Some bones form directly from mesenchymal tissue without a cartilage model:

• Ossification Center: Mesenchymal cells differentiate into osteoblasts and secrete osteoid.

• Bone Spicules: Grow and fuse, trapping blood vessels.

• Remodeling: Produces compact bone and periosteum.

Clinical Module: Abnormalities in Bone Growth

Disorders Causing Shortened Bones

• Pituitary Growth Failure: Inadequate growth hormone, reduced epiphyseal cartilage activity, short bones.

• Achondroplasia: Slow growth of epiphyseal cartilage, early replacement by bone, short limbs.

Disorders Causing Lengthened Bones

• Marfan's Syndrome: Excessive cartilage formation, tall stature, long limbs, cardiovascular risks.

• Gigantism: Overproduction of growth hormone before puberty, extreme height.

Other Skeletal Growth Abnormalities

• Acromegaly: Overproduction of growth hormone after epiphyseal plates close, thickened bones.

• Fibrodysplasia Ossificans Progressiva (FOP): Bone deposition around skeletal muscles, heterotopic bones.

• Congenital Talipes Equinovarus (Clubfoot): Developmental abnormality, feet turn medially and are inverted.

Bones as Mineral Reservoirs

Bone Composition and Mineral Storage

Bones are major reservoirs for minerals, especially calcium:

• Inorganic Components: 67% (calcium, phosphate, carbonate, etc.)

• Organic Components: 33% (primarily collagen)

• Calcium: 99% of body’s calcium is stored in bones.

Calcium Regulation

Calcium levels are maintained by coordinated actions of the intestines, bones, and kidneys:

• Intestines: Absorb calcium and phosphate under hormonal control.

• Bones: Osteoclasts release calcium, osteoblasts deposit calcium.

• Kidneys: Regulate calcium and phosphate loss in urine.

Hormones Regulating Calcium Ion Metabolism

Increasing Blood Calcium Levels

• Parathyroid Hormone (PTH): Stimulates osteoclasts, increases intestinal absorption, reduces urinary loss.

Decreasing Blood Calcium Levels

• Calcitonin: Inhibits osteoclasts, decreases intestinal absorption, increases urinary excretion.

Clinical Module: Bone Fractures

Fracture Repair Process

Bone fractures heal through a four-step process:

1. Fracture Hematoma Formation: Clot closes injured vessels.

2. Callus Formation: Internal (spongy bone) and external (cartilage and bone) callus stabilize fracture.

3. Spongy Bone Formation: Replaces cartilage of external callus.

4. Compact Bone Formation: Remodeling eliminates evidence of fracture.

Types of Fractures

• Closed (Simple) Fracture: Internal, no break in skin.

• Open (Compound) Fracture: Projects through skin, risk of infection.

• Specific Types: Transverse, spiral, displaced, compression, greenstick, comminuted, epiphyseal, Pott’s, Colles.

Additional info: This study guide covers the structure, function, classification, growth, and clinical aspects of the skeletal system, providing a comprehensive overview for Anatomy & Physiology students.