Skeletal System: Structure, Function, and Development

General Functions of Bone & the Skeletal System

Main Functions

The skeletal system provides the framework for the body, supports and protects organs, enables movement, and serves as a reservoir for minerals and blood cell production.

• Support: Bones form the rigid structure that supports the body and maintains its shape.

• Protection: Vital organs such as the brain, heart, and lungs are protected by bones (e.g., skull, rib cage).

• Movement: Bones act as levers for muscles, facilitating movement.

• Mineral Storage: Bones store minerals, primarily calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Production: Hematopoiesis occurs in the red bone marrow, producing red and white blood cells.

• Energy Storage: Yellow bone marrow stores fat as an energy reserve.

Structural Components – Microscopic Anatomy

Cell Types and Extracellular Matrix (ECM)

Bones are composed of specialized cells embedded in an extracellular matrix (ECM) that provides strength and flexibility.

• Osteoblasts: Cells responsible for bone formation; they secrete bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Cells that break down bone tissue during remodeling.

• ECM: Consists of organic components (collagen fibers) and inorganic components (hydroxyapatite crystals).

Structural Organization of Compact Bone

• Osteon (Haversian System): The basic structural unit of compact bone, consisting of concentric lamellae surrounding a central canal.

• Central Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Connect central canals to each other and to the periosteum.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

Types of Cartilage Tissues in the Skeletal System

• Hyaline Cartilage: Most abundant; found in articular surfaces, costal cartilages, and respiratory tract.

• Fibrocartilage: Found in intervertebral discs and pubic symphysis; resists compression.

• Elastic Cartilage: Found in the ear and epiglottis; provides flexibility.

Additional info: Cartilage is avascular and relies on diffusion for nutrient supply.

Structural Components – Gross Anatomy

Long Bone Anatomy

Long bones have distinct regions and structural features that support their function and growth.

• Diaphysis: The shaft of the bone, composed mainly of compact bone.

• Epiphysis: The ends of the bone, containing spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains the growth plate (epiphyseal plate).

• Articular Cartilage: Covers joint surfaces, reducing friction.

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Endosteum: Lines the inner surfaces of bone, including the medullary cavity.

• Medullary Cavity: Central cavity containing yellow marrow.

Physiology of Embryonic Bone Formation (Ossification, Osteogenesis)

Bone Formation Processes

Bone develops through two main processes: intramembranous and endochondral ossification.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms flat bones like the skull.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; responsible for the formation of long bones.

Primary vs. Secondary Bone

• Primary (Woven) Bone: First bone formed; irregular arrangement of collagen fibers; temporary.

• Secondary (Lamellar) Bone: Mature bone; organized collagen fibers; replaces primary bone.

Physiology of Bone Growth, Repair, & Remodeling

Growth and Development

Bones grow in length and width through coordinated cellular activity and hormonal regulation.

• Longitudinal Growth: Occurs at the epiphyseal plate via endochondral ossification.

• Appositional Growth: Increases bone diameter by adding new bone to the surface.

• Hormonal Regulation: Growth hormone, parathyroid hormone, and calcitriol regulate bone growth and remodeling.

Bone Remodeling

• Remodeling: Continuous process of bone resorption and formation, adapting bone to stress and repairing microdamage.

• Osteoclasts: Resorb bone matrix.

• Osteoblasts: Form new bone matrix.

Additional info: Remodeling is essential for calcium homeostasis and bone health throughout life.

Organization and Development of the Skeletal System

Axial and Appendicular Skeleton

The skeletal system is divided into two main parts: axial and appendicular skeleton.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic).

General Bone Structure

• Flat Bones: Thin and broad; e.g., skull, ribs.

• Long Bones: Longer than wide; e.g., femur, humerus.

• Short Bones: Cube-shaped; e.g., carpals, tarsals.

• Irregular Bones: Complex shapes; e.g., vertebrae, facial bones.

• Sesamoid Bones: Embedded in tendons; e.g., patella.

Gross Anatomy of the Skeletal System

Identification of Bones

Each region of the skeleton contains specific bones with unique features and functions.

• Skull: Frontal, parietal, temporal, occipital, sphenoid, ethmoid, facial bones (maxilla, zygomatic, mandible).

• Vertebral Column: Cervical, thoracic, lumbar vertebrae, sacrum, coccyx.

• Thoracic Cage: Sternum, ribs.

• Upper Limb: Clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, phalanges.

• Lower Limb: Pelvic bones, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Anatomical Classification of Joints

Structural Types

Joints are classified based on their structure and the type of tissue connecting the bones.

• Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures of the skull).

• Cartilaginous Joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

• Synovial Joints: Bones separated by a fluid-filled cavity; freely movable (e.g., knee, shoulder).

Functional Classification

• Synarthrosis: Immovable joints.

• Amphiarthrosis: Slightly movable joints.

• Diarthrosis: Freely movable joints.

Additional info: Structural and functional classifications are related; synovial joints are typically diarthroses.

Synovial Joints

Structural Components

Synovial joints have a complex structure that allows for a wide range of movements.

• Articular Cartilage: Covers bone surfaces within the joint.

• Joint (Synovial) Cavity: Space filled with synovial fluid.

• Synovial Membrane: Produces synovial fluid for lubrication.

• Fibrous Capsule: Encloses the joint and provides stability.

• Accessory Structures: Include bursae (fluid-filled sacs), tendon sheaths, and ligaments.

Types of Synovial Joints

There are six main types of synovial joints, each allowing specific movements.

Type	Features	Location	Movements Allowed
Plane (Gliding)	Flat surfaces; sliding movements	Intercarpal joints	Gliding
Hinge	Convex/concave surfaces	Elbow, knee	Flexion, extension
Pivot	Rounded surface rotates within ring	Proximal radioulnar joint	Rotation
Condyloid (Ellipsoid)	Oval surface fits into depression	Wrist	Flexion, extension, abduction, adduction
Saddle	Both surfaces concave/convex	Thumb (carpometacarpal joint)	Flexion, extension, abduction, adduction, opposition
Ball-and-Socket	Ball fits into cup-like socket	Shoulder, hip	All movements (multiaxial)

Example: Knee Joint

• Type: Hinge joint

• Features: Articular cartilage, menisci, ligaments, bursae

• Movements: Flexion, extension, slight rotation

Additional info: Synovial joints are the most common and versatile joints in the body, allowing for complex movements essential for daily activities.