General Functions of Bone & the Skeletal System

Main Functions

The skeletal system provides the framework for the body, supports and protects organs, enables movement, stores minerals, and produces blood cells.

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

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

• Movement: Bones act as levers for muscles, enabling movement at joints.

• Mineral Storage: Bones store minerals such as 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 and platelets.

Structural Components – Microscopic Anatomy

Cell Types and Extracellular Matrix (ECM)

Bones are composed of specialized cells and an extracellular matrix that provides strength and flexibility.

• Osteoblasts: Cells responsible for bone formation.

• Osteocytes: Mature bone cells that maintain bone tissue.

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

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

Functions of Components:

• Collagen provides tensile strength and flexibility.

• Hydroxyapatite provides hardness and resistance to compression.

Structural Components – Gross Anatomy

Long Bone Structure

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

• Diaphysis: Shaft of the bone, composed mainly of compact bone.

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

• Metaphysis: Region between diaphysis and epiphysis, includes the epiphyseal plate (growth plate).

• Articular Cartilage: Covers joint surfaces, reduces friction and absorbs shock.

• Periosteum: Dense connective tissue covering the bone, contains nerves and blood vessels.

• Endosteum: Thin membrane lining the medullary cavity.

• Medullary Cavity: Central cavity containing yellow marrow (fat storage).

Key Structural Features:

• Central canal, perforating canals, lamellae, lacunae, canaliculi, osteocytes.

Types of Cartilage Tissue in the Skeletal System

Cartilage Types and Functions

Cartilage is a flexible connective tissue found in various parts of the skeletal system.

• Hyaline Cartilage: Most common type; found in articular surfaces, epiphyseal plates, and respiratory tract.

• Fibrocartilage: Found in intervertebral discs and pubic symphysis; provides strength and shock absorption.

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

Function: Cartilage supports soft tissues, provides a smooth surface for joint movement, and enables growth at epiphyseal plates.

Bone Formation (Ossification)

Embryonic Bone Formation

Bone formation occurs through two main processes: intramembranous and endochondral ossification.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone develops from a cartilage model (e.g., long bones).

Osteogenesis: The process of bone tissue formation.

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 woven bone.

Bone Growth, Repair, and Remodeling

Growth and Development

Bones grow in length and width through specific processes regulated by hormones and mechanical stress.

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

• Appositional Growth: Increase in bone diameter by adding new bone tissue at the surface.

Hormonal Regulation:

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

Remodeling: Continuous process of bone resorption and formation throughout life.

Organization of the Skeletal System

Axial and Appendicular Skeleton

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

• Axial Skeleton: Skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Limbs and girdles (pectoral and pelvic).

General Bone Structure: Each bone has unique features and functions depending on its location and type.

Gross Anatomy of Bones

Identification and Classification

Bones are classified by shape and location, and each has distinct anatomical features.

• Long Bones: Femur, humerus, tibia, fibula.

• Short Bones: Carpals, tarsals.

• Flat Bones: Skull, ribs, sternum, scapula.

• Irregular Bones: Vertebrae, facial bones.

• Sesamoid Bones: Patella.

Examples: Identify cranial bones (frontal, parietal, occipital, temporal, sphenoid, ethmoid), facial bones (maxilla, zygomatic, nasal, mandible), and vertebral column regions (cervical, thoracic, lumbar, sacrum, coccyx).

Anatomical Classification of Joints

Structural and Functional Classification

Joints are classified based on their structure and function.

• Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures in 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), amphiarthrosis (slightly movable), diarthrosis (freely movable).

Synovial Joints

Structural Components

Synovial joints have specialized structures that allow for a wide range of movements.

• Articular Cartilage: Covers bone surfaces at the joint.

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

• Synovial Membrane: Produces synovial fluid for lubrication.

• Accessory Structures: Bursae, tendon sheaths, ligaments.

Types of Synovial Joints:

• Plane (gliding), hinge, pivot, condyloid, saddle, ball-and-socket.

Movements Allowed: Flexion, extension, abduction, adduction, rotation, circumduction.

Table: Types of Synovial Joints and Their Features

Key Equations and Concepts

Bone Growth Rate

The rate of bone growth can be influenced by hormonal and nutritional factors.

• Example Equation:

Additional info: The above equation is a simplified representation; actual bone growth involves multiple hormones and signaling pathways.

Calcium Homeostasis

Calcium levels in the blood are regulated by bone resorption and deposition.

• Example Equation:

Additional info: Parathyroid hormone increases bone resorption, while calcitonin promotes bone deposition.