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, stores minerals, and produces blood cells.

• 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 and platelets.

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

Structural Components – Microscopic Anatomy

Cell Types and Extracellular Matrix

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

• Osteoblasts: Cells responsible for bone formation.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Cells that resorb (break down) bone tissue.

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

Structural Components of Compact Bone:

• Osteon (Haversian system): Cylindrical structures containing a central canal surrounded by concentric lamellae.

• Central canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) canals: Connect central canals across osteons.

• Lacunae: Small spaces housing osteocytes.

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

Types of Cartilage Tissues:

• Hyaline cartilage: Most common, found in joints, nose, and respiratory tract.

• Fibrocartilage: Strong, found in intervertebral discs and pubic symphysis.

• Elastic cartilage: Flexible, found in ear and epiglottis.

Types of Bone Cells: Osteoblasts, osteocytes, osteoclasts.

Structural Components – Gross Anatomy

Long Bone Structure

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

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

• Epiphysis: Ends of the bone, mostly spongy bone covered by articular cartilage.

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

• Medullary cavity: Central cavity containing yellow bone marrow.

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

• Endosteum: Thin membrane lining the medullary cavity.

Additional info: The epiphyseal plate is responsible for longitudinal bone growth during childhood and adolescence.

Physiology of Embryonic Bone Formation (Ossification, Osteogenesis)

Bone Development Processes

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 forms by replacing hyaline cartilage (e.g., long bones).

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

Comparison Table: Primary vs. Secondary Bone

Feature	Primary Bone	Secondary Bone
Structure	Woven, irregular	Lamellar, organized
Location	Fetal development, fracture repair	Adult skeleton
Strength	Weaker	Stronger

Physiology of Bone Growth, Repair, & Remodeling

Growth and Regulation

Bones grow and remodel throughout life, adapting to stress and repairing damage.

• Chondroblasts: Cells that produce cartilage during bone growth.

• Osteoblasts: Form new bone during growth and repair.

• Osteoclasts: Resorb bone during remodeling.

• Hormonal regulation: Growth hormone, parathyroid hormone, and calcitriol regulate bone growth and calcium homeostasis.

• Remodeling: Continuous process of bone resorption and formation.

Bone Growth Equation:

$\text{Bone Growth Rate} = \text{Osteoblast Activity} - \text{Osteoclast Activity}$

Additional info: Remodeling allows bones to adapt to mechanical stress and repair microdamage.

Organization and Anatomy 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, rib cage, and sternum.

• Appendicular skeleton: Limbs and girdles (shoulder and pelvic).

General Bone Structure: Each bone has a unique shape and function, classified as long, short, flat, irregular, or sesamoid.

Gross Anatomy of Bones

Identification and Classification

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

• Long bones: Femur, humerus, tibia.

• Short bones: Carpals, tarsals.

• Flat bones: Skull, ribs, sternum.

• Irregular bones: Vertebrae, facial bones.

• Sesamoid bones: Patella.

Facial Bones: Maxilla, zygomatic, nasal, mandible.

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

Additional info: The vertebral column supports the head and trunk and protects the spinal cord.

Classification of Joints

Structural and Functional Classification

Joints are classified by their structure and function, determining their range of movement.

• 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).

Additional info: Most joints in the limbs are synovial and allow a wide range of movements.

Synovial Joints

Structure and Types

Synovial joints are characterized by a joint cavity, articular cartilage, and accessory structures.

• Components: Articular cartilage, synovial membrane, joint capsule, synovial fluid, ligaments, bursae, tendon sheaths.

• Types of synovial joints:

  • Plane (gliding) joints

  • Hinge joints

  • Pivot joints

  • Condyloid joints

  • Saddle joints

  • Ball-and-socket joints

Example: The knee is a hinge joint, allowing flexion and extension.

Synovial Joint Type	Movement Allowed	Location Example
Hinge	Flexion/Extension	Knee, elbow
Ball-and-socket	Multiaxial movement	Shoulder, hip
Pivot	Rotation	Atlas/axis (neck)
Plane	Gliding	Intercarpal joints
Saddle	Flexion/extension, abduction/adduction	Thumb (carpometacarpal joint)
Condyloid	Flexion/extension, abduction/adduction	Wrist

Additional info: Bursae and tendon sheaths reduce friction in synovial joints.