Skeletal System: Structure, Function, and Development – Study Notes

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

Cellular and Extracellular Components of Bone Tissue

Bone tissue consists of specialized cells embedded in an extracellular matrix (ECM) that provides strength and flexibility.

• Cells:

  • Osteoblasts: Bone-forming cells responsible for synthesizing the bone matrix.

  • Osteocytes: Mature bone cells that maintain bone tissue.

  • Osteoclasts: Bone-resorbing cells that break down bone matrix.

  • Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

• Extracellular Matrix (ECM): Composed of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components, providing tensile strength and hardness.

Structural Organization of Compact Bone

Compact bone is organized into repeating structural units called osteons (Haversian systems).

• Osteon: Cylindrical structure 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, allowing nutrient and waste exchange.

Types of Cartilage Tissues in the Skeletal System

Cartilage is a flexible connective tissue found in joints, rib cage, ear, nose, and respiratory tract.

• Hyaline Cartilage: Most common type; provides support and flexibility (e.g., articular surfaces, costal cartilage).

• Fibrocartilage: Contains dense collagen fibers; resists compression (e.g., intervertebral discs, pubic symphysis).

• Elastic Cartilage: Contains elastic fibers; maintains shape (e.g., external ear, epiglottis).

Structural Components – Gross Anatomy

Gross Anatomy of a Long Bone

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

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

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

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

• Articular Cartilage: Hyaline cartilage covering joint surfaces.

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

• Endosteum: Membrane lining the medullary cavity.

• Medullary Cavity: Central cavity containing yellow marrow.

Physiology of Embryonic Bone Formation (Ossification, Osteogenesis)

Ossification 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, vertebrae).

Osteogenic Cells

Osteogenic cells are stem cells that differentiate into osteoblasts, initiating bone formation.

Physiology of Bone Growth, Repair, & Remodeling

Bone Growth

Bones grow in length and thickness through cellular activity at the epiphyseal plate and periosteum.

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

• Appositional Growth: Increase in bone thickness by adding new bone tissue at the periosteum.

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption by osteoclasts and formation by osteoblasts.

• Hormonal Regulation: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts; vitamin D promotes calcium absorption.

• Remodeling Cycle: Old bone is replaced with new bone to maintain strength and mineral homeostasis.

Bone Repair

Bone repair follows a sequence: hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Organization 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 (ribs and sternum).

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

Gross Anatomy of Bones

Identification and Classification

Bones are classified by shape and location, each with 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.

Key bones to identify: frontal, parietal, temporal, occipital, sphenoid, ethmoid, facial bones (maxilla, zygomatic, mandible), vertebrae (cervical, thoracic, lumbar, sacrum, coccyx), ribs, sternum, scapula, clavicle, humerus, radius, ulna, carpals, metacarpals, phalanges, pelvis, femur, tibia, fibula, tarsals, metatarsals, phalanges.

Anatomical Classification of Joints

Structural Types of Joints

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

• 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 a complex structure allowing 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.

• Accessory Structures: Bursae (fluid-filled sacs), tendon sheaths, 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	Intercarpal joints	Sliding/gliding
Hinge	Convex/concave surfaces	Elbow, knee	Flexion/extension
Pivot	Ring-shaped bone and another bone	Atlantoaxial joint	Rotation
Condyloid (Ellipsoid)	Oval articular surface	Wrist	Flexion/extension, abduction/adduction
Saddle	Concave/convex surfaces	Thumb (carpometacarpal joint)	Flexion/extension, abduction/adduction, opposition
Ball-and-Socket	Ball-shaped head fits into cup-like socket	Shoulder, hip	All movements (multiaxial)

Example: Knee Joint

The knee is a hinge synovial joint, allowing flexion and extension, and contains accessory structures such as bursae and ligaments for stability.

Additional info: Academic context and terminology have been expanded for clarity and completeness.