Skeletal system

The skeletal system is a complex framework of bones, cartilage, ligaments, and joints that provides structural support to the human body. It plays a critical role in movement, protection of internal organs, and metabolic functions such as mineral storage and blood cell production. Understanding the anatomy and physiology of the skeletal system is essential for comprehending human development, mobility, and disease processes.

Introduction

The human skeletal system consists of 206 bones in the adult body, along with associated cartilage, ligaments, and tendons. It serves as the structural scaffold that maintains body shape and posture while allowing mobility through articulations at joints. In addition to mechanical functions, the skeletal system participates in mineral homeostasis and hematopoiesis, making it vital for overall health and physiological balance.

Overview of the Skeletal System

Definition

The skeletal system is the organized structure of bones and connective tissues that forms the framework of the body. It supports soft tissues, facilitates movement through muscular attachments, and provides protection for vital organs such as the brain, heart, and lungs.

Main Components

• Bones: Rigid structures that form the primary framework and support for the body.
• Cartilage: Flexible connective tissue that provides cushioning at joints and contributes to bone formation.
• Ligaments: Fibrous tissues connecting bones to other bones, providing joint stability.
• Tendons: Strong connective tissues that attach muscles to bones, enabling movement.
• Joints: Articulations between bones that allow mobility and flexibility of the skeleton.

Functions of the Skeletal System

• Support and Structure: Bones provide a rigid framework that supports the body’s weight and maintains shape and posture.
• Protection of Vital Organs: The skull protects the brain, the vertebral column safeguards the spinal cord, and the rib cage shields the heart and lungs.
• Movement and Locomotion: Bones act as levers and points of attachment for muscles, facilitating voluntary and involuntary movements.
• Mineral Storage and Homeostasis: Bones serve as a reservoir for calcium, phosphate, and other minerals, releasing them into the bloodstream as needed for physiological processes.
• Hematopoiesis: Red bone marrow within certain bones produces red blood cells, white blood cells, and platelets essential for oxygen transport, immunity, and clotting.
• Endocrine Regulation: Bones secrete hormones such as osteocalcin, which influences energy metabolism and glucose regulation.

Bone Anatomy

Macrostructure

• Bone Types: Long bones, short bones, flat bones, irregular bones, and sesamoid bones are classified based on shape and function.
• Diaphysis and Epiphysis: Long bones consist of a central shaft (diaphysis) and terminal ends (epiphyses) that contain spongy bone and red marrow.
• Bone Surfaces: Processes, cavities, and projections serve as attachment sites for muscles, ligaments, and tendons.

Microstructure

• Compact Bone: Dense outer layer providing strength and protection.
• Spongy (Trabecular) Bone: Porous inner layer that reduces bone weight and contains red marrow.
• Osteons and Haversian System: Structural units of compact bone consisting of concentric lamellae surrounding central canals with blood vessels and nerves.
• Bone Cells: Osteoblasts build bone, osteocytes maintain bone tissue, and osteoclasts resorb bone for remodeling.

Bone Marrow

• Red Marrow: Hematopoietic tissue responsible for producing blood cells.
• Yellow Marrow: Fat-storing tissue that can convert to red marrow under increased hematopoietic demand.

Cartilage and Connective Tissue

• Types of Cartilage:
  • Hyaline cartilage: Provides smooth surfaces for joint movement and forms the embryonic skeleton.
  • Elastic cartilage: Offers flexibility and maintains shape, found in the ear and epiglottis.
  • Fibrocartilage: Resists compression and absorbs shock, located in intervertebral discs and menisci.
• Functions and Locations: Cartilage reduces friction at joints, supports soft tissues, and contributes to bone growth.
• Ligaments and Tendons:
  • Ligaments connect bones to other bones, stabilizing joints and preventing excessive movement.
  • Tendons attach muscles to bones, transmitting force to enable movement and maintain posture.

Joints

Classification

• Fibrous joints: Immovable or slightly movable, connected by dense connective tissue (e.g., sutures of the skull).
• Cartilaginous joints: Allow limited movement, connected by cartilage (e.g., intervertebral discs, pubic symphysis).
• Synovial joints: Freely movable joints with a synovial cavity (e.g., knee, shoulder, hip).

Structure of Synovial Joints

• Joint capsule and synovial membrane: Encloses the joint cavity and secretes synovial fluid.
• Synovial fluid: Lubricates the joint, reducing friction and facilitating smooth movement.
• Articular cartilage: Covers bone surfaces at the joint, absorbing mechanical stress.
• Ligaments and menisci: Provide stability and distribute load within the joint.

Movement Types

• Flexion and extension: Decreasing and increasing the angle between bones.
• Abduction and adduction: Moving limbs away from or toward the midline of the body.
• Rotation and circumduction: Circular or twisting movements around an axis.
• Special movements: Elevation, depression, pronation, supination, inversion, and eversion.

Axial Skeleton

• Skull: Composed of cranial bones protecting the brain and facial bones forming the structure of the face.
• Vertebral Column: Consists of cervical, thoracic, lumbar, sacral, and coccygeal vertebrae, providing support, flexibility, and protection for the spinal cord.
• Thoracic Cage: Includes ribs and sternum, protecting the heart, lungs, and major blood vessels while assisting in respiration.

Appendicular Skeleton

• Pectoral Girdle and Upper Limbs: Consists of the clavicle and scapula, and the bones of the arm, forearm, wrist, and hand, enabling a wide range of upper limb movements.
• Pelvic Girdle and Lower Limbs: Comprises the hip bones, femur, tibia, fibula, ankle, and foot bones, providing weight-bearing support and facilitating locomotion.
• Special Bones: Patella and sesamoid bones enhance joint stability and improve the mechanical efficiency of tendons.

Bone Development and Growth

Ossification

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue, primarily in flat bones like the skull and clavicle.
• Endochondral Ossification: Bone formation by replacement of a cartilage template, occurring in long bones and most of the skeleton.

Bone Remodeling

• Osteoblast and Osteoclast Activity: Osteoblasts synthesize new bone, while osteoclasts resorb old or damaged bone, maintaining skeletal integrity.
• Response to Mechanical Stress: Bones adapt to physical forces by modifying density and structure, a process known as Wolff’s law.

Physiology and Biochemistry of Bone

• Calcium and Phosphate Metabolism: Bones store minerals and release them into the bloodstream to maintain physiological balance.
• Hormonal Regulation: Parathyroid hormone, calcitonin, and vitamin D control bone formation, resorption, and mineral homeostasis.
• Bone Matrix Composition: Organic components include collagen and proteoglycans, providing tensile strength; inorganic components such as hydroxyapatite crystals provide hardness and compressive strength.

Disorders of the Skeletal System

• Osteoporosis and Osteopenia: Conditions characterized by decreased bone mass and increased fracture risk.
• Osteomalacia and Rickets: Softening of bones due to vitamin D deficiency, affecting adults and children respectively.
• Fractures and Bone Injuries: Disruption of bone continuity due to trauma, stress, or pathological conditions.
• Congenital Skeletal Disorders: Genetic anomalies such as cleidocranial dysplasia, dwarfism, and skeletal dysplasias affecting bone formation and growth.
• Arthritis: Degenerative (osteoarthritis) and inflammatory (rheumatoid arthritis) conditions affecting joints and surrounding bone structures.

Diagnostic Approaches

• Imaging: X-ray, computed tomography (CT), magnetic resonance imaging (MRI), and dual-energy X-ray absorptiometry (DEXA) to assess bone integrity, density, and structural abnormalities.
• Laboratory Tests: Measurement of serum calcium, phosphate, alkaline phosphatase, and vitamin D levels to evaluate bone metabolism.
• Bone Biopsy and Histology: Examination of bone tissue for cellular composition, remodeling activity, and pathological changes.

Treatment and Management

• Pharmacological Interventions: Supplementation with calcium and vitamin D, bisphosphonates, and other medications to enhance bone density and prevent fractures.
• Surgical Management: Procedures including fracture fixation, joint replacement, spinal fusion, and corrective osteotomies to restore skeletal function and stability.
• Physical Therapy and Rehabilitation: Exercise programs, weight-bearing activities, and mobility training to maintain bone strength, improve joint function, and prevent deformities.
• Lifestyle Modifications: Adequate nutrition, avoidance of smoking and excessive alcohol, and fall prevention strategies to reduce skeletal risk factors.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Martini FH, Nath JL, Bartholomew EF. Fundamentals of Anatomy & Physiology. 11th ed. Boston: Pearson; 2018.
3. Shier D, Butler J, Lewis R. Hole’s Human Anatomy & Physiology. 15th ed. New York: McGraw-Hill; 2021.
4. Roodman GD. Cell biology of bone remodeling. Exp Hematol. 1999;27(5):563-573.
5. Rizzoli R, Reginster JY, Arnal JF, et al. Quality of life in patients with osteoporosis. Osteoporos Int. 2011;22(8):2349-2360.
6. Cooper C, Melton LJ. Epidemiology of osteoporosis. Trends Endocrinol Metab. 1992;3(6):224-229.
7. Parfitt AM. Bone remodeling and its regulation. In: Bilezikian JP, Raisz LG, Martin TJ, editors. Principles of Bone Biology. 4th ed. San Diego: Academic Press; 2012. p. 3-15.
8. Currey JD. Bones: Structure and Mechanics. 3rd ed. Princeton: Princeton University Press; 2006.
9. Clarke B. Normal bone anatomy and physiology. Clin J Am Soc Nephrol. 2008;3(Suppl 3):S131-S139.
10. Seeman E, Delmas PD. Bone quality–the material and structural basis of bone strength and fragility. N Engl J Med. 2006;354(21):2250-2261.