The Skeletal System

Introduction

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. Understanding its anatomy and physiology is essential for comprehending how the body maintains homeostasis and responds to injury or disease.

Key Terms Related to the Skeletal System

Importance of Terminology

• Bone Anatomy: The study of the structure of bones, including their shapes, markings, and internal organization.

• Bone Remodeling: The continuous process of bone renewal involving resorption and formation, crucial for bone health and repair.

• Osteoporosis: A disease characterized by decreased bone mass and increased fracture risk.

• Homeostasis: The maintenance of stable internal conditions, in which the skeletal system plays a vital role, especially in mineral balance.

Application: Knowledge of bone anatomy and physiology is essential for diagnosing and treating bone diseases such as osteoporosis and fractures.

Cartilage

Overview of Cartilage in the Skeletal System

Cartilage is a resilient, flexible connective tissue that forms the initial skeleton in embryos and persists in specific adult locations requiring flexibility. It is avascular and consists primarily of water, which contributes to its ability to withstand compression.

• Chondrocytes: The cells found in cartilage, residing in small cavities called lacunae within the extracellular matrix.

• Extracellular Matrix: The non-cellular component that provides structural support and flexibility.

Types of Skeletal Cartilage

Classification and Locations

There are three main types of skeletal cartilage, each with distinct properties and locations in the body.

1. Hyaline Cartilage

• Definition: Most abundant type; provides support, flexibility, and resilience.

• Composition: Contains only collagen fibers.

• Locations: Articular cartilage (joints), costal cartilage (ribs), respiratory cartilage (larynx), nasal cartilage (nose tip).

2. Elastic Cartilage

• Definition: Similar to hyaline cartilage but contains more elastic fibers, allowing greater flexibility.

• Locations: External ear and epiglottis.

3. Fibrocartilage

• Definition: Contains thick collagen fibers, providing tensile strength and the ability to withstand pressure and stretch.

• Locations: Intervertebral discs, menisci of the knee.

Growth and Development of Cartilage

• Appositional Growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface.

• Interstitial Growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding cartilage from within.

• Calcification: Occurs during normal bone growth in youth and sometimes in old age, but calcified cartilage is not bone.

Functions of Bone

Primary Roles of Bone in the Body

• Support: Provides a framework for the body and supports soft organs.

• Protection: Shields vital organs such as the brain, spinal cord, and heart.

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores minerals, especially calcium and phosphorus, and releases them as needed.

• Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities of certain bones.

• Fat Storage: Yellow marrow stores triglycerides for energy.

• Hormone Production: Bones secrete osteocalcin, which regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

Grouping Bones by Location and Shape

• Axial Skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).

• Appendicular Skeleton: Includes bones of the limbs and girdles attaching them to the axial skeleton.

Types of Bones by Shape

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., wrist and ankle bones); includes sesamoid bones (e.g., patella).

• Flat Bones: Thin, flat, and slightly curved (e.g., sternum, scapulae, ribs, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Structure of a Long Bone

Components and Organization

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding the medullary cavity (contains yellow marrow in adults).

• Epiphyses: Bone ends; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the epiphyseal plate, where bone growth occurred during childhood.

• Membranes: Periosteum (outer double-layered membrane) and endosteum (internal membrane lining the medullary cavity and trabeculae).

• Bone Marrow: Red marrow (hematopoietic tissue) and yellow marrow (fat storage).

Major Types of Bone Cells

Cell Types and Their Functions

• Osteogenic (Osteoprogenitor) Cells: Mitotic stem cells that differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), rich in collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells that monitor and maintain the bone matrix, acting as stress or strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces, help maintain the matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Derived from hematopoietic stem cells; responsible for bone resorption (breakdown), have ruffled borders to increase surface area for enzyme release.

Compact vs. Spongy Bone

Structural and Functional Differences

• Compact Bone: Dense outer layer; organized into osteons (Haversian systems) that resist stress and twisting. Contains blood vessels and nerves.

• Spongy Bone: Honeycomb structure of trabeculae; spaces filled with red or yellow marrow. Arranged along lines of stress for strength.

Composition: Both types consist of organic (cells, osteoid) and inorganic (hydroxyapatite, mineral salts) components.

Bone Development and Growth

Osteogenesis and Growth Mechanisms

• Osteogenesis: The process of bone tissue formation, beginning in the embryo and continuing through adulthood.

• Longitudinal Growth: Occurs at the epiphyseal plate via interstitial growth; increases bone length.

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

• Hormonal Regulation: Growth hormone stimulates epiphyseal plate activity; thyroid hormone modulates growth; sex hormones induce growth spurts and plate closure.

Equation:

Bone Remodeling and Repair

Continuous Renewal and Healing

• Remodeling: 5-7% of bone mass is recycled weekly; involves bone deposit (by osteoblasts) and resorption (by osteoclasts).

• Regulation: Controlled by hormonal (parathyroid hormone, calcitonin) and mechanical factors (stress, muscle pull).

• Calcium Homeostasis: Blood calcium levels are regulated by a negative feedback loop involving PTH and calcitonin.

Equation:

Bone Repair Process

1. Hematoma Formation: Blood vessels rupture, forming a clot at the fracture site.

2. Fibrocartilaginous Callus Formation: New blood vessels and cells form a soft callus.

3. Bony Callus Formation: Spongy bone replaces the soft callus.

4. Bone Remodeling: Compact bone is laid down, restoring the bone's original structure.

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