Skeletal System: Structure, Function, and Bone Tissue

Skeletal System Overview

Divisions of the Skeletal System

The human skeleton is divided into two main divisions, each containing specific groups of bones.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage. It provides support and protection for the brain, spinal cord, and organs in the thorax.

• Appendicular Skeleton: Comprises the bones of the limbs and girdles (shoulder and pelvic girdles) that attach them to the axial skeleton.

Functions of the Skeletal System

The skeletal system performs several vital functions necessary for human survival and movement.

1. Support: Provides a structural framework for the body.

2. Storage: Stores minerals (such as calcium and phosphorus) and lipids in yellow bone marrow.

3. Blood Cell Production: Hematopoiesis occurs in red bone marrow, producing red and white blood cells and platelets.

4. Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

5. Leverage: Acts as levers for muscles to produce movement.

Bone Classification and Structure

Types of Bones

Bones are classified by their shapes, each serving different functions and locations in the body.

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).

• Sutural Bones: Small, irregular bones found between the flat bones of the skull.

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).

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

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

Example: The femur is a long bone, while the patella is a sesamoid bone.

Bone Markings and Surface Features

Bones have various markings that serve as sites for muscle, ligament, and tendon attachment, or as pathways for nerves and blood vessels.

• Elevations or Projections: Raised areas that serve as attachment points (e.g., tuberosity, crest, spine).

• Depressions, Grooves, or Tunnels: Indentations or holes that allow the passage of nerves and blood vessels (e.g., fossa, sulcus, foramen).

Example: The foramen magnum in the skull allows passage of the spinal cord.

Parts of a Long Bone

Long bones have distinct regions, each with specific functions.

• Epiphysis: The expanded ends of the bone, containing spongy bone and red marrow.

• Metaphysis: The region between the diaphysis and epiphysis; site of bone growth during development.

• Diaphysis: The shaft of the bone, composed mainly of compact bone and containing the medullary cavity.

Bone Tissue and Cells

Types and Functions of Bone Marrow

• Red Bone Marrow: Site of hematopoiesis (blood cell formation); found mainly in flat bones and the epiphyses of long bones.

• Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones.

Note: Bones with high levels of blood vessels contain more red marrow, supporting active blood cell production.

Cells Found in Bone Tissue

Bone tissue contains four main types of cells, each with specialized functions.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts. Location: Inner layer of periosteum and endosteum.

• Osteoblasts: Bone-forming cells that secrete bone matrix. Location: Surfaces of new bone.

• Osteocytes: Mature bone cells that maintain bone tissue. Location: Lacunae within the bone matrix.

• Osteoclasts: Large cells that break down bone matrix (bone resorption). Location: Bone surfaces undergoing remodeling.

Bone Structure and Growth

Structure of an Osteon

An osteon (Haversian system) is the fundamental unit of compact bone, providing strength and structure.

• Central Canal: Contains blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

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

Strength: The organization of osteons allows compact bone to withstand compressive forces.

Spongy Bone

Spongy bone (cancellous bone) is less dense than compact bone and contains trabeculae, which provide structural support and house red marrow.

• Organization: Trabeculae form a lattice-like network.

• Difference from Compact Bone: Lighter, contains spaces for marrow, and lacks osteons.

• Function: Reduces bone weight and supports marrow.

Types of Bone Growth

Bones grow in length and diameter through two main processes.

• Appositional Growth: Increases bone diameter by adding new bone tissue to the surface. Cells Involved: Osteoblasts in the periosteum.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. Direction: Growth occurs from the epiphyseal plate toward the diaphysis. Cells Involved: Chondrocytes (cartilage cells) and osteoblasts.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue. Location: Flat bones of the skull, clavicle.

Minerals and Bone Health

Minerals in Bone Tissue

• Calcium: The most abundant mineral in bone, essential for bone strength and metabolic functions.

• Importance: Calcium is vital for muscle contraction, nerve transmission, and blood clotting.

• Transport: Absorbed from the diet in the intestines, stored in bones, and regulated by the kidneys.

Hormonal Regulation of Calcium

• Low Blood Calcium: Parathyroid hormone (PTH) is released, stimulating bone resorption, increasing intestinal absorption, and reducing renal excretion.

• High Blood Calcium: Calcitonin is released, inhibiting bone resorption and increasing calcium excretion by the kidneys.

Organs Involved: Bones, intestines, kidneys.

Bone Fractures

A fracture is a break in the bone, which can occur due to trauma, overuse, or disease.

• Types: Simple (closed), compound (open), comminuted, greenstick, etc.

• Healing: Involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Summary Table: Types of Bone Cells

Cell Type	Function	Location
Osteogenic Cells	Stem cells that differentiate into osteoblasts	Periosteum, endosteum
Osteoblasts	Form new bone matrix	Bone surfaces
Osteocytes	Maintain bone tissue	Lacunae within bone matrix
Osteoclasts	Resorb (break down) bone matrix	Bone surfaces

Key Equations

• Calcium Homeostasis:

$\text{Ca}^{2+}_{\text{blood}} \xleftrightarrow{\text{PTH, Calcitonin}} \text{Ca}^{2+}_{\text{bone}}$

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology content.