Bone Tissue

Tissues of the Skeletal System

The skeletal system is composed of several tissue types, each contributing to the structure and function of bones as organs.

• Osseous tissue: The main tissue forming bones, providing strength and support.

• Cartilage: Flexible connective tissue found at joints and in growth plates.

• Blood: Supplies nutrients and removes waste from bone tissue.

• Adipose tissue: Stores fat within bone marrow.

• Nervous tissue: Provides sensory and regulatory functions.

• Fibrous connective tissue: Forms membranes such as periosteum and endosteum.

Each bone is considered a separate organ due to its complex structure and multiple tissue types.

Osseous Tissue

Structure and Properties

Osseous tissue is a specialized connective tissue with a hard matrix primarily composed of calcium phosphate. The mineralization of collagen fibers creates a rigid structure, making bone the hardest substance in the human body.

• Matrix: Hard due to calcium phosphate and collagen fiber mineralization.

• Function: Provides structural support and protection.

Types of Osseous Tissue

• Compact bone:

  • Organized into osteons (Haversian systems).

  • Concentric lamellae encircle a central canal.

  • Forms the outer surface of all bones.

• Spongy bone:

  • Organized into trabeculae.

  • Lamellae arranged as rods or plates to form trabeculae.

  • Contains red bone marrow in spaces between trabeculae.

Cartilages

Types and Structure

Cartilage is a flexible connective tissue found in various locations throughout the body. It is classified into three main types:

• Hyaline cartilage: Most common; found at articular surfaces, growth plates, and respiratory tract.

• Elastic cartilage: Contains elastic fibers; found in the ear and epiglottis.

• Fibrocartilage: Contains dense collagen fibers; found in intervertebral discs and pubic symphysis.

Chondroblasts are cartilage-forming cells, while chondrocytes are mature cells residing in lacunae. The perichondrium is a dense connective tissue membrane surrounding cartilage (except at articular surfaces).

Comparison of Cartilage and Bone

The following table summarizes the main differences between cartilage and bone:

Structure of Bones

Structure of Flat Bones

Flat bones, such as those in the skull, have a unique structure that provides protection and strength.

• External and internal surfaces: Composed of compact bone.

• Diploe: Spongy bone layer sandwiched between compact bone layers.

• No marrow cavity: Marrow is present in the trabeculae of spongy bone.

• Function: Protection of vital organs (e.g., brain).

Structure of Long Bones

Long bones, such as the femur and humerus, have a complex structure that supports movement and growth.

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

• Epiphysis: Enlarged ends containing spongy bone; increases surface area for articulation.

• Articulation: Where one bone meets another; covered by articular cartilage (hyaline cartilage) and lubricated by synovial fluid.

• Endosteum: Internal membrane lining the medullary cavity and covering trabeculae; contains osteogenic cells, osteoblasts, and osteoclasts.

• Periosteum: External membrane with an outer dense irregular connective tissue layer and an inner osteogenic layer; absent over articular cartilage.

• Epiphyseal growth plate: Hyaline cartilage plate separating epiphysis and diaphysis in growing bones; site of bone elongation. After puberty, it becomes the epiphyseal line (calcified).

Bone Microanatomy

Key Structures

The microscopic structure of bone is essential for its function and remodeling.

• Osteon: The structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix within osteons.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing communication and nutrient exchange between osteocytes.

• Central canal: Contains blood vessels and nerves.

Chemical Composition of Bone

Organic and Inorganic Components

Bone tissue is composed of both organic and inorganic materials, each contributing to its properties.

• Organic components:

  • Cells: Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

  • Osteoid: Unmineralized bone matrix, mostly collagen fibers, made and secreted by osteoblasts; provides tensile strength (resistance to bending and stretching).

• Inorganic components:

  • 85% hydroxyapatite (crystallized calcium phosphate salt):

  • 15% other minerals (e.g., magnesium, sodium, potassium).

  • Mineralization provides hardness and allows bone to resist compression.

Functions of the Skeletal System

Major Roles

The skeletal system performs several vital functions in the human body:

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

• Anchorage/Levers: Facilitates movement by serving as attachment points for muscles.

• Blood cell formation (Hematopoiesis): Occurs in red bone marrow.

• Mineral storage: Reservoir for calcium and phosphate.

• Triglyceride storage: Fat stored in yellow bone marrow.

• Hormone production: Osteocalcin regulates bone formation and energy metabolism.

Additional info:

• Some details, such as the full process of bone remodeling, hormonal regulation, and clinical conditions (e.g., osteoporosis), are covered in later sections of the original notes and are essential for a complete understanding of bone physiology.