Bones and Skeletal Tissues

Bones and Skeletal Tissues

Overview of Bone Tissue

• Chapter 6: This chapter explores the structure and function of bones and skeletal tissues, detailing various components and classifications.

Perichondrium

• Definition: a dense layer of connective tissue that surrounds cartilage, consisting of collagen fibers and cells, which aids in the nourishment and growth of cartilage tissue.

Types of Cartilage

• Types:
  • Hyaline Cartilage:
  • Characteristics: Glossy, smooth appearance; provides support and flexibility.
  • Elastic Cartilage:
  • Characteristics: Contains more fibers, gives flexibility to structures like the ear.
  • Fibrocartilage:
  • Characteristics: Tough, contains dense bundles of collagen, provides support and absorbs shock in intervertebral discs.

Types of Cartilage Structures

• Chondrocyte: A type of cell found in cartilage that maintains the cartilaginous matrix.
• Lacuna: Small cavities within the matrix that contain chondrocytes.
• Matrix: The intercellular substance of the cartilage that contains collagen and elastic fibers.

Bone Structure

Gross Anatomy of Bone

• Compact Bone:
  • Dense outer layer that appears smooth and solid.
• Spongy Bone:
  • Contains a honeycomb of trabeculae (pieces of bone), filled with bone marrow, and lacks osteons.

Classification of Bones

• By Shape:
  1. Long Bones: Longer than they are wide (e.g., limb bones).
  2. Short Bones: Cube-shaped (e.g., wrist and ankle bones).
  3. Flat Bones: Thin and flat (e.g., sternum).
  4. Irregular Bones: Have complex shapes (e.g., vertebrae).
  5. Sesamoid Bones: Formed within tendons (e.g., patella).

Bone Features

• Articular Cartilage: Silk-like covering on bone ends, providing a smooth surface at joints.
• Endosteum: Thin membrane lining the internal surfaces of bones, covering trabeculae.
• Periosteum: Double-layered connective tissue surrounding bone; important for bone growth and repair.

Bone Marrow Types

• Red Marrow: Found within trabecular cavities and is responsible for blood cell formation (hematopoiesis).
• Yellow Marrow: Found in the medullary cavities; stores fat; can convert to red marrow if needed.

Bone Membranes

• Functions:
  • Nutritional support and homeostasis provided by the periosteum.
  • The endosteum facilitates bone growth and repair.

Bone Cells

Five Major Bone Cell Types

1. Osteogenic Cells:
   • Stem cells found in periosteum and endosteum.
   • Differentiate into osteoblasts or bone-lining cells.
2. Osteoblasts:
   • Bone-forming cells that secrete osteoid (unmineralized matrix).
   • Osteoid is primarily composed of collagen and calcium-binding proteins.
3. Osteocytes:
   • Mature bone cells that maintain the bone matrix, not mitotic; monitor the environment in the bone.
4. Bone-Lining Cells:
   • Flat cells that help maintain the bone surface.
5. Osteoclasts:
   • Large cells responsible for bone resorption (breaking down bone).

Compact Bone Microscopic Anatomy

• Central Canal: Houses blood vessels and nerve fibers; runs through the center of each osteon.
• Canaliculi: Small canals connecting lacunae and allowing for nutrient exchange between osteocytes.
• Lamellae: Layers of compact bone around the central canals, contributing to the strength of bones.
• Interstitial Lamellae: Remnants of old osteons that fill gaps between newer osteons.
• Circumferential Lamellae: Surround the entire bone and resist twisting and bending.

Spongy Bone Microscopic Anatomy

• Trabeculae: Structural units that confer strength and resistance to stress; contain red bone marrow.
• No osteons present.

Chemical Composition of Bone

Organic Components

• Comprises osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.
• Organic matrix provides flexibility and tensile strength.
• The resilience of bones is due to sacrificial bonds that dissipate energy during impact.

Inorganic Components

• Hydroxyapatites (Mineral Salts):
  • Comprise approximately 65% of bone mass, providing hardness and structural integrity.

Bone Development and Growth

Ossification

• Definition: The process of bone tissue formation. Occurs during fetal development and postnatal growth.

Types of Ossification

1. Endochondral Ossification:
   • Most common; forms bone by replacing hyaline cartilage, responsible for forming long bones.
2. Intramembranous Ossification:
   • Forms directly from mesenchymal tissue, responsible for flat bones like the skull.

Growth in Length of Long Bones

• Epiphyseal Plate Zones:
  1. Resting Zone: Inactive cartilage.
  2. Proliferation Zone: Rapidly dividing cells pushing epiphysis away from diaphysis.
  3. Hypertrophic Zone: Enlarged chondrocytes; cartilage lacunae enlarge.
  4. Calcification Zone: Cartilage matrix calcifies; chondrocytes die.
  5. Ossification Zone: Newly formed bone replaces degraded cartilage.

Growth in Width (Thickness)

• Involves appositional growth; bones widen as they lengthen.

Factors Affecting Bone Growth

Hormonal Control

• Growth Hormone: Stimulates epiphyseal plate activity.
• Thyroid Hormone: Modulates growth hormone effects.
• Sex Hormones (Testosterone/Estrogens): Promote growth spurts and cause epiphyseal plate closure during puberty.

Response to Mechanical Stress

• Wolff's Law: Bone remodels in response to mechanical stress placed upon it.

Bone Healing Process

Stages of Fracture Repair

1. Hematoma Formation: Swelling and inflammation at the fracture site.
2. Fibrocartilaginous Callus Formation: New tissue bridges the gap between broken bones.
3. Bony Callus Formation: Bone replaces fibrocartilaginous callus, converting it to spongy bone over several weeks.
4. Bone Remodeling: The bone may return to its original shape over time.

Types of Fractures