Skeletal System: Histology of Bone and Cartilage

Overview

The skeletal system is composed of specialized connective tissues, primarily bone and cartilage, which provide structural support, protection, and facilitate movement. Understanding the histological components of these tissues is essential for comprehending their functions and roles in the human body.

Classification of Connective Tissue

Types of Connective Tissue

• Connective Tissue Proper

  • Loose connective tissue (e.g., areolar, adipose, reticular)

  • Dense connective tissue (e.g., dense regular, dense irregular, elastic)

• Specialized Connective Tissue

  • Supporting connective tissue (cartilage and bone)

  • Fluid connective tissue (blood and lymph)

Additional info: Connective tissues are characterized by cells embedded in an extracellular matrix composed of fibers and ground substance.

Cartilage

Histological Features

• Cells: Chondrocytes, located in spaces called lacunae.

• Matrix: Composed of collagen and elastic fibers embedded in chondroitin sulfate (a gel-like ground substance).

• Perichondrium: Dense irregular connective tissue covering cartilage (except at articular surfaces).

• Properties: Avascular (lacks blood vessels), lacks nerves, can withstand more stress than other connective tissues.

Types of Cartilage

• Hyaline Cartilage

  • Most common type; provides support with flexibility.

  • Locations: Articular surfaces of bones, nose, trachea, larynx, costal cartilages.

  • Matrix appears glassy; contains fine collagen fibers.

• Fibrocartilage

  • Contains thick bundles of collagen fibers, providing rigidity and strength.

  • Locations: Intervertebral discs, menisci of the knee, pubic symphysis.

  • Functions as a shock absorber.

• Elastic Cartilage

  • Contains a network of elastic fibers, allowing flexibility and shape retention.

  • Locations: Auricle of the ear, epiglottis.

Bone (Osseous Tissue)

Histological Components

• Cells:

  • Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells that secrete matrix and collagen fibers.

  • Osteocytes: Mature bone cells that maintain bone tissue.

  • Osteoclasts: Large, multinucleated cells responsible for bone resorption.

• Matrix: Contains collagen fibers and mineral salts, primarily hydroxyapatite () and calcium carbonate.

Functions of the Skeletal System

• Support: Provides a framework for the body.

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

• Movement: Serves as attachment sites for muscles.

• Mineral Storage: Stores and releases minerals, especially calcium and phosphate.

• Blood Cell Production: Hemopoiesis occurs in red bone marrow.

• Triglyceride Storage: Yellow bone marrow stores fat.

Classification of Bones by Shape

• Long Bones: Greater length than width (e.g., femur, humerus).

• Short Bones: Nearly equal in length, width, and depth (e.g., carpals, tarsals).

• Flat Bones: Thin, composed of two parallel plates (e.g., sternum, cranial bones).

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

• Sesamoid Bones: Develop in tendons (e.g., patella).

Structure of a Typical Long Bone

• Diaphysis: Shaft or body of the bone.

• Epiphyses: Proximal and distal ends.

• Metaphysis: Region between diaphysis and epiphysis; contains the epiphyseal (growth) plate in growing bones.

• Articular Cartilage: Hyaline cartilage covering joint surfaces.

• Medullary Cavity: Hollow space within the diaphysis containing yellow marrow.

• Periosteum: Tough membrane covering the bone (except at articular cartilage); consists of an outer fibrous layer and an inner cellular layer.

• Endosteum: Thin membrane lining the medullary cavity.

Types of Bone Tissue

Compact (Cortical) Bone

• Composed of concentric rings (lamellae) surrounding a central (Haversian) canal.

• Contains osteons (Haversian systems), the structural unit of compact bone.

• Provides strength for weight-bearing and protection.

• Found in the shaft of long bones and the outer layer of all bones.

Spongy (Cancellous, Trabecular) Bone

• Composed of a network of thin plates called trabeculae, aligned along lines of stress.

• Spaces between trabeculae are filled with red bone marrow, where blood cells develop.

• Found at the ends of long bones and inside flat bones (e.g., sternum, ribs, skull).

Bone Formation (Ossification)

Types of Ossification

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone forms by replacing a hyaline cartilage model (e.g., most bones of the body).

Bone Growth

• Lengthwise Growth: Occurs at the epiphyseal (growth) plate by proliferation of cartilage cells on the epiphyseal side and replacement by bone on the diaphyseal side.

• Appositional Growth (Width): Osteoblasts in the periosteum add new bone tissue to the outer surface, while osteoclasts resorb bone from the inner surface.

Bone Remodeling

• Continuous process of bone resorption (by osteoclasts) and bone deposition (by osteoblasts).

• Allows bone to adapt to stress, repair microdamage, and maintain mineral homeostasis.

• Example: Orthodontic tooth movement involves remodeling of alveolar bone in response to pressure from braces.

Blood and Nerve Supply of Bone

• Bones have a rich blood supply, especially in areas of active growth and remodeling.

• Nerves accompany blood vessels and are present in the periosteum, carrying pain sensations.

Aging and Bone Tissue

• With aging, there is a loss of bone mass due to decreased production of matrix components and minerals.

• Results in increased risk of osteoporosis and fractures.

Summary Table: Types of Cartilage