Cartilage Overview and Importance

Cartilage Overview

• Cartilage is a specialized connective tissue that provides support and resilience
• It has a rigid structure, capable of withstanding pressure
• It is composed of cells (chondrocytes and chondroblasts) and a matrix (fibers and ground substance)
• Fibers include collagen type II (mostly), and sometimes type I and/or elastic
• Ground substance consists of proteoglycans (GAGs), glycoproteins, and water
• Found in body areas needing elasticity and rigidity

Importance of Cartilage

• Capable of bending and returning to its original shape
• Provides rigidity and support to soft tissues
• Offers a smooth gliding surface for articulation
• Enables the development and growth of long bones

Cartilage Cells

• Chondroblasts are young cells that secrete the cartilage matrix around them
• Chondrocytes are mature cartilage cells, residing in lacunae within the matrix
• Mesenchymal (chondrogenic) cells differentiate into chondroblasts
• As cartilage grows, chondroblasts become trapped in the matrix compartments (lacunae), maturing into chondrocytes
• Cartilage calcification leads to chondrocyte death due to nutrient deprivation

Ground Substance of Cartilage

• Composed of proteoglycans (GAGs), glycoproteins, and water
• Proteoglycans (chondroitin sulfate and keratan sulfate) attract water molecules, forming a gel-like consistency
• Glycoproteins (chondronectin and chondrocalcin) bind to GAGs and collagen fibers, holding cartilage cells in place
• Water (tissue fluid) hydrates the matrix

Characteristic Features of Cartilages

• Typically surrounded by perichondrium (dense irregular connective tissue)
• Structure comprises outer fibrous and inner cellular layers
• Fibrous layer contains collagen fibers and fibroblasts
• Cellular layer includes chondrogenic cells that differentiate into chondroblasts, producing the cartilage matrix
• Articular and fibrocartilage lack perichondrium
• Cartilage provides nutrient and support

Cartilage Properties

• Avascular: Nourishment occurs through diffusion via the ground substance
• Slow repair/regeneration
• No lymphatics: Homogenous cartilage transplantation is possible without rejection
• No nerves: Insensitive

Cartilage Growth

• Appositional growth: New cartilage is added to the surface (beneath the perichondrium) by chondroblasts, increasing cartilage width
• Interstitial growth: Existing cartilage is added internally through chondrocyte proliferation, increasing cartilage size

Types of Cartilage

• Based on the fiber type within their matrix:
  • Hyaline cartilage (provides structural support)
  • Elastic cartilage (confers flexibility)
  • Fibrocartilage (provides tensile strength)

Hyaline Cartilage Locations

• Found in various locations: nose, larynx cartilages (thyroid, cricoid, lower arytenoid), trachea and bronchi, costal cartilages of ribs, articular cartilages in joints

Hyaline Cartilage Functional Significance

• In young individuals: Present in epiphyseal plates, enabling bone growth in length
• In adults: Mostly replaced with bone
• Articular cartilage (hyaline) has no perichondrium in joints, preventing friction and allowing smooth movement

Hyaline Cartilage Structure

• Covered by perichondrium
• Contains a glossy, homogenous, transparent, basophilic matrix containing collagen fibers (type II)
• Chondrocytes reside within lacunae, often arranged in isogenous groups

Hyaline Cartilage Matrix

• Territorial matrix: Surrounding lacunae, more basophilic
• Interterritorial matrix: Located between lacunae, less basophilic
• Refractive index of collagen in the matrix is similar to that of ground substance, creating a glossy appearance

Elastic Cartilage

• Highly flexible
• Found in: ear pinna, part of the auditory tube, and cartilages of the larynx (epiglottis, apex of arytenoid, corniculate, and cuneiform cartilages)

Elastic Cartilage Structure

• Covered by perichondrium
• Chondroblasts synthesize elastic fibers
• Matrix contains elastic fibers with branching and anastomoses, few type II collagen fibers
• Ground substance is basophilic
• Chondrocytes are mostly singly located within lacunae

Fibrocartilage

• Bears weight and resists compression
• Found in: secondary cartilaginous joints (intervertebral discs, symphysis), articular disk of the temporomandibular joint, glenoid labrum and acetabular labrum, menisci of the knee joint

Fibrocartilage Structure

• Not covered by perichondrium
• Matrix has thick bundles of collagen fibers (type I) arranged interlacingly
• Fewer chondrocytes, often arranged in a single row between bundles

Osteoarthritis

• Due to articular cartilage degeneration
• Lack of perichondrium means no regeneration capacity
• Degeneration causes difficulty walking (hip and knee involvement)
• Joint involvement (hand) can be painful

Self Learning

• Identify photomicrographs of different cartilage types (A, B, C)
• List histological differences
• Note locations of cartilage types
• Identify component responsible for high water content in cartilage (proteoglycan)

This quiz provides an in-depth look at cartilage, a vital connective tissue that offers support and resilience in the body. It covers its structure, types of cartilage cells, and the roles they play in maintaining body function. Test your knowledge on the significance and functionality of cartilage.