Cartilage

Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints. It is a crucial component of the skeletal system, providing structural support to various parts of the body, including the ears, nose, and joints. Unlike bone, cartilage does not contain blood vessels; it relies on the perichondrium, a dense layer of connective tissue, for diffusion of nutrients. This unique structure allows cartilage to function effectively as a cushion for joints, aiding in the reduction of friction and serving as a shock absorber during movement. Understanding cartilage and its function is essential for exploring how our bodies maintain mobility and manage everyday physical stresses.

What is Cartilage?

Cartilage is a firm, flexible connective tissue found in various parts of the human body including the joints, rib cage, ear, nose, bronchial tubes, and intervertebral discs. Unlike bone, cartilage is more elastic and lacks the hard mineralization, making it an essential component for cushioning and structural support. This tissue is made up primarily of chondrocytes, the cells responsible for producing the large amounts of collagen and proteoglycan that form its matrix.

Cartilage Structure

Cartilage is a specialized form of connective tissue that plays a critical role in the human body. It provides support and cushioning at joints, maintains the shape of certain structures like the nose and ears, and allows for smooth movements between bones. The structure of cartilage is unique and designed to fulfill its various functions effectively. Here’s a detailed look at the structure of cartilage:

Matrix Composition

The bulk of cartilage is made up of extracellular matrix, which is rich in collagen fibers, proteoglycans, and elastin fibers. The type and arrangement of these fibers vary depending on the type of cartilage, which in turn affects its flexibility and strength.

• Collagen Fibers: These provide tensile strength, ensuring that the cartilage can withstand elongation without tearing.
• Proteoglycans: Consisting of proteins and polysaccharides, proteoglycans are responsible for retaining water, which contributes to the resilience of cartilage, allowing it to return to its original shape after compression.
• Elastin Fibers: Found primarily in elastic cartilage, these fibers grant cartilage the ability to stretch and bounce back.

Chondrocytes

Chondrocytes are the cells that produce and maintain the cartilaginous matrix. These cells are located within spaces in the matrix called lacunae. Chondrocytes derive from precursor cells called chondroblasts, which become embedded in the matrix they produce and then mature into chondrocytes.

Perichondrium

Except in the articular cartilage covering joints, most cartilage is surrounded by a dense layer of connective tissue called the perichondrium. This layer serves two primary functions:

• Growth and Repair: The perichondrium contains cells that can differentiate into chondroblasts, which are essential for cartilage growth and repair.
• Nutrient Supply: Since cartilage is avascular (lacks blood vessels), the perichondrium plays a crucial role in the diffusion of nutrients to the cartilage.

Functions of Cartilage

1. Support and Structure

Cartilage provides structure and support to various body parts without the rigidity of bone. It is crucial in the development and shaping of a child’s skeleton and in areas such as the nose, ear, and trachea where a mix of flexibility and rigidity is essential.

2. Facilitating Movement

Cartilage acts as a cushion between bones at a joint and prevents their friction, which can lead to damage. By covering the ends of bones where they meet to form joints, cartilage enables smooth and pain-free movement. This is particularly evident in load-bearing joints such as the knees and hips.

3. Shock Absorption

Cartilage serves a vital role in absorbing shock in joints that carry the weight of the body. The flexibility and resilience of cartilage help it absorb impact from walking, running, or any activity that puts stress on the bones.

4. Growth and Development

Cartilage plays an essential role in the growth and lengthening of bones in children and adolescents. This is primarily through the process of endochondral ossification, where cartilage is progressively replaced by bone in the growth plate, leading to an increase in bone length until early adulthood.

5. Contributing to Respiratory Functions

In the respiratory system, cartilage helps to keep the trachea and bronchi open and flexible. This flexibility ensures that the airway is resistant to collapse and can adjust its diameter, facilitating effective air passage during breathing.

6. Protection of Underlying Tissue

Cartilage also protects underlying tissues by providing a tough, but yielding barrier to external forces. This function is crucial in areas such as the knee, where the cartilage protects the ends of the bones from mechanical damage.

Types of Cartilage

Cartilage is classified into three types, each with distinct properties and locations:

1. Hyaline Cartilage

• Description: This is the most common type of cartilage, characterized by its glossy, bluish-white appearance. It consists of a large number of collagen fibers and is less flexible than elastic cartilage but more flexible than fibrocartilage.
• Locations:
  • Articular Cartilage: Covers the ends of bones in major joints like the knees, shoulders, and hips.
  • Costal Cartilage: Connects the ribs to the sternum (breastbone), contributing to the rib cage’s elasticity and expansion during breathing.
  • Respiratory Cartilages: Forms part of the larynx and reinforces the trachea and bronchi within the respiratory tract.
  • Nasal Cartilages: Supports the external structure of the nose.

2. Elastic Cartilage

• Description: Contains elastic fibers along with collagen, making it more flexible than hyaline cartilage. It can withstand repeated bending, so it is found in structures that require flexibility.
• Locations:
  • Ear (Pinna): Supports the shape of the ear.
  • Epiglottis: A flap in the throat that prevents food and drink from entering the windpipe during swallowing.
  • Eustachian Tube: Connects the middle ear to the throat and helps in equalizing ear pressure.

3. Fibrocartilage

• Description: The toughest and most durable type of cartilage, fibrocartilage contains a dense matrix of collagen fibers. It is designed to withstand heavy loads and high tension, making it ideal for areas that undergo intense pressure.
• Locations:
  • Intervertebral Discs: Acts as cushions between the vertebrae in the spine.
  • Menisci: Crescent-shaped cartilage in the knee that distributes weight and improves the fit between the femur and tibia.
  • Pubic Symphysis: Located between the pubic bones in the pelvis, aiding in pelvic movement and absorption of pressure during locomotion.

Stages of Cartilage Formation

1. Mesenchymal Cell Aggregation

• Process: Chondrogenesis begins with the aggregation of mesenchymal stem cells, which are multipotent stromal cells that can differentiate into a variety of cell types. These cells are derived from the mesoderm layer during embryonic development.
• Result: The aggregated cells form dense cellular condensations in specific areas where cartilage is to be formed.

2. Differentiation into Chondroprogenitor Cells

• Process: The cells within the condensations differentiate into chondroprogenitor cells. These cells are the early stage of cells that specifically commit to becoming chondrocytes.
• Result: Chondroprogenitor cells begin to express early cartilage markers, such as SOX9, a transcription factor crucial for promoting chondrogenesis.

3. Maturation into Chondrocytes

• Process: Under the influence of regulatory factors like SOX9, BMPs (Bone Morphogenetic Proteins), and other growth factors, chondroprogenitor cells mature into chondrocytes. These chondrocytes start to produce the extracellular matrix components characteristic of cartilage, including type II collagen and proteoglycans.
• Result: The maturing chondrocytes become embedded in the cartilage matrix they produce, which is crucial for the structural integrity and function of cartilage.

4. Secretion of Extracellular Matrix

• Process: As chondrocytes mature, they secrete large amounts of extracellular matrix, which consists of collagen fibers, elastin fibers, and a gel-like substance made up of proteoglycans and water.
• Result: The secretion of the extracellular matrix gives cartilage its unique properties of resilience and flexibility, capable of withstanding compressive forces.

Types of Cartilage Growth

Cartilage growth can occur in two main ways during development and throughout life:

1. Interstitial Growth

• Description: This type of growth involves the division of existing chondrocytes within the cartilage, leading to an increase in the volume of cartilage from within.
• Importance: Interstitial growth is significant during the early stages of cartilage formation and in the growth of long bones in children and adolescents.

2. Appositional Growth

• Description: New chondrocytes form from the differentiation of perichondrial cells located on the surface of the cartilage, contributing to the thickness and repair of cartilage.
• Importance: Appositional growth is crucial for the surface expansion of cartilage and ongoing maintenance and repair throughout an individual’s life.

Anatomy of Cartilage

The anatomy of cartilage is uniquely suited to its functions. It is primarily made up of chondrocytes, the only type of cells found in cartilage, which produce and maintain the cartilaginous matrix. This matrix is rich in collagen fibers and proteoglycans, which attract water to provide a hydrated, gel-like environment that resists compression.

Structural Features

• No Blood Vessels or Nerves: Cartilage is avascular, meaning it lacks blood vessels. Nutrients are diffused through the matrix, which slows down its metabolic activity and regenerative capabilities.
• Perichondrium: A dense layer of connective tissue that surrounds the cartilage (except at articulations). This layer provides oxygen and nutrients to the cartilage and facilitates its growth and repair.

Blood Supply and Lymphatics of Cartilage

Cartilage is distinct in its composition and function, particularly notable for its avascular nature, meaning it lacks its own blood vessels. This characteristic of cartilage has significant implications for its nourishment and repair mechanisms. Here’s a closer look at the blood supply and lymphatics of cartilage:

Avascular Nature of Cartilage

Cartilage does not contain blood vessels within its matrix. This lack of direct blood supply poses a unique challenge for the delivery of nutrients and the removal of waste products. The avascular nature of cartilage results in slower metabolic processes, and consequently, a reduced capacity for repair and regeneration.

Nutrient Supply Through Diffusion

Since cartilage is avascular, the nutrients required for the chondrocytes—the cells within the cartilage that produce and maintain the cartilaginous matrix—are supplied by diffusion. This process occurs through the following:

• Perichondrium: For the types of cartilage enveloped by a perichondrium (a dense layer of connective tissue), such as hyaline cartilage and elastic cartilage, nutrients diffuse through this outer layer. The perichondrium contains blood vessels, and nutrients diffuse from these vessels through the dense connective tissue into the cartilage.
• Synovial Fluid: In the case of articular cartilage, such as that found in movable joints, nutrients are derived from the synovial fluid. This fluid fills the joint cavity and serves as a lubricant to reduce friction between articular bones during movement. Synovial fluid also provides the necessary nutrients and removes waste from the cartilage.

Absence of Lymphatic Vessels

Similarly to the absence of blood vessels, cartilage also lacks lymphatic vessels. The removal of waste products from cartilage also primarily occurs through diffusion. In articular cartilage, metabolic wastes are expelled into the synovial fluid, which is then circulated away when the joint moves, facilitating the exchange of waste and nutrients.

Implications of Avascularity

The avascular nature of cartilage has important clinical implications, especially in terms of healing and repair. Since the repair process is slower due to limited nutrient supply and waste removal, damage to cartilage, whether due to injury or disease like osteoarthritis, is often challenging to treat and can lead to long-term joint problems.

Common Disorders of Cartilage

1. Osteoarthritis (OA)

• Description: This is the most common form of arthritis, characterized by the breakdown and eventual loss of cartilage in one or more joints. Osteoarthritis commonly affects the hands, knees, hips, and spine.
• Symptoms: Pain, swelling, reduced range of motion, and sometimes a grating sensation during joint movement.
• Cause: The wear and tear of cartilage over time, though genetic factors, obesity, and joint injuries can exacerbate the condition.

2. Rheumatoid Arthritis (RA)

• Description: Unlike osteoarthritis, RA is an autoimmune disorder where the immune system mistakenly attacks the joints, primarily affecting the joint lining and cartilage.
• Symptoms: Tender, warm, swollen joints and morning stiffness that may last for hours.
• Cause: The exact cause of RA is unknown, but it involves a combination of genetic, environmental, and hormonal factors.

3. Traumatic Injuries

• Description: Sudden injuries to the cartilage can occur during sports or due to accidents, leading to tears or breaks. Common examples include meniscus tears in the knee and labral tears in the shoulder.
• Symptoms: Pain, swelling, and instability of the affected joint.
• Cause: Direct impact or sudden, sharp movements that exceed the tissue’s tolerance.

4. Chondromalacia Patellae

• Description: Also known as “runner’s knee,” this condition involves the softening and breakdown of the cartilage on the underside of the kneecap (patella).
• Symptoms: Knee pain and a sensation of grinding when the knee is extended.
• Cause: Overuse, misalignment, and other factors that increase stress on the knee joint.

5. Osteochondritis Dissecans

• Description: A joint condition whereby bone underneath the cartilage of a joint dies due to lack of blood flow. This can lead to small pieces of bone and cartilage breaking off and causing joint pain and dysfunction.
• Symptoms: Pain, swelling, and joint catching or locking.
• Cause: Often occurs in young athletes; however, the exact cause is not well understood and may involve genetic factors, repetitive trauma, or abnormalities in bone formation.

FAQs

Is Cartilage a Small Bone?

No, cartilage is not a small bone; it is a flexible connective tissue that cushions joints and supports structures like ears and nose.

What is Cartilage Short?

Cartilage is a tough, flexible tissue that supports and cushions bones at joints without blood vessels.

Where is Cartilage Located?

Cartilage is found in joints, rib cage, ear, nose, throat, and between intervertebral discs in the spine.

What is an Example of a Cartilage Bone?

The term “cartilage bone” refers to bones that originally form from cartilage, such as the femur in the leg.

Are All Bones Cartilage?

No, not all bones are cartilage. Bones are rigid, while cartilage is flexible and lacks mineralization.