Cartilage Histology

1. Cells (5%)
2. Extracellular matrix (95%)

1. Hyaline cartilage
2. Elastic cartilage
3. Fibrocartilage

They permit easy diffusion of nutrients through the cartilage, which is avascular and receives nutrition by diffusion from adjacent tissues.

The perichondrium is a fibrocellular covering that externally supports cartilages, except for fibrocartilages. It has outer fibrous and inner cellular layers.

Hyaline cartilage is found in the fetal skeleton, articular cartilages, nose cartilage, costal cartilages, laryngeal cartilages (thyroid, cricoid, and arytenoids), trachea, bronchi, and developing bone (epiphyses).

Hyaline cartilage has a glass-like (transparent) matrix, which is why it is called hyaline cartilage (from Greek hyalos meaning glass).

Chondrocytes receive their nutrition through diffusion from adjacent tissues, as cartilage is avascular.

Hyaline cartilage can undergo ossification, meaning it can be converted into bone in later age.

The matrix of hyaline cartilage consists of:

• 60-80% water
• 15% collagen fibers (mainly type II collagen)
• Approximately 15% proteoglycans and glycoproteins
• 3-5% cells

The glassy appearance of hyaline cartilage is due to the homogeneous refractive indices of collagen fibers and the ground substance, which makes collagen fibers invisible in histological sections.

The ground substance of hyaline cartilage contains three main types of proteoglycans:

• Hyaluronan
• Chondroitin sulfate
• Keratan sulfate

Proteoglycans provide firmness to hyaline cartilage and contribute to its structural integrity. Acidic proteoglycans also take on a hematoxylin (violet) color, giving the matrix a violet appearance.

Collagen fibers, primarily type II collagen, provide strength and stability to hyaline cartilage by forming a meshwork within the ground substance.

A proteoglycan monomer, such as aggrecan, consists of a core protein with peripherally attached chondroitin and keratan sulfate molecules arranged in a bottle-brush configuration.

The main components of hyaline cartilage include:

1. Cells/Chondrocytes:

   • Lie in lacunae
   • Form cell nests/isogenous groups
   • Found in fetal skeleton, articular cartilage, nose, costal cartilages, laryngeal cartilages, trachea, bronchi, and developing bone.

2. Matrix:

   • Contains Type II Collagen fibers, Proteoglycans (Chondroitin sulfate, Keratan sulfate), and Glycoproteins.
   • Has three types: Capsular matrix, Territorial matrix, and Interterritorial matrix.

3. Perichondrium:

   • Irregular fibrous connective tissue with outer fibrous and inner cellular layers.

The aggrecan molecule is negatively charged, allowing it to hold water molecules, which contributes to the hydrated matrix. This hydration facilitates the diffusion of metabolites and provides resilience to cartilage.

The matrix shows varied staining properties with hematoxylin based on the concentration of sulfated proteoglycans:

1. Capsule/Pericellular matrix: Densely staining due to high content of negatively charged sulfated proteoglycans and type VI collagen fibrils.
2. Territorial matrix: Stains less intensely than the capsular matrix due to lower quantity of sulfated proteoglycans.
3. Interterritorial matrix: Stains lighter than the territorial matrix, containing even less sulfated proteoglycans.

The three zones of the matrix in hyaline cartilage based on staining properties are:

1. Capsule/Pericellular matrix: Densely staining, high in sulfated proteoglycans.
2. Territorial matrix: Surrounds the capsular matrix, stains less intensely.
3. Interterritorial matrix: Surrounds the territorial matrix, stains lighter.

The two types of matrix are:

1. Capsular matrix: Surrounds each chondrocyte.
2. Territorial matrix: Surrounds the capsular matrix.

The perichondrium serves as a protective layer and has two layers:

1. Outer fibrous layer: Composed of dense irregular fibrous connective tissue.
2. Inner cellular layer: Contains cells that divide and give rise to new chondrocytes. It also has capillaries that provide nutrients to the cartilage through diffusion.

The two types of growth in cartilage are:

1. Interstitial growth: Addition of new cartilage throughout the substance of cartilage before maturation.
2. Appositional growth: Formation of new cartilage on the surface by division of chondrocytes after maturation, facilitated by the cellular layer of perichondrium.

Osteoarthritis is a common joint disease characterized by:

• Chronic joint pain, especially in weight-bearing joints (e.g., hip and knee).
• Decrease in proteoglycan content and water content of the cartilage matrix.
• Increased production of interleukin 1 and tumor necrotic factor by chondrocytes, along with decreased production of type II collagen and proteoglycans.
• It has no cure and typically progresses with age.

Chondrocytes are the cells of hyaline cartilage that:

• Divide and deposit matrix around themselves.
• Occupy lacunae in the matrix.
• Form isogenous groups or cell nests during development, which are groups of daughter chondrocytes derived from a single mother chondrocyte.

Elastic cartilage consists of ground substance, elastic fibers, and chondrocytes. The ground substance contains proteoglycans and glycoproteins. Chondrocytes are the cells that produce collagen and ground substance.

The perichondrium surrounds elastic cartilage and has two layers:

1. Outer fibrous layer - consists of dense irregular fibrous connective tissue.
2. Inner cellular layer - contains cells that divide and give rise to new chondrocytes, aiding in the growth and repair of cartilage.

Elastic cartilage is located in several areas including:

• Pinna of the external ear
• Walls of the external acoustic meatus
• Auditory tube
• Epiglottis
• Tips of arytenoids, corniculate, and cuneiform cartilages of the larynx.

In elastic cartilage, chondrocytes are larger than those in hyaline cartilage and can be found singly or in groups of two. Additionally, the cells in elastic cartilage occupy lacunae that appear empty due to shrinkage during slide preparation, whereas in living tissue, they fill the lacunae completely.

The elastic fibers in elastic cartilage form a dense meshwork that provides resilience, pliability, and elasticity to the cartilage, allowing it to return to its original shape after deformation. These fibers also impart a yellow color to the cartilage.

Elastic cartilage provides elasticity, resilience, and firmness to the structure.

The matrix of elastic cartilage consists of a meshwork of branching anastomosing elastic fibers and a ground substance that includes proteoglycans.

The perichondrium surrounding elastic cartilage has two layers: an outer fibrous layer and an inner cellular layer.

Elastic cartilage is found in locations such as the pinna of the external ear, external acoustic meatus, auditory tube, epiglottis, and the corniculate and cuneiform cartilages of the larynx.

Fibrocartilage is characterized by rows of chondrocytes embedded in a matrix of thick collagen fibers.

Fibrocartilage consists of type I and type II collagen fibers in varying proportions, forming dense regular connective tissue.

Fibrocartilage is found in locations such as intervertebral discs, pubic symphysis, articular discs of joints, menisci, and at tendon attachment sites with bones.

Fibrocartilage consists of:

1. Fibers: Bundles of thick collagen fibers that form dense regular connective tissue.
2. Cells/Chondrocytes: Lie singly or in rows within lacunae surrounded by matrix.
3. Matrix: Composed of proteoglycans and glycoproteins.

Additionally, fibrocartilage is characterized by the absence of perichondrium.

The primary functions of fibrocartilage include:

1. Shock absorption
2. Withstanding compression and shearing forces

Fibrocartilage is typically found in the following locations:

• Intervertebral discs
• Pubic symphysis
• Articular disc of sternoclavicular and temporomandibular joints
• Menisci
• Glenoidal labrum
• Acetabular labrum
• Articular disc of wrist joint

Fibrocartilage can be histologically differentiated from tendon by the presence of chondrocytes that lie singly or in isogenous groups between bundles of collagen fibers, and the absence of a perichondrium in fibrocartilage.

Proteoglycans secreted by chondrocytes, such as aggrecan, are significant for maintaining the structure and function of the matrix in fibrocartilage, contributing to its ability to withstand compressive forces.

With aging, calcium salts get deposited in the matrix of hyaline cartilage, which reduces diffusion through the cartilage and can lead to the death of chondrocytes. This process can result in the replacement of cartilage by bone (ossification), impacting joint function.

Achondroplasia is an autosomal dominant genetic disorder characterized by short legs and arms with a normal torso.

• Cause: Mutation of the fibroblast growth factor receptor 3 (FGFR3) gene.
• Pathogenesis: Defective proteins interfere with the conversion of cartilage into bone.
• New concept: Achondroplasia is transmitted only from the father.

Chondrosarcoma is a malignant tumor of chondroblasts that typically develops in the bones of the axial skeleton.