The perichondrium is a layer of dense, irregular connective tissue that encases most of the cartilage found within the body. It functions as a protective and life-sustaining membrane. This tissue is responsible for enveloping both hyaline and elastic cartilage, forming a distinct border around these structures.
Composition and Layers
The perichondrium is composed of two distinct layers. The outer layer is a tough, fibrous membrane composed primarily of fibroblast cells and type I collagen. This fibrous layer provides mechanical support and protection to the underlying cartilage. It also houses the blood vessels and nerves that service the tissue.
Directly adjacent to the cartilage surface is the inner, or chondrogenic, layer. This layer is less dense and more cellular in nature. It contains specialized progenitor cells that develop into chondroblasts—the cells responsible for forming new cartilage. The chondrogenic layer is the active site for cartilage growth and maintenance.
Primary Functions
A primary role of the perichondrium is to nourish the cartilage it surrounds. Cartilage is avascular, meaning it does not have its own blood supply. The dense network of capillaries within the perichondrium’s outer fibrous layer facilitates the diffusion of oxygen and essential nutrients to the chondrocytes (cartilage cells) embedded deep within the cartilage matrix.
The perichondrium is also involved in how cartilage grows. It facilitates a process known as appositional growth, where new cartilage is added to the surface of existing cartilage, increasing its overall diameter. The chondroblasts originating from the inner chondrogenic layer secrete a new cartilage matrix, effectively building upon the outer surface of the cartilage structure.
This connective tissue sheath also provides a secure anchoring point for other structures. It serves as an attachment site for muscles, tendons, and ligaments. This function allows for the effective transmission of forces between cartilage and adjacent tissues.
Role in Cartilage Repair
When cartilage sustains an injury, the perichondrium plays a direct part in the healing process. The progenitor cells located in the inner chondrogenic layer are activated by the trauma. These cells then differentiate into chondroblasts and begin to produce new cartilage matrix, attempting to repair the damaged area.
This repair function, however, can be complex. In some instances of injury, the outer fibrous layer can generate fibrous tissue that covers the wound, which may prevent the inner layer from effectively forming new cartilage. A common example of perichondrial injury occurs in the ear, where trauma can lead to a perichondrial hematoma, or “cauliflower ear.” This condition arises when the blood supply from the perichondrium is disrupted, causing a collection of blood that separates the perichondrium from the cartilage, leading to tissue death and deformity.
Locations Without Perichondrium
Not all cartilage in the body is covered by a perichondrium. Articular cartilage, the smooth, resilient tissue found on the surfaces of bones within synovial joints like the knee, lacks this sheath. Instead of relying on a perichondrium for nutrients, articular cartilage is nourished by the synovial fluid that fills the joint cavity. This fluid circulates during movement, providing the necessary oxygen and nutrients.
Fibrocartilage is another type of cartilage that does not have a perichondrium. This tough, shock-absorbing tissue is found in structures such as the intervertebral discs of the spine and the menisci in the knee.
The absence of a perichondrium has clinical implications for these specific types of cartilage. Without the regenerative cells supplied by the chondrogenic layer, both articular cartilage and fibrocartilage have a very limited ability to heal after being damaged. This is why injuries to the cartilage in joints often result in long-term damage, contributing to degenerative conditions such as osteoarthritis.