The perichondrium is a specialized layer of dense connective tissue that acts as a protective sheath surrounding most types of cartilage in the body. Derived from the Greek words for “around” (peri) and “cartilage” (chondros), this covering is an active biological structure that facilitates the health and maintenance of the underlying tissue. Without the perichondrium, cartilage would be unable to receive necessary nutrients and would lack a mechanism for growth and repair.
Anatomical Layers and Distribution
The perichondrium is organized into two distinct layers. The outermost is the fibrous layer, composed of dense irregular connective tissue featuring Type I collagen fibers and fibroblasts. This layer provides mechanical strength, acting as a sturdy casing that anchors the cartilage to surrounding soft tissues and offers structural protection.
Directly beneath the fibrous layer is the inner cellular layer, also known as the chondrogenic or cambium layer. This region is rich in progenitor cells that mature into chondroblasts, the cells responsible for forming new cartilage tissue. This dual-layer structure is present around elastic cartilage (e.g., the external ear and epiglottis) and most hyaline cartilage (e.g., the costal cartilages of the ribs and the trachea).
A significant exception is the absence of the perichondrium on articular cartilage, the hyaline cartilage covering the ends of bones within movable joints. Fibrocartilage, found in structures like the intervertebral discs and menisci, also lacks this sheath. In these locations, the tissue relies on other mechanisms, such as synovial fluid in joints, for nourishment and maintenance.
Primary Functions Supporting Cartilage
The primary function of the perichondrium is to ensure the survival and growth of the underlying cartilage, which is avascular and lacks its own direct blood supply. The outer fibrous layer is highly vascularized, containing blood vessels that deliver oxygen and nutrients. These substances then diffuse from the perichondrium through the dense matrix to reach the cartilage cells, a process essential for cell metabolism.
The inner cellular layer is responsible for appositional growth, the process of adding new cartilage matrix to the surface of existing cartilage. Progenitor cells in this layer differentiate into chondroblasts, which secrete the components of the cartilage matrix, increasing the structure’s girth or width. This mechanism is especially important during skeletal development and growth.
Beyond nutrition and growth, the perichondrium also plays a mechanical role in maintaining the integrity of the cartilage. The dense nature of the outer layer provides structural support, preventing deformation of the cartilage, particularly in areas subjected to frequent movement or pressure. Its anchoring function ensures that structures like the cartilage of the nose or ear remain securely connected to adjacent tissues.
Role in Cartilage Repair and Injury
The ability of cartilage to heal itself after injury is limited due to its lack of blood vessels and the low metabolic rate of its mature cells. The perichondrium serves as the primary source of biological activity in response to damage. When an injury occurs, the progenitor cells within the inner layer are activated and proliferate rapidly to bridge the defect.
These activated cells can differentiate into chondroblasts to form new cartilage, though the resulting repair tissue is often a less organized fibrocartilage scar, especially in adults. The regenerative capacity of the perichondrium is so pronounced that it is frequently utilized in surgical procedures. Perichondrial grafts, often harvested from the rib or ear, are transplanted to repair defects in articular cartilage or to reconstruct facial features.
Damage to this sheath itself can lead to severe complications, such as perichondritis, an inflammation or infection that compromises the nutrient supply to the cartilage. The resulting lack of nourishment can cause the underlying cartilage to deteriorate and die, as seen in the permanent deformity known as “cauliflower ear” following trauma. Preserving the integrity of the perichondrium is a major consideration in the surgical management of cartilage injuries.