Histology of Hyaline Cartilage
Hyaline cartilage is the most common of the three types of carticlage. In its fresh state, it is homogeneous and semi-transparent. In adults, hyaline cartilage is located in the articular surfaces of movable joints, in the walls of the respiratory tracts (nose, larynx, trachea, and bronchi), in the costal cartilages, and in the epiphyseal plates of long bones.
During embryonic development, hyaline cartilage serves as temporary cartilage models that are essential precursors to the formation of most of the axial and appendicular skeleton. This article will focus on important features of hyaline cartilage, namely its matrix, chondrocytes, and perichondrium.
The extracellular matrix (ECM) of hyaline cartilage is homogeneous and glassy, rich in in type II collagen, proteoglycans such as aggrecan, and structural glycoproteins such as chondronectin. Aggrecan is the most abundant proteoglycan in hyaline cartilage; hundreds of these aggrecans are bound noncovalently by link proteins to long polymers of hyaluronic acid. Chondronectin is a structural multi-adhesive glycoprotein. It binds specifically to glycosaminoglycans, collagen type II fibers, as well as integrins, and mediates the adherence of chondrocytes to the ECM.
Chondrocytes occupy relatively little of the hyaline cartilage mass. They are embedded in an extensive matrix and are located in matrix cavities known as lacunae, which appear as tiny white lakes under a light microscope. Chondrocytes are important in synthesizing and maintaining components of the ECM.
At the periphery, young chondrocytes (or chondroblasts) have an elliptic shape, with their long axis parallel to the surface. At the core, mature chondrocytes have a round shape and often appear as a group of up to eight cells. These groups are known as isogenous aggregates, which originate from the mitotic divisions of a single chondrocyte. Once the chondrocytes become more active in secreting ECM components, they become pushed apart and each occupy a lacuna.
Notably, chondrocytes tend to have irregular shapes under a light microscope. That is because cartilage cells and the matrix often shrink during routine histologic preparations. In living tissue and in properly prepared sections, chondrocytes completely fill their lacunae.
All cartilage is avascular and receives nutrients by diffusion from capillaries originating from the perichondrium. Perichondrium is a layer of dense connective tissue that surrounds all hyaline cartilage, except in the articular cartilage of movable joints. It is essential for the growth and maintenance of cartilage, as it harbors its vascular supply, as well as its nerves and lymphatic vessels. Although the articular cartilage of movable joints is not covered by perichondrium, they are sustained by diffusion of oxygen and nutrients from synovial fluid. The perichondrium consists mostly of fibroblasts, chondroblasts, and collagen type I fibers.
Osteoarthritis is a chronic condition that generally occurs with aging. It involves the gradual loss or changes in the physical properties of the hyaline cartilage that covers the articular ends of movable joints. Hyaline cartilage in the joints that are weight-bearing (knees and hips) or that are heavily used (wrist and fingers) are most prone to degeneration. Wear-and-tear to the articular cartilage releases fragments that secrete matrix metalloproteinases. These exacerbate damage and are the source of pain and inflammation within the joint.