Chondrocytes are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although the word chondroblast is commonly used to describe an immature chondrocyte, the term is imprecise, since the progenitor of chondrocytes (which arise in the bone marrow, in a form of stem cell) can differentiate into various cell types, depending on the need. When they differentiate into cartilage cells, they start out as chondroblasts, actively producing secretions of chondrin, the primary substance in cartilage, to build and repair the tissue. In this article, we focus on the structure, function and markers of chondrocytes.
Chondrocytes are the only type of cells found in cartilage. Chondrocytes have different morphologies due to their different locations in cartilage. Juvenile chondrocytes are located in the surface layer of cartilage tissue. They are distributed individually with a small volume and an oval shape. The long axis is parallel to the surface of the cartilage. Cytoplasm is weakly basophilic, and different numbers of lipid droplets are common.
Mature chondrocytes are mostly distributed in cartilage pits in groups of 2-8. Under the electron microscope, chondrocytes had protrusions and wrinkles, and there were a large number of rough endoplasmic reticulum, a developed Golgi complex, and a small amount of mitochondria in the cytoplasm. Chondrocytes are buried in the cartilage interstitium. It is located in a small cavity called a cartilage lacuna (Figure 1).
Figure 1. The Structure of Chondrocyte
Chondrocytes are mature cells found in cartilage. They make up the cellular matrix of cartilage, performing a number of functions within the tissue, including facilitating the exchange of fluids through the gelatinous layers. Because cartilage lacks vascularization, it relies on this exchange to receive nutrients and express waste materials. Fully mature chondrocytes tend to be round, and they may cluster together in small groups within the network of the cartilage.
Taking the articular cartilage (AC) as an example, chondrocytes of the AC perform different functions compared to chondrocytes of the epiphyseal growth plates. Chondrocytes of the AC aid in joint articulation, while chondrocytes of the growth plate regulate the growth of the epiphyseal plates. Since this review focuses on osteoarthritis (OA), we will relate only to chondrocytes of the AC. Chondrocytes are metabolically active cells that synthesize and turnover a large volume of extra cellular matrix (ECM) components such as collagen, glycoproteins, proteoglycans, and hyaluronan  .
As mentioned before, chondrocytes play an irreplaceable role in the metabolism of cartilage. Running, jumping and other actions in daily life will cause the wear of joint cartilage. Knee arthritis is caused by the wear of articular cartilage. Articular cartilage is the most important structure in the joint. It plays a series of joint protection functions such as buffering stress and reducing friction. With the increase of age, repetitive sports injuries and changes in the body's conditions, the accumulated microtrauma and the accumulation of harmful substances in the joints will cause cartilage damage, cracking, and shedding, and eventually knee arthritis.
In the last section of this article, we list the most common markers of chondrocyte on the following table. Chondrocyte markers refer to several special proteins which distinguish chondrocyte from other cell types.
 Hemanth Akkiraju and Anja Nohe. Role of Chondrocytes in Cartilage Formation, Progression of Osteoarthritis and Cartilage Regeneration [J]. J Dev Biol. 2015;3(4):177-192.
 Archer CW, Francis-West P. The chondrocyte [J]. Int. J. Biochem. Cell Biol. 2003;35:401–404.
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