CHONDROCYTES: CHARACTERISTICS, HISTOLOGY, FUNCTIONS, CULTURE - BIOLOGY - 2025

The chondrocytes are the main cells of the cartilage. They are responsible for the secretion of the extracellular matrix of cartilage, made up of glycosaminoglycans and proteoglycans, collagen fibers and elastic fibers.

Cartilage is a special type of tough, elastic, whitish connective tissue that forms the skeleton or is added to certain bones of some vertebrate animals.

Section of cartilaginous tissue, the number 2 indicates the location of a chondrocyte (Source: Guido Fregapani via Wikimedia Commons)

Cartilage also contributes to the shape of various organs such as the nose, ears, larynx, and others. According to the type of fibers included in the secreted extracellular matrix, cartilage is classified into three types: (1) hyaline cartilage, (2) elastic cartilage, and (3) fibrocartilage.

The three types of cartilage have two common building blocks: cells, which are chondroblasts and chondrocytes; and the matrix, made up of fibers and a fundamental substance similar to a gel that leaves small spaces called "gaps" where cells are located.

The cartilaginous matrix does not receive blood vessels, lymphatic vessels, or nerves and is nourished by diffusion from the surrounding connective tissue or, in the case of synovial joints, from synovial fluid.

characteristics

Chondrocytes are present in all three types of cartilage. They are cells derived from mesenchymal cells, which, in the areas where cartilage is formed, lose their extensions, round off and congregate to form dense masses called "chondrification" centers.

In these chondrification centers, the progenitor cells differentiate into chondroblasts, which begin to synthesize the cartilaginous matrix that little by little surrounds them.

In a similar way to what happens with osteocytes (bone cells), the chondroblasts that are included in the so-called "gaps" of the matrix, differentiate into chondrocytes.

The chondrocytes within their lacunae can divide, forming clusters of about four or more cells. These clusters are known as isogenic groups and represent the divisions of the original chondrocyte.

Cartilage growth and chondroblast differentiation

As each cell of each cluster or isogenic group forms a matrix, they move away from each other and form their own separate lagoons. As a consequence, the cartilage grows from the inside, calling this form of cartilage growth interstitial growth.

In the peripheral regions of developing cartilage, mesenchymal cells differentiate into fibroblasts. These synthesize a dense irregular collagenous connective tissue called the perichondrium.

The perichondrium has two layers: an external fibrous vascularized layer composed of type I collagen and fibroblasts; and another inner cell layer formed by chondrogenic cells that divide and differentiate into chondroblasts, which form the matrix that is added peripherally.

Through this differentiation of the cells of the perichondrium, the cartilage also grows by peripheral apposition. This growth process is called appositional growth.

Interstitial growth is typical of the initial phase of cartilage development, but it also occurs in articular cartilage that does not have perichondrium and in the epiphyseal plates or growth plates of long bones.

In the rest of the body, on the other hand, cartilage grows by apposition.

Histology

Three types of chondrogenic cells can be found in cartilage: chondroblasts and chondrocytes.

Chondrogenic cells are thin and elongated in the shape of a spindle and originate from differentiation of mesenchymal cells.

Their nucleus is ovoid, they have little cytoplasm and a poorly developed Golgi complex, scarce mitochondria and rough endoplasmic reticulum, and abundant ribosomes. They can differentiate into chondroblasts or osteoprogenitor cells.

The chondrogenic cells of the inner layer of the perichondrium, as well as the mesenchymal cells of the chondrification centers, are the two sources of chondroblasts.

These cells have a highly developed rough endoplasmic reticulum, numerous ribosomes and mitochondria, a well-developed Golgi complex, and numerous secretory vesicles.

Chondrocytes in cartilage tissue

Chondrocytes are chondroblasts surrounded by extracellular matrix. They can have an ovoid shape when they are near the periphery, and a more rounded shape with about 20 to 30 µm in diameter when they are found in deeper regions of the cartilage.

Young chondrocytes have a large nucleus with a prominent nucleolus and abundant cytoplasmic organelles such as Golgi complex, rough endoplasmic reticulum, ribosomes, and mitochondria. They also have abundant cytoplasmic glycogen stores.

Old chondrocytes have few organelles, but abundant free ribosomes. These cells are relatively inactive, but can be reactivated by increasing protein synthesis.

Chondrocytes and types of cartilage

The arrangement of chondrocytes varies according to the type of cartilage where they are found. In hyaline cartilage, which is pearly white and translucent, chondrocytes are found in many isogenic groups and arranged in large gaps with very few fibers in the matrix.

Hyaline Articular Cartilage (Source: Eugenio Fernández Pruna via Wikimedia Commons)

Hyaline cartilage is the most abundant in the human skeleton and contains type II collagen fibers.

In elastic cartilage, which has abundant branched elastic fibers intertwined with type II collagen fibers distributed throughout the matrix, chondrocytes are abundant and uniformly distributed among the fibers.

This type of cartilage is typical of the pinna, the Eustachian tubes, some laryngeal cartilage and the epiglottis.

In fibrocartilage, there are few chondrocytes lined up between its thick, densely distributed type I collagen fibers in the matrix.

This type of cartilage is located in the intervertebral discs, in the symphysis pubis, in the areas of insertion of the tendons and in the knee joint.

Features

The fundamental function of chondrocytes is to synthesize the extracellular matrix of the different types of cartilage. Like chondrocytes, together with the matrix, they are the constitutive elements of cartilage and share its functions with it (as a whole).

Among the main functions of cartilage are those of cushioning or absorbing shocks or blows and compressions (thanks to its resistance and flexibility).

In addition, they provide a smooth articular surface that allows joint movements with minimal friction and, ultimately, shape different organs such as the pinna, nose, larynx, epiglottis, bronchi, etc.

Crops

Hyaline cartilage, which is the most abundant in the human body, can be subject to multiple injuries due to diseases, but, above all, from sports.

Since cartilage is a highly specialized tissue with relatively little self-healing capacity, its injuries can cause irreversible damage.

Many surgical techniques have been developed in order to repair articular cartilage injuries. Although these techniques, some more invasive than others, can improve injuries, the repaired cartilage is formed as fibrocartilage and not as hyaline cartilage. This means that it does not have the same functional characteristics as the original cartilage.

In order to obtain adequate repair of damaged articular surfaces, autologous culture techniques (from own cartilage) have been developed to achieve in vitro growth of cartilage and its subsequent transplantation.

These cultures have been developed by isolating chondrocytes from a healthy cartilage sample from the patient, which are then cultured and transplanted.

These methods have proven to be efficient for the growth and development of hyaline articular cartilage and, after a period of approximately two years, they achieve the definitive recovery of the articular surface.

Other techniques involve the cultivation of cartilage in vitro on a matrix or gel of fibrin and alginic acid or other natural or synthetic substances currently under study.

However, the objective of these cultures is to provide material for transplantation of the injured joint surfaces and their definitive recovery.

References

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