Cartilaginous tissue

The cartilaginous tissue, or cartilage, is a type of specialized, elastic connective tissue, devoid of blood vessels, formed mainly by extracellular matrix and scattered cells called chondrocytes The outer part of the cartilage, called the perichondrium, is in charge of providing life support to the chondrocytes.

Cartilage is found covering joints, at the junctions between the ribs and the sternum, as reinforcement in the trachea and bronchi, in the external ear and in the nasal septum. It is also found in embryos of vertebrates and cartilaginous fish.

Cartilages serve to accommodate the surfaces of the femoral condyles to the glenoid cavities of the tibia, to cushion shocks from walking and jumping, to prevent abrasion and, therefore, to allow movements of the joint. It is a support structure and gives some mobility to the joints.

Classification

There are 3 types of cartilage tissue:

Hyalin cartilage microphotography.

• Hialino cartilage: It has condrocitos arranged in groups (isogenic groups) each group surrounded by the territorial matrix, and among them there are interterritorial matrix. These matrices contain mainly type II collagen figurines, which interacts with proteoglucans. It is surrounded by perichondria (except the articular cartilage). It is the most abundant type of cartilage in the body. It looks bluish white. It is found in cartilage of the respiratory system: the nasal skeleton, the larynx, the trachea, the bronchus; and in the coastal arches (coasles), the joint ends of the bones and the temporary skeleton of the embryo. It is avascular, nourished by diffusion from the synovial fluid. It's a few fibers.

• Fibrous cartilage or fibrocartilage: It is a form of transition between regular dense connective tissue and hialin cartilage. It is composed of condrocytes and fibroblasts, surrounded by type I collagen fibers. He's usually avascular. It is found in intervertebral disks, joint edges, joint discs and meniscuses, sternoclavicular joints, jaw, pubic synfisis, as well as in the ligaments and tendons insertion sites.

• Elastic cartilage: Formed by condrocitos surrounded by the territorial and interterritorial matrices, which contain type II collagen, which interacts with proteoglucans and elastic fibers. It is surrounded by perichondria. It's avascular. It forms the epiglotis, cartilage corniculate or of Santorini, cuneiforme or Wrisberg, in the larynx, the external ear (acoustic) and on the walls of the external auditory duct and the Eustaquio tube. Form the ear pavilion. It is yellowish and has greater elasticity and flexibility than hialin. Its main difference with the latter is that the matrix presents a dense interwoven of fine elastic fibers that are basophils and stained with hematoxylin and eosine, as well as orcein. It has more axial and porferous isogen groups.

Cells of cartilage tissue

The cells of this tissue are called chondrocytes, which come from the chondroblasts present in the perichondrium. They have a well-developed rough endoplasmic reticulum and a large Golgi apparatus, as well as many vesicles, which are indicative of their secretory activity. Intermediate filaments, composed of vimentin, appear in abundance. Cells often contain glycogen and not infrequently also lipid inclusions, partly large.

• Condroblastos: they are responsible for the synthesis and secretion of the cartilaginous matrix. They are in the perichondria. They're small and fusiform. They divide with a certain frequency and are isolated. By differentiating they grow and become more spherical, they develop the Golgi and RER apparatus, and they go into the cartilaginous matrix, differing in condrocitos.
• Condrocitos: they are mature, larger, according to the center of the matrix are more rounded. Young people still divide. They keep the matrix, secreting collagen and glucosaminoglucans (GAG). They are grouped in isognonic groups, each surrounded by territorial matrix, and in turn are isolated by the extracellular matrix, called interterritorial matrix.

Hyaline cartilage

Hyaline cartilage is covered externally by a fibrous membrane called the perichondrium, except at the articular ends of bones and also where it lies directly under the skin, ie, the ears and nose. This membrane contains vessels that provide nutrition to the cartilage.

If you look at a thin layer under the microscope, it will be found to be made up of round-shaped or pointless cells, in groups of two or more in a granular or almost homogeneous biological matrix. Under the light microscope, no fibrillar network is observed in the extracellular matrix, however, when using polarized light, networks of fibrils can be visualized. This optical effect is due to the fact that the refractive index of the fibrils is similar to that of the matrix in which they are immersed.

The extracellular matrix is a very complex 3D network. The cells are found in cavities in the matrix, called cartilage lacunae; around these the matrix is arranged in concentric lines, as if it had been formed in successive portions around the cartilage cells. This constitutes the so-called space capsule.

Each lacuna is usually occupied by a single cell, but during cell division it may contain two, four, or eight cells, constituting an isogenic group or group of isogenic cells.

Hyaline cartilage also contains chondrocytes which are the cartilage cells that produce the matrix. The hyaline matrix of cartilage is composed primarily of type II collagen and chondroitin sulfate, both of which are also found in elastic cartilage. Formed mainly by type I collagen fibrils. It has chondrocytes arranged in groups. There is a perichondrium. It is the most abundant in the body. It has a bluish-white appearance. It is found in the nasal skeleton, larynx, trachea, bronchi, costal arches (ribs), and the articular ends of bones. It is avascular, being nourished by diffusion from the synovial fluid. It is of few fibers.

Cartilaginous matrix

Chondrocytes produce the broad matrix consisting of type II collagen (forms characteristic fine fibrils), type IX collagen (binds type II fibrils), type X collagen (surrounds hypertrophic cells), type XI collagen (function unknown), hyaluronan and the proteoglycan aggregate attached to it. In particular, the keratin sulfate and chondroitin sulfate chains of aggrecan bind water, a fundamental requirement for the creation of the characteristic elastic consistency of cartilage. Hyaluronan and the large number of aggrecan molecules attached to it form a giant molecular complex as it can reach a size of 3-4 mm. These aggregates make up the bulk of cartilage and give it its cartilaginous consistency. The morphological stability of cartilage also has its origin in these aggregates.

In the form of articular cartilage, this unique material can support the entire weight of the body. An important link between the matrix and cartilage cells is chondronectin present in the chondrocyte membrane, a fibronectin-like protein.

Characteristically, chondrocytes that have arisen from a parent cell by mitotic division are located in small contiguous groups (isogenous cell groups). In the immediate environment of the cartilage cells, the matrix contains highly sulfated glycosaminoglycans and is called the territorial matrix. This matrix stains deep violet blue in slides stained with H-E. The group of chondrocytes and their matrix form a cartilaginous territory (= chondrone). The matrix between the territories is called the interterritorial matrix. It is cellless and stains pale.

The chondrocytes are located in spaces ("lacunae" or chondroplasts) of the matrix whose wall, that is, the immediate environment of the cartilage cells, is also called the chondrocyte capsule. The capsular region often has a pericellular layer with a special protective function against compression and traction. In histological preparations, chondrocytes often appear artificially retracted into their lacunae.

Growth occurs by secretion of matrix inside the cartilage pieces (interstitial growth or by intussusception) or by new formation on the periphery (growth by apposition).

Mature cartilage is nerveless and in most cases avascular. The nutrition of the cartilage cells occurs by diffusion through the matrix provided with abundant water. The metabolism is anaerobic in a considerable proportion.