What is connective tissue? Definition, types and examples

The connective tissuealso called conjunctive tissueis one of the four basic types of tissue in the human body, along with muscle tissue, epithelial tissue and nervous tissue.

connective tissue is the most abundant of all. It develops from mesoderm and is distributed among other tissues and organs throughout the body, being specialized in the functions of support and protection of other body structures.

Although it had already been identified as a type of tissue in the 18th century, the term "connective tissue" was introduced in 1830 by the German physiologist and anatomist Johannes Peter Mueller (1801 – 1858).

Definition: components of connective tissue

Within the connective tissue or conjunctive tissue, several very different tissue types are included, for example, the meninges that protect the central nervous system are formed by connective tissue. Also cartilage or bones.

The components and characteristics that are usually used to define connective tissue are:

  • cells dispersed in an extracellular fluid or matrix
  • ground substance: translucent and gelatinous material composed of proteins and hydrated glycosaminoglycans.
  • fibers: collagen, reticular, elastic fibers and microfibrils.
  • Matrix: made up of ground substance and fibers

connective tissue cells

Except for some cases with slight movement, the connective tissue cells are sessile (motionless) cells. Some types of cells are present in several types of connective tissue, while others are specialized cells characteristic of a specific type of connective tissue.

Among the cells that can be found in the connective tissue we can find the following:

  • mesenchymal cells: they are the cell line from which the other specialized cells of the connective tissue arise. These cells are typical of the embryonic and fetal stages.
  • fibroblasts: they are resident cells of the connective tissue, since they are born and die here. They have a large amount of endoplasmic reticulum and are dedicated to synthesizing connective tissue fibers and maintaining the matrix in good condition.
  • adipocytes: are the characteristic cells of adipose tissue, a type of connective tissue specialized in storing fat for various purposes (energy reserve, thermoregulator, etc.)
  • reticular cells: they are a type of fibroblast specialized in the synthesis of type I collagen fibers. They can be found in various tissues, such as the spleen or lymph nodes.
  • immune cells: lymphocytes, macrophages, mast cells, plasma cells and other cells of the immune system.
  • blood components: erythrocytes, megakaryocytes, platelets.

ground substance

The so-called fundamental substance is a material formed by Water and various suspension components, especially proteins and glycosaminoglycans (proteoglycans, mucopolysaccharides). often set Mineral saltsespecially sulfates and sodium ions (Na+), and presents a gelatinous consistency.

Among the most common glycosaminoglycans in connective tissue are chondroitin sulfateheparan sulfate, keratan sulfate and the hyaluronic acid. They are hydrophilic substances that attract and retain water and mineral salts.

fibers

The connective tissue fibers are protein fibers that, together with the ground substance, form the matrix in which connective tissue cells are dispersed.

They can be of various types:

  • collagen fibers: give resistance to connective tissue and are especially abundant in tendons, ligaments and fascia.
  • reticular fibers: bind connective tissue with other tissues and organs. It is usual covering organs. They are actually type III collagen fibers.
  • elastic fibers: are fibers composed of elastin and fibrillin abundant in loose connective tissues. They are very elastic fibers. They can increase in size by up to 50% and then return to their normal size. They are abundant, for example, in the trachea, vocal cords or blood vessels.
  • microfibrils: Microfibrils are elastin-associated fibrillin fibers characteristic of the basal lamina of epithelial tissue.

types of connective tissue

Connective tissue is usually classified into two large groups, unspecialized connective tissue or connective tissue properand the specialized connective tissue.

Unspecialized connective tissues (loose and dense)

Unspecialized connective tissues are differentiated into several subtypes based on the ratio of ground substance to fibers.

  1. loose connective tissue: large amount of ground substance and few fibers.
  2. dense connective tissue: higher proportion of fibers.

loose connective tissue

Loose connective tissue is composed primarily of ground substance and relatively little fibrous component. It is the most abundant type of connective tissue in most vertebrates.

It performs functions of fixing organs in their position and anchoring the epithelia to the underlying tissues.

In addition, it has a very important role as diffusion medium for oxygen and nutrients from the capillaries into the cells, and carbon dioxide and metabolic wastes from the cells into the capillaries.

They are loose connective tissue:

  • mucous connective tissue: Composed mostly of amorphous ground substance.
  • reticular connective tissue: a type of connective tissue with a high proportion of ground substance and type III collagen fibers. They form the stroma (functional part) of hematopoietic tissue (in the bone marrow) and lymphatic tissues (spleen, lymph nodes, thymus).
  • mesenchymal tissue: loose connective tissue characteristic of the developing embryo and organs. It is rich in mesenchymal cells that can be differentiated into other connective tissue cells.

dense or fibrous connective tissue

Dense connective tissue is made up primarily of fibroblasts and fibersespecially collagen fibers.

  • regular dense connective tissue: is the connective tissue with collagen fibers parallel typical of tendons, ligaments and aponeurosis. It is also abundant in other highly specialized organs, for example the cornea, and other structures that must resist traction, for example in the walls of blood vessels or in the alveoli of the lungs.
  • irregular dense connective tissue: In this type of dense connective tissue, the collagen fibers appear with a distribution random. It is part of the capsules of almost all organs, for example the liver, the kidneys, the small intestine or the dermis.

specialized connective tissues

Specialized connective tissues are those whose main function lies not only in the composition of fibers and ground substance, but also in highly specialized cells typical of these tissues.

They are considered specialized connective tissue, among others:

  • adipose tissue: it is a type of connective tissue of loose consistency with abundant adipocytes, cells specialized in the storage of triglycerides. It also has endocrine and thermoregulatory functions (brown adipose tissue).
  • Cartilage or cartilaginous tissue: cartilage is a type of connective tissue formed by highly packed type II collagen fibers in a matrix of chondroitin sulfate synthesized by the chondrocytes.
  • Woven boneIt is a very hard mineralized connective tissue formed by the osteoblasts.
  • hematopoietic tissue: responsible for the production of blood cells. The main hematopoietic tissue is located in the bone marrow of long bones, but also in the spleen, thymus and lymph nodes.
  • Lymphatic tissue or lymph
  • Blood tissue or blood

Most sources consider blood and lymph to be connective tissue, since they derive from the mesoderm, they are composed of a protein matrix (blood plasma) in which cells are distributed, and both fluids have a support function for other tissues and organs. . In both cases, the matrix would be liquid.

However, there are authors who consider that blood, lacking the fibrous component, could not be classified as a type of connective tissue. In the case of lymph, the fibrous component would be mainly in the fibrous network of the lymph nodes.

connective tissue functions

As we have seen, there are many types of connective tissue, each of which performs a different function. Broadly speaking, the general functions of connective tissue could be classified into the following categories:

  • structural support
  • metabolic support
  • nutritional support
  • storage of energy reserves
  • mechanical protection
  • immune protection
  • inflammation
  • injury repair

medical implications

Connective tissue is involved in many diseases with very different causes and origins. Together they are known as connective tissue diseasesbetween them:

  • Neoplasms: for example, sarcomas.
  • congenital diseases: For example, Marfan syndrome and Ehlers-Danlos syndrome.
  • mitral valve prolapse: coronary valve disease due to excess connective tissue in the heart valves.
  • Mixed connective tissue disease: autoimmune disease characterized by the presence of anti-RNP (ribonucleoprotein) antibodies. The adjective "mixed" is due to the fact that it produces clinical symptoms similar to other autoimmune diseases of the connective tissue such as systemic lupus erythematosus (SLE), scleroderma and polymyositis.
  • Systemic lupus erythematosus: is one of the main autoimmune diseases that affects the connective tissue.
  • Scurvy: disease caused by deficiency of vitamin C, a vitamin that acts as a coenzyme in the synthesis of collagen.
  • Rheumatological diseases: for example, arthritis, osteoarthritis, costochondritis, bursitis, etc.
  • mucocele: connective tissue swelling due to mucosal extravasation. Usually associated with trauma to the salivary gland ducts.
Go up