LOOSE CONNECTIVE TISSUE: CHARACTERISTICS, HISTOLOGY, TYPES, FUNCTIONS - BIOLOGY - 2025

The connective tissue loose, loose connective also called, is a type of connective tissue fibers and characterized by having cells suspended in a gelatinous ground substance. It is the most widespread and common fabric, considered the filling material of the body.

This tissue is ubiquitous, since it is found in all organs and is part of the stroma of many of them, among which the liver, kidney and testicles stand out. Loose connective tissue (LCT), together with fibrous (dense) tissue, are part of what is known as proper connective tissue.

Loose areolar connective tissue. Taken and edited from: علاء.

The TCL is mainly made up of an abundant extracellular matrix and fibroblasts. This tissue is divided into three types of tissues: adipose, reticular and areolar. It is considered as a non-specialized fabric.

It has multiple blood vessels, secretory cells, and even nerve cells. This tissue is in charge of fixing blood vessels, nerves and organs; Among other functions, they reserve fluids, nourish organs, regenerate tissues and participate in immune reactions in the body.

characteristics

Loose connective tissue, as its name suggests, is part of the connective tissues, that is, it shares characteristics with all connective tissues. This implies that it is a fabric of support, support and protection.

As connective tissue, it also has an abundant extracellular matrix where cells are found and is composed of proteoglycans and glycosamicoglycans.

Connective tissues are classified, according to various authors, into specialized tissue and tissue itself. The latter is characterized by being a group of tissues with a wide body distribution filling spaces between organs and having fibroblasts as the main group of cells.

The tissue itself is divided into dense tissue and loose tissue. The TCL is the one with the greatest body distribution of all connective tissues. This is present in all organs, and has even been found in areas that do not require resistance to mechanical stress, which is rare.

The TCL presents an extracellular matrix with dispersed and disordered fibers between the fibroblasts. It is also characterized by being a soft tissue, not very resistant, foldable and with a certain elasticity.

Histology

This tissue is derived or originates from cells of the embryonic mesenchyme. It presents fibroblasts as main cells. These cells are elongated, irregular, and sometimes spindle-shaped. They have an oval nucleus with 2 nucleoli and a cytoplasm that is generally not very visible.

Fibroblasts are the cells responsible for the production and release of substances in the extracellular matrix. Other types of cells can be part of loose connective tissue, such as macrophages, monocytes, basophils, plasma cells or adipocytes, among others. The presence and number of these cells will depend on the type of TCL.

Fibroblasts and other cells in this tissue are not clearly organized, but are found dispersed in an abundant extracellular matrix composed of also dispersed collagen, elastic and reticular fibers (these in fewer numbers than the rest).

It should be noted that this tissue has a relatively high vascularization (presence of blood vessels), as well as nerve extensions and exocrine glands.

Types

Loose connective tissue is subdivided into three types of tissues that, depending on the authors, may or may not be considered within loose connective tissue: areolar, reticular, and adipose.

Areolar

Considered a relatively simple tissue, it is the one with the greatest body distribution of the three tissues that make up the TCL. It presents a homogeneous, translucent and gelatinous extracellular matrix, composed of mucin, glycoproteins, chondroitin sulfate and hyaluronic acid.

It has fibers arranged loosely, leaving areoles, that is, spaces between the fibers, a characteristic that gives this fabric its name. It can be found in the form of continuous layers under the skin, filling spaces between muscles, peritoneum and organs.

Reticulate

Sometimes, it is described as a proper tissue but independent of dense and loose tissues. However, other authors do include it within the TCL and some even consider it to be a modified areolar tissue.

Loose reticular connective tissue is composed of a significant number of star-shaped reticular fibroblast cells, which are observed floating in the matrix. The fibers formed by these cells (reticulin) are occasional in other connective tissues but are more abundant in this one.

Cross section of reticular loose connective tissue. Taken and edited from: Berkshire Community College Bioscience Image Library.

Reticulins or reticular fibers are mainly made up of type III collagen. These fibers are basically about 150 nanometers (nm) in diameter, branched, braided or anastomosed, and high in carbohydrates.

The branched appearance of these fibers is a diagnostic characteristic that allows them to be separated from other fibers composed of type I and II collagen. In addition, these can become so fine that they are difficult to observe with non-electronic microscopy. This tissue is found in bone marrow and lymphoid tissue.

Adipose

Some authors consider it a specialized tissue or even an organ, while others consider it proper or non-specialized tissue. Within this classification it is frequently described as a modified areolar loose tissue, but which presents a large number of adipocytic cells.

Adipocytes are cells of variable size, sometimes spherical or oval, with a lipid content that can exceed 80% (in some, 95%) of the cell and which causes the nucleus to move towards the peripheries of the cell. In adipose tissues, adipocytes can be found singly or in small groups.

Cross section of loose adipose connective tissue. Taken and edited from: Berkshire Community College Bioscience Image Library.

Until very recently, scientists recognized three types of adipocytes (white, brown or brown and beige), however currently the existence of at least one other type (pink) is recognized and the existence of a fifth type called adipocyte has been proposed. yellow.

These adipose cells make up two main types of adipose TCL, white adipose tissue and brown or brown. White adipose tissue is the most abundant and can represent up to one fifth (men) or one fourth (women) of normal total body weight.

It is found in various parts of the body, but most abundantly forming subcutaneous fat. It can also be found surrounding numerous organs. On the other hand, brown or brown adipose tissue is more abundant in newborns and it was believed that in adults it completely disappeared.

It is also abundant in mammals that go through the hibernation process. In humans, this tissue is located mainly in the cervical and supraclavicular regions, although it can also be found in the middle region of the intestine and in the adrenal.

Features

- Areolar

The areolar TCL has the function of joining the skin with internal muscle tissues. The macrophage cells of this tissue are responsible for engulfing bacteria, dead or damaged cells. In addition, this tissue produces anticoagulant (heparin) and pro-inflammatory (histamine) substances, it also has the ability to produce antibodies.

Another of its functions is that of storage, this tissue stores nutrients in the form of lipids and also stores body fluids in the fundamental substance. Provides support and padding to organs and tissues.

- Reticulate

Reticular cells and fibers have the primary function of providing support and support to other cells. This function is extremely important in organs such as the kidney, arterial walls, spleen, liver and tonsils, where this type of tissue is more abundant.

Reticular cells are capable of phagocytizing other cells and perform this function mainly when they are part of the walls of lymphatic tissue (lymphatic sinus) or of special capillary blood vessels (blood sinusoid). They also participate in the immune reactions of the body.

- Adipose

Adipose tissue has multiple functions, which is why some researchers suggest considering it as an organ rather than a tissue. Among these functions, the best known is to store reserve energy for metabolic processes during periods of low caloric intake. They also have an important hormonal activity.

Among the hormones secreted by adipose tissue are leptin, resistin, adiponectin, as well as angiotensin. And among its functions are to regulate the appetite, promote lipolysis, modulate the immune system and reduce adipogenesis. they also possess pro-inflammatory activity.

White adipose tissue

Its main function is to store energy in the form of lipid droplets, it also functions as a cushioning tissue and provides the body with a certain thermal resistance of the thermo-insulating type. As connective tissue, it is also responsible for filling spaces.

Due to the influence of sex hormones, this tissue is able to shape the body surface. For example, in males it accumulates in the nape, buttocks and in the seventh cervical vertebra; while in women it does it in the breasts, buttocks and front of the thighs.

Brown adipose tissue

This tissue is capable of generating more body heat than white adipose tissue, especially in newborn humans. In adult organisms, its thermal function is minimal. In animals, such as mammals that go through periods of hibernation, this tissue functions as an easily accessible reserve of caloric energy.

It has been determined that other animals that do not necessarily hibernate, can present this tissue and thus fulfill the function of providing a source of heat. They can also help prevent obesity by burning excess energy.

References

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