ADIPOSE TISSUE: CHARACTERISTICS, FUNCTIONS, TYPES, DISEASES - BIOLOGY - 2025

The adipose tissue, also called fatty tissue is connective tissue formed by a loose type of cells called adipocytes together by intercellular connections. It contains the largest fuel reserves of all tissues in animals.

An average person has a food reserve in the adipose tissue of his body that reaches up to 40 days. This reserve of energy is stored in the form of triglycerides.

Adipose tissue (Source: OpenStax College via Wikimedia Commons)

Adipocytes or lipocytes are so named because they store lipids and fatty acids in their cytosol. It is estimated that more than 90% of these cells are composed of lipids, mostly in the form of triglycerides.

The main physiological function of adipose tissue is to maintain the levels of free fatty acids in the blood, as this ensures the body a constant supply of oxidable substrates for cellular respiration and other metabolic processes.

All the fatty acids that are synthesized in the liver or absorbed by the digestive system are assimilated and stored in the form of triglycerides in the fatty tissues within adipocytes.

Storing calories in the form of triglycerides has a better energy yield for the body than if it were stored in the form of protein and carbohydrates.

The oxidation of carbohydrates and proteins generate approximately 4 Kcal / g, while the oxidation of fatty acids generates 9 Kcal / g. Furthermore, the storage of proteins and carbohydrates within the cell requires the accumulation of intracellular water, while triglycerides can occupy about 90% of the net weight of adipocytes.

General characteristics

Adipose tissue is made up of adipocytes linked together. Each adipocyte is perfectly adapted to store fatty acids in the form of unique triglyceride droplets within the cytosol.

Adipocytes are highly specialized cells, so much so that they fulfill three functions: (1) store energy, (2) release energy and endocrine substances, and (3) inform the central nervous system how many calories are stored.

An adult of average height and weight has between 25 and 30 trillion adipocytes in his body. However, this number is sensitive to the weight gain of the person, since, after an increase in weight, the size and number of adipocytes in the body increases.

Histological section of adipose tissue (Source: Ganymede via Wikimedia Commons)

All adipose tissue is highly vascularized, this allows it to effectively comply with the rapid metabolic changes of the body. Furthermore, blood flow is greater in adipose tissue than in resting skeletal muscle.

The origin of adipose tissue is mesenchymal, that is, it originates from embryonic tissue. Adipocytes originate from a pluripotential mesenchymal precursor cell.

This pluripotential cell differentiates first into an adipoblast, later into a preadipocyte, and finally into an adipocyte. Newly differentiated adipocytes have the appearance of fibroblasts and, over time, they mature and acquire the enzymatic apparatus typical of adipocytes.

Adipose tissues are distributed in multiple places in the body of animals at the dermal, subcutaneous, mediastinal, mesenteric, perigonadal, perirenal and retroperitoneal levels. In mammals it can be found in two different types, white adipose tissue and brown adipose tissue. Both types of adipose tissue have marked differences between them, both morphological and of distribution, as well as genetic and functional.

Features

For many years it was considered that the only and main function of adipose tissue was the storage of lipids after excessive energy consumption; in addition to providing energy-rich substrates when necessary for the other organs of the body.

However, for some years it has been found that adipose tissue also has an important active secretory function for the body of animals. Therefore, adipose tissue is now also considered an endocrine tissue.

Today, adipose tissue is considered a “master” of energy storage in the form of lipids and, through the secretion of protein factors called adipokines, it is a powerful regulator of many other processes.

Among those processes regulated by adipose tissue are energy metabolism, inflammations and pathophysiological changes such as cancer and infectious diseases.

Many scientists link the secretion of tumor necrosis factor by adipose tissue with a rampant increase in obesity and type 2 diabetes.

The endocrine function of adipose tissue is of such importance to doctors, that they think that in most cases obesity causes a malfunction of adipose tissue and this causes many of the metabolic and cardiovascular diseases associated with this condition.

In many animals, adipose tissue represents a protective mechanism against mechanical impacts and an insulator against extreme cold conditions. Marine animals such as seals have large layers of adipose tissue to insulate themselves from the cold of the environment.

Morphology

In most animals, adipose tissue is an association of adipocytes that are delimited with other tissues by collagen fibers. To a lesser extent, vascular stromal cells are found.

These vascular stromal cells include fibroelastic connective tissue cells, some white blood cells, macrophages, and pre-adipocytes. The latter are waiting to be filled with triglycerides to transform into mature adipocytes.

According to their morphology, two types of adipocytes can be distinguished within adipose tissue, unilocular and multilocular adipocytes. Uniloculars contain a single drop of triglycerides that compresses the cell nucleus against the cell plasma membrane.

These cells, if observed under a microscope, have a ring shape and are characteristic of white adipose tissue, their size ranges from 25 to 200 microns. The mitochondria of these cells are found in the thickest portion of the cytosolic border, near the nucleus.

On the other hand, multilocular adipocytes are generally found in brown adipose tissue and contain many small lipid droplets dispersed in their cytosol. These cells can reach a maximum of 60 microns; while lipid droplets can be larger than 25 microns.

Composition

The "brown" color of brown adipose tissue is due to increased vascularization and packaging of the mitochondria. In contrast, white adipose tissue is practically made up of pure lipids, with a proportion of 90 to 99% of triglycerides.

Small amounts of free fatty acids such as diglycerides, cholesterol, phospholipids, and small amounts of cholesterol esters and monoglycerides are also found in adipocytes that make up white adipose tissue.

Furthermore, white adipose tissue is not as vascularized as brown adipose tissue, but each adipocyte in white adipose tissue is in contact with at least one blood capillary.

Almost 90% of the lipid mix of all adipocytes consists of six fatty acids, namely: myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid and linoleic acid.

However, the fatty acid composition of adipose tissue varies depending on the composition of the diet. The remaining weight of adipose tissue is made up of water in 5 to 30% and proteins in 2 to 3% of the remaining composition to lipids.

Types

As mentioned earlier, there are two different types of adipose tissue, white adipose tissue and brown adipose tissue.

White adipose tissue

This is the body's energy reservoir par excellence, it is found in a much greater amount than brown adipose tissue, and it is distributed subcutaneously throughout almost the entire body of animals.

White adipose tissue represents a highly dynamic tissue. This, depending on the energy needs of the individual, is capable of degrading or storing triglycerides in the form of lipid drops.

Distribution of white adipose tissue in the human body (Source: Cook, A. and Cowan, C., Adipose (March 31, 2009), StemBook, ed. The Stem Cell Research Community, StemBook, doi / 10.3824 / stembook.1.40. 1, http://www.stembook.org. Via Wikimedia Commons)

In addition, white adipose tissue represents an important mechanical support for the positioning of organs such as the kidneys and the eyes. Also, it works as an elastic shock absorber in places subject to high mechanical stress, as in the case of the soles of the feet and the palms of the hands.

It can be seen that white adipose tissue is divided into two parts, mature adipose tissue and stomatal adipose tissue. The latter has many immune cells like macrophages and lymphocytes, endothelial cells and fibroblasts.

White adipose tissue is not uniformly distributed throughout the body, each fatty deposit varies in composition, micro vascularization, nerve innervation, metabolic characteristics, extracellular composition and the amount of secreted adipokines.

It is in white adipose tissue that several hormones are synthesized that play important roles in metabolism and the endocrine system. Some of these hormones are adiponectin, leptin and resistin, all of which are involved in energy metabolism.

Brown adipose tissue

Brown adipose tissue is specialized in the thermogenesis of animals that are able to maintain the temperature of their internal body more or less constant (homeotherms) through the hydrolysis and oxidation of fatty acids inside adipocytes.

This tissue is present more frequently in newborn animals and, instead of storing energy, it consumes it to generate heat. In humans, as infants grow older, the percentage of brown adipose tissue in the body decreases.

However, in some animals, especially those that go through hibernation events during their life cycle, brown adipose tissue is found in adult organisms and is of great importance for their survival.

Brown adipose tissue (Source: Lucasmcorso, via Wikimedia Commons)

An example of these animals are brown and black bears that eat excess fat before winter to store lipids in their brown adipose tissue. During hibernation your body temperature decreases and your metabolism slows down.

To wake up from this state, the brown adipose tissue begins to consume lipids and release heat. This release of heat causes the individual to wake up and to come out of their dormant state.

Brown adipose tissue has a red or brown color, that is, it is more or less brown. This is due to the rich vascularization and the increased presence of mitochondria within the adipocytes. These mitochondria can vary in size and shape.

References

1. Coelho, M., Oliveira, T., & Fernandes, R. (2013). Biochemistry of adipose tissue: an endocrine organ. Archives of medical science: AMS, 9 (2), 191.
2. Lee, YH, Mottillo, EP, & Granneman, JG (2014). Adipose tissue plasticity from WAT to BAT and in between. Biochimica et Biophysica Acta (BBA) -Molecular Basis of Disease, 1842 (3), 358-369.
3. Marcela, RJ (2012). Biological characteristics of adipose tissue: the adipocyte as an endocrine cell. Las Condes Clinical Medical Journal, 23 (2), 136-144
4. Scherer, PE (2006). Adipose tissue: from lipid storage compartment to endocrine organ. Diabetes, 55 (6), 1537-1545.
5. Trayhurn, P. (2007). Adipocyte biology. Obesity reviews, 8, 41-44.
6. Villarroya, F., Cereijo, R., Villarroya, J., & Giralt, M. (2017). Brown adipose tissue as a secretory organ. Nature Reviews Endocrinology, 13 (1), 26.