Adipose tissue: what it is, types and characteristics

From a biochemical point of view, fat encompasses several classes of lipids, organic molecules constituted mainly by carbon and hydrogen, whose function is usually the storage of energy in the living beings. Lipids, generally in the form of triacylglycerols, have a very high caloric content (10 kcal/gram). Therefore, they are a compact and excellent way to store energy for times of stress and exercise.

According to the World Health Organization (WHO), dietary fat intake should not exceed 30% of the total amount of organic matter ingested. For its part, the percentage of carbohydrates should be between 55-60% and protein 15%. Contrary to what is said in popular culture, fats are not bad: on the contrary, they help us to function at times when resources are scarce, both for humans and for the rest of the animals.

Moving a little away from dietary concepts, today we come to show you some of the most important characteristics of human adipose tissue. We anticipate that these go far beyond energy storage, so it is better not to take anything for granted.

• Related article: "Types of fats (good and bad) and their functions"

What is adipose tissue?

Adipose tissue is a type of tissue made up of cells called adipocytes, where a large amount of energy is stored in the form of fat. This cellular organization is responsible for many functions, however, an excess of this tissue can cause serious health problems. It is generally associated with energy storage work, but it also produces a large number of peptides and factors with both endocrine and paracrine function.

In healthy individuals, adipose tissue accounts for between 10 and 30% of the total mass, depending on Body Mass Index (BMI), ethnicity, gender and other factors. In patients with morbid obesity, the percentage of fat can reach up to 80% of their weight. When a person moves within these values, the risks of ischemic heart disease, gastric cancer, accelerated cellular aging and many other pathologies multiply.

Adipose tissue is made up of many cells. Adipocytes are the dominant and main bodies, since they account for 80-90% of the total tissue volume and 60-70% of the total amount of cells (as far as numbers are concerned). Looking at the ultrastructure of this tissue, we will see that adipose tissue is 18% water, 80% triglycerides (fat) and 2% protein, approximately. In obese subjects, the water content decreases and fats occupy a larger fraction of the total.

Adipocytes and their relationship with weight

As its name indicates, adipocytes are the cell types that characterize adipose tissue. Before continuing with more complex terminologies, it is necessary to dedicate a few lines only to these characteristic cell bodies. we are before rounded cells with a diameter of 10-200 microns, whose internal content is 95% represented by lipid components.

Very interesting studies have explored the idea that, from the age of 20, the number of adipocytes in humans remains practically constant even if weight is lost. Obesity and overweight are due to 2 main events at the cellular level: the increase in the size of existing adipocytes (hypertrophy) and the increase in the number of existing adipocytes (hyperplasia). The prevalence of one process or another depends a lot on age.

Until 2008, it was believed that newly synthesized adipocytes during weight gain were lost when losing weight. However, it seems that the cell number is constant during adult life: what varies is the diameter and the amount of fat stored in each adipocyte, not the number of them. Interesting, right?

• You may be interested in: "Main Cell Types of the Human Body"

The types of adipose tissue in humans

In the human body, lipids are stored in 2 types of adipose tissue: white and brown. We tell you about its particularities in the following lines.

1. White adipose tissue (TAB)

Its main location is in the subcutaneous, abdominal, inguinal, perirenal section (around the kidneys), retroperitoneal, gonadal, around organs and in other physiologic sites so scattered. The TAB is the main energy reserve tissue of the body and, in addition, the functionality of protecting many vital organs and structures from mechanical injuries is attributed to it.

This type of tissue is called "unilocular", since each adipocyte has a single drop of fat, which occupies the vast majority of its cytoplasm.. In any case, this characteristic is not immovable, since sometimes very well-fed animals can present multilocular white adipocytes during their development, although the fat droplets will merge into one when they mature. Both the nucleus and the rest of the cytoplasmic components occupy a thin peripheral space, close to the plasmatic membrane.

Historically the TAB was believed to be an inactive tissue, but research has shown in recent decades that it is a highly dynamic cellular conglomerate, since it secretes numerous factors, not only lipids but also protein. Without going any further, it has been detected that white adipose tissue secretes steroid hormones, so it is considered an essential component of the human endocrine system.

The molecules synthesized in this tissue are called adipokines.. An example of these is leptin, synthesized almost exclusively in adipocytes, although it is also expressed in the hypothalamus, the ovary, and the placenta. Among many other functions, this hormone affects food intake in humans, acting on the central nervous system (CNS). Other lesser known but equally important adipokines are resistin and adiponectin.

2. Brown or brown adipose tissue (TAM)

Biology buffs may be familiar with this type of tissue, as it is abundant in hibernating mammals. During this delicate period of physiological "break", brown or brown fat is responsible for thermogenesis, or what is the same, the production of heat in response to abnormally cold temperatures. Since the rest of the animal's metabolism is at a standstill (except for vital functions), energy has to trickle out of specialized tissues, like this one.

The human being also presents it, but mainly in the neonatal stage, during lactation and in very young children. The proportion of this tissue decreases markedly after 8 weeks of life, until reaching 1% of the total mass of the individual. Brown adipose tissue is found in the axillary, perineal, paravertebral, and cervical areas, scattered in arteries and around organs.

Brown fat is made up of adipocytes that contain several lipid droplets, that is, it is multilocular. These are smaller than those that make up white fat and, in addition, have a brownish tone when dry (hence their name). It is striking to learn that this tone is due to the abundant presence of mitochondria in the cytoplasm, something in line with their function of burning energy and producing heat.

It is also curious to report that, in studied mammals, the appearance of brown adipose tissue varies based on environmental conditions. When the specimens are at temperate temperatures and have ample food availability, the adipocytes closely resemble those present in white fat. On the other hand, when the animal is deprived of food and the weather becomes inclement, they return to adopt its natural "cluster" shape, with multiple drops of fat inside well differentiated.

Summary

As you may have seen, adipose tissue goes far beyond storing energy in the form of fat, both in humans and in other animals. It also has an important endocrine function (since it synthesizes and releases hormones that act on other organs), promotes thermogenesis and survival in times of need (the case of brown fat in mammals) and protects organs from mechanical damage, among many other things things.

Fat is not bad in itself, because without it, mammals could not live. The problem comes when many more calories than necessary are ingested and, therefore, the adipocytes increase in size and generate overweight or obesity.

Bibliographic references:

• Atlas of plant and animal histology: adipose tissue. Collected on February 12 in https://mmegias.webs.uvigo.es/guiada_a_adiposo.php
• Hernandez, H. d. B., Guillén, L. F. M., Guillén, M. F. M., Lemus, L. F. M., Ferrufino, G. J. Q., & Vidaur, N. TO. (2019). Adipose tissue and its endocrine function. Scientific Journal of the University School of Health Sciences, 6(2), 49-60.
• Hita, m. G., Ramirez, B. AND. B., Madrigal, B. R., Godínez, S., & Panduro, A. (2002). Endocrine functions of the adipose cell. Journal of endocrinology and nutrition, 10(3), 140-146.
• Marcano, Y., Torcat, J., Ayala, L., Verdi, B., Lairet, C., Maldonado, M., & de Vegas, J. (2006). Endocrine functions of adipose tissue. Venezuelan Journal of Endocrinology and Metabolism, 4(1), 15-21.
• Rafols, M. AND. (2014). Adipose tissue: cellular heterogeneity and functional diversity. Endocrinology and Nutrition, 61(2), 100-112.
• Vega-Robledo, G. B., & Rico-Rosillo, M. g. (2019). Adipose tissue: immune function and alterations induced by obesity. Allergy Magazine Mexico, 66(3), 340-353.