ADIPOCYTES: ORIGIN, CHARACTERISTICS, TYPES AND FUNCTIONS - BIOLOGY - 2025

The adipocytes are round or polygonal cells characterized by storing large amounts of lipids. These cells, also known as lipocytes, or adipose cells, are cells derived from primitive mesenchymal tissue and constituents of adipose tissue.

The lipids stored in adipocytes come from three fundamental sources: fats that come from food and are circulating in the bloodstream, triglycerides synthesized in the liver, as well as triglycerides synthesized inside the adipocytes from glucose.

Adipocytes. Taken and edited from: M. Oktar Guloglu.

In recent years, the scientific community has shown a greater interest in the knowledge of adipocytes and adipose tissues, due to the alarming increase in the incidence of obesity in industrialized countries.

Until a few years ago, the existence of only two types of adipocytes was known, whose main functions are related to the storage of reserve substances in the form of fats and the control of body temperature. However, today other types of adipose cells are recognized as well as their glandular function.

Origin (Adipogenesis)

The origin of both adipose cells and adipose tissues is not fully known and many stages of the process have yet to be described. White adipose tissue makes its appearance immediately after birth, and proliferates rapidly due to both the increase in the number and size of adipocytes.

Some studies, with multipotent clonal cell lines, suggest that the adipocyte lineage comes from a precursor branch of embryonic cells that have the ability to differentiate into adipocytes, chondrocytes, osteoblasts, and myocytes.

During embryonic development, some of these multipotent embryonic cells will give rise to the precursors of adipocytes, which are called adipoblasts. As a result of the division of these adipoblasts, immature preadipocytes are obtained, which must undergo a series of transformations until they become mature adipocytes.

During the maturation phase the cell becomes spherical, accumulates fat droplets and progressively acquires the morphological and biochemical characteristics of a mature adipocyte.

This maturation phase is characterized by chronological changes in the expression of many genes, which is reflected by the appearance of early, intermediate and late mRNA markers, as well as by the accumulation of triglycerides.

Beige adipocytes originate within white adipose tissue, apparently by transdifferentiation of white adipocytes.

Pink adipocytes, for their part, arise during the gestation process from transdifferentiation of white adipocytes, and remain in the mammary gland during the lactation period, to later be reabsorbed.

Optical microscopy image with 200X magnification, the different stages of adipogenesis are observed in adipocytes obtained from human fat pre-adipocytes stimulated with a cocktail of hormones. Taken and edited from: Arodmur.

characteristics

The defining characteristics of apidocytes are that they are cells capable of storing large amounts of fat, they have glandular activity producing adipokines and are subject to hormonal regulation of their functioning by these same adipokines.

On the other hand, adipocytes can have a rounded or polygonal shape; its cytoplasm may be abundant or sparse, with a nucleus that may or may not be displaced from the center; They have a variable content of mitochondria depending on the type of adipocyte and their size can vary depending on the amount of fats contained within.

Types

White adipocyte

The white adipocyte is a spherical cell of mesodermal origin that has a very variable size. This size depends on the volume of accumulated lipids, which represent up to 95% of the cell mass and which can increase or decrease depending on the functional state of the adipocyte.

Lipids are accumulated in droplet forms that fuse to form a single fat droplet that occupies almost the entire cytoplasm. The nucleus of the adipocyte is compressed and displaced to one side of the fat drop, like the rest of the cellular organelles.

In this cell, the cytoplasm is restricted to a thin ring around the fat blob. The adipocyte is the main actor in the processes of lipogenesis and lipolysis, which are regulated by different types of hormones. Additionally, it is the main cell producing resistin, adiponectin and leptin in adipose tissue.

Brown adipocyte

The brown adipocyte is also called the brown adipocyte. It is shaped like a polygon and has a greater amount of cytoplasm than the white adipocyte. The nucleus is rounded and only slightly offset from the center of the cell. The cytoplasm, for its part, acquires a brown color due to the high content of mitochondria.

Fats are stored in multiple small vesicles and not in a large central vacuole. The brown adipocyte also stores glycogen granules in higher concentrations than the white adipocyte.

Another difference between these two types of adipocytes is that the latter expresses the uncoupling protein-1 (uncoupledprotein 1; UCP-1) and the white adipocyte does not.

Although its origin is also mesodermal, it is independent from that of the white adipocyte, since it comes from the Myogenic Factor 5+ (Myogenic Factor 5+; MF5 +). In humans these cells are more abundant in the early stages of development and until recently it was believed that they disappeared in adults.

Beige adipocyte

The beige adipocyte is a cell that presents characteristics of both the white adipocyte and the brown adipocyte. It is a cell of mesenchematic origin from cell precursors close to white adipocytes.

It has numerous vacuoles but never as many as the brown adipocyte. Their fat deposits are of the paucilocular type because they present an intermediate deposit between unilocular and multilocular. They differ from brown adipocytes by expressing lower levels of UCP-1.

Pink adipocyte

The pink adipocyte is a milk-secreting cell. It is caused by a transdifferentiation of white adipocytes from white adipose tissue. This tissue develops in the mammary glands during pregnancy and lactation.

In addition to producing milk, the pink adipocyte produces leptin, a hormone that promotes proliferation of the mammary epithelium. In addition, it serves to prevent obesity in infants and participates in the maturation of the central nervous system of the infant.

Yellow adipocyte

Recently (March 2019) Camille Attané and collaborators from the University of Toulouse, France, proposed the existence of a new type of adipocyte that they named yellow adipocyte.

This cell is located in the adipose tissue of the bone marrow, which constitutes about 10% of the total adipose tissue of the body and was known as adipocyte of the bone marrow.

This adipocyte is morphologically similar to the subcutaneous white adipocyte, but exhibits a very specific lipid metabolism, oriented towards cholesterol metabolism. Another characteristic of the yellow adipocyte is that it increases in volume under conditions of caloric restriction.

Adiposites in pig adipose tissue. Taken and edited from: Laurararas.

Where are they found? (Topography)

Adipocytes form adipose tissue that is deposited in different parts of the body. The main of these deposits is the subcutaneous one, which is located over a large part of the body's surface, especially in the proximal area of ​​the lower extremities and in the abdomen.

In mammals there are two main types of adipose tissue: white adipose tissue and brown (also called brown or brown). White adipose tissue constitutes up to 20% (men) or 25% (women) of total body weight in normal individuals.

This tissue is mainly made up of white fat cells, but it can also have beige fat cells.

Brown adipose tissue, for its part, is made up of brown adipocytes, as well as adipocyte progenitor cells. In the past, scientists believed that in humans it was restricted to the neonatal period, however today they have shown that they persist in the adult state.

Deposits of this type of tissue are located in the cervical, supraclavicular, adrenal, paravertebral and mid-intestinal regions. Clusters of brown adipocytes have also been found in adult striated muscle tissue.

Perivisceral adipose tissue is found surrounding the coronary artery, aorta, mesentery, kidney, and muscles. It may have characteristics of white or brown fat tissue. The adipose tissue of the bone marrow contains yellow adipocytes, which, as already noted, have characteristic particles that differentiate it from other adipocytes.

The mammary adipose tissue has white adipocytes and beige adipocytes. During the gestation and lactation period, some white adipocytes transform into pink adipocytes, which are capable of secreting milk.

Other important fat deposits are represented by the adipose tissue of the face, the joints, the soles of the feet and the palms of the hands.

Features

The main function of white adipocytes is to store energy in the form of fat droplets, but it also acts as a thermal insulator and as a shock-absorbing layer.

Brown adipocytes play a fundamental role in regulating body temperature and also in burning excess energy, preventing obesity.

These two types of adipocytes, as well as the others, have hormonal activity. Adipocytes secrete substances that collectively are called adipokines.

Adipokines can have autocrine activity, that is, their secretions directly affect the cells of adipose tissue. They can have paracrine activity by affecting the organs adjacent to the adipose tissue. They can also have endocrine activity as they are transported through the bloodstream and affect target cells.

Leptin was the first adipokine described. This hormone has multiple functions, such as: regulating appetite and energy expenditure; promote lipolysis in adipose tissue; inhibit insulin secretion by the pancreas, as well as insulin-induced steroid synthesis in the ovary. It also has immunomodulatory activity.

Resistin is secreted by cells other than adipocytes and is a pro-inflammatory protein. Adiponectin, on the other hand, has anti-inflammatory activity and is also anorexogenic.

The function of angiotensin appears to be to decrease adipogenesis and stimulate adipocyte hypertrophy, and chemokines are responsible for promoting the interaction of white blood cells with the endothelium of blood vessels.

For all these functions, some authors point out that adipose tissue should be considered an organ, which would allow us to understand the importance of adipocytes, as well as the pathophysiological processes in which these cells intervene.

References

1. PR Weather, HG Burkitt & VG Daniels (1987). Functional Histology. 2 ^nd edition. Churchill Linvingstone.
2. FG Gregoire, CM Smas & HS Sul (1998). Understanding adipocyte differentiation. Physionogical Reviews.
3. Adipogenesis. On Wikipedia. Recovered from en.wikipedia.org.
4. JM Moreno-Navarrete & JM Fernández-Real (2011). Adipocyte differentiation. In: M. Symmonds, Ed. Adipose Tissue Biology. Springer.
5. M. Reyes (2012). Biological characteristics of adipose tissue: The adipocyte as an endocrine cell. Las Condes Clinical Medical Journal.
6. JC Sánchez, CR Romero, LV Muñoz, RA Rivera (2016). The adipose organ, a rainbow of metabolic and endocrine regulation. Cuban Journal of Endocrinology
7. Attané, D. Estève, K. Chaoui, J. Iacovoni, J. Corre, M. Moutahir, P. Valet, O. Schiltz, N. Reina & C. Muller (2019). Yellow adipocytes comprise a new adipocyte 1 sub-type present in human bone marrow. BioRxiv. Preprint. Recovered from: biorxiv.org