Adipose tissue is a specialized connective tissue consisting of lipid-rich cells called adipocytes. As it comprises about 20-25% of total body weight in healthy individuals, the main function of adipose tissue is to store energy in the form of lipids (fat). Based on its location, fat tissue is divided into parietal (under the skin) and visceral (surrounding organs). Depending on adipocyte morphology, there are two types of adipose tissue:
Besides energy storing, fat tissue has several other important functions in the human body. These include thermal isolation, cushioning the organs, an endocrine role, and production of numerous bioactive factors. This article will discuss the histology of the adipose tissue.
Adipose tissue is distributed within two compartments of the human body:
Like every other tissue, adipose tissue consists of cells and extracellular matrix. The cells are the most abundant structural elements of this tissue, predominating over the small amount of extracellular matrix. The main cells that compose adipose tissue are called adipocytes. Besides adipocytes, several other cell types are present; preadipocytes, fibroblasts, capillary endothelial cells, macrophages and stem cells. These non-adipocyte cells collectively form the stromal vascular fraction, and their main function is to support and protect the adipose tissue.
The extracellular matrix is produced by both adipocytes and stromal cells. It consists of a fine network of reticular fibers (type III collagen), whose function is to hold the cells in place. Adipose tissue is richly supplied with blood vessels and unmyelinated nerve fibers. On histology slides, these structures are usually found within the meshwork that separates neighboring adipocytes. Mast cells are also present here.
Adipocytes (adipose cells, fat cells), are the building blocks of adipose tissue. There are three types of adipocytes that constitute two different types of adipose tissue;
These cell types differ in their morphology and function.
White adipocytes are mainly present in white adipose tissue. Their shapes range from spherical (when isolated) to oval or polyhedral (as part of adipose tissue). The largest part of the cell is filled with a single (unilocular) lipid droplet that pushes and flattens the nucleus to the periphery of the cell. The cytoplasm forms a thin sheath around the droplet and contains a few mitochondria inside. The lipid droplets usually get lost during routine preparation of histological slides, which makes white adipose tissue appear as a delicate net of polygonal structures. These cells store fat.
In contrast to white adipocytes, brown adipocytes are smaller in size and have the lipids contained in multiple lipid droplets (multilocular morphology). The droplets surround the centrally positioned nucleus. Brown adipocytes have plenty of mitochondria dispersed between the droplets which give these cells their brown appearance. The cytoplasm also contains Golgi apparatus, and only a small amount of ribosomes and endoplasmic reticulum. These cells produce heat (thermogenic adipocytes). Same as in white adipocytes, the lipid droplets get lost in brown adipocytes as well during routine histological preparation. They are seen as a network of cells filled with numerous empty vacuoles.
Beige adipocytes are a distinct type of brown-like thermogenic adipocytes with multilocular morphology. They exist mainly in subcutaneous fat, but a small portion can also be found in visceral fat.
Every adipocyte is surrounded by a thick basal lamina containing collagen IV as a major component, similar to the cells of bone and cartilage. The strong external membrane of adipocytes is of key importance for resilience to mechanical stress and disruption.
The adipocytes in white adipose tissue are organized into lobules by connective tissue septa. The septa contain collagen fibers, nerve endings, blood and lymph capillaries. Extracellular matrix of white adipose tissue is made of reticular fibers and contains non-residential cells of adipose tissue (e.g. inflammatory cells).
On a histology slide, the adipocytes appear empty with a thin rim of cytoplasm close to the basal lamina. This is described as “signet ring” appearance of the unilocular tissue. This is because the intracellular fat droplet gets dissolved when dyed with standard histology staining methods (H&E staining).
White adipose tissue is the predominant type in adult humans. The greatest portion of this tissue is located in the hypodermis of the skin. This layer of subcutaneous fat is also known as panniculus adiposus. The thickness of this layer depends mainly on localization and gender. For example, women have more unilocular adipose tissue in the thigh and breast regions. In comparison, men have more abdominal fat. White adipose tissue can be found in other parts of the human body such as the retroperitoneal space, greater omentum, mesentery and surrounding the organs (for example kidney, heart, eyeballs). It’s also present in the bone marrow and other tissues where it usually fills in the spaces between the cells.
In contrast to white adipocytes, brown adipocytes have the appearance of a sponge due to the multiple droplets in the cytoplasm. Groups of adipocytes are divided into lobules by connective septa, which contain a substantial amount of blood vessels and unmyelinated nerve fibers. The extracellular matrix between individual cells within the lobules is sparse.
Brown adipose tissue is usually located in the body of the newborns, and it makes about 5% of their body mass. Newborns have much less subcutaneous fat than adults, which is why they are predisposed for hypothermia. To prevent lethal hypothermia, newborns have a large amount of brown adipose tissue, which has a great capacity for thermogenesis. With age, the amount of brown adipose tissue decreases, but it remains widely distributed throughout the body until puberty. Finally, in adults, the brown fat disappears from most sites. It remains in some regions only, such as; retroperitoneal space, around major vessels, deep cervical and supraclavicular regions of the neck, interscapular, paravertebral regions of the back and mediastinum.
The most important role of white adipocytes is energy storage. They store fat in the form of triglycerides inside their cytoplasmic lipid droplets, which helps to maintain free fatty acid levels in the blood.
For a long time, adipose tissue has been considered only as a passive fuel reservoir. Now, it is also considered to be an endocrine organ which secretes several bioactive factors (hormones, growth factors, cytokines). The most important adipose tissue hormones include leptin (satiety factor) and adiponectin. These biofactors circulate through the organism and carry information to other metabolically active organs such as liver, pancreas, muscle, and brain. These factors are of key importance in the pathophysiology of many metabolic disorders (e.g. type 2 diabetes mellitus).
Different localizations of the adipose tissue have different roles in the human body. For example, abdominal fat has a different metabolic profile than the rest of the fat in the body, and it has the biggest influence in inducing insulin resistance. Parietal fat has an important role in thermoregulation, while visceral fat provides the cushion-like support for internal organs, protecting them from mechanical injuries. During reduced caloric intake, the amount of parietal adipose tissue decreases, while the visceral fat remains undiminished.
In contrast to white, brown adipose tissue transforms chemical energy into heat. That way it prevents obesity, other metabolic disorders, and hypothermia.