Adipose tissue

Adipocytes

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; 

• White adipocytes - main cells of the white adipose tissue
• Brown adipocytes - chief cells of the brown adipose tissue 
• Beige adipocytes - recently discovered type, found dispersed within white fat 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.

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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. 

Test yourself on the main cell and tissue types of the body with this interactive quiz.

White adipose tissue

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.  

Brown adipose tissue

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.