We often view fat as something negative, but fat is simply adipose tissue with a range of important functions. In the realm of biomedical applications, white adipose tissue (WAT) is incredibly useful in reconstruction and tissue engineering applications. Why is that?
Adipose tissue is one of the most flexible types of cells in the body because of its ability to change into a new cell type. The process of one less specialized cell becoming a specialized type of cell is called differentiation. Now adipose tissue is already differentiated but we can take fat and tell it to become stem cell-like again. You may hear these dedifferentiated adipose cells called adipose-derived stem cells (ASCs) or induced pluripotent stem cells (iPSCs). Stem cells are known for their ability to become many different types of cells (multi or pluripotent). Once in the stem cell-like state, we can give these cells new instructions to redifferentiate into a totally different type of specialized cell. Fat is among the best sources for generating stem cells from adult tissue. Not only is adipose tissue incredibly flexible and multipotent, but it's also a breeze to collect compared to other stem cell sources. Imagine that—doctors taking away some of your fat could be a step towards groundbreaking medical treatments!
Biomedical Examples
Personal medication models derived from ASCs could revolutionize patient treatments that require multiple drugs. If doctors can test multiple drug cocktails on different tissues that represent your various body systems, they can find the most effective medication combination for you with fewer side effects and less trial and error.
Tissue repair could be aided by taking a little fat from your body. The risk of graft rejection is reduced significantly by using your own cells to make grafts, as well as faster healing and reduced pain during recovery for some patients.
But there is more to fat than WAT. Our bodies house a diverse range of adipose tissues, each with its own unique characteristics. Some are named after their colors, while others take their names from where they reside. One thing all adipose tissue has in common is that they are endocrine tissue. This allows them to receive and send signals to the rest of the body, which we are still unraveling. I will not go too deep into these functions in this post because it would be incredibly long otherwise. However, I will try to mention some interesting functions as I go through the main types of adipose tissue.
The main types of adipose tissue we currently recognize are white, brown, beige, pink, and bone adipose tissue.
White adipose tissue (WAT)
WAT is also called common, yellow, or unilocular adipose tissue. WAT is the type of fat we often associate with our bodies. It is a white-yellow color and is the most abundant type of fat in the body generally. It serves as an essential energy storage system, similar to a battery. WAT cells contain one large lipid droplet that displaces other organelles. Mitochondria, responsible for energy production, adapt to this space constraint by elongating and becoming thinner. WAT can be subdivided into other sub-types by the locations it is found, such as:
Visceral adipose tissue (VAT) plays a vital role in our bodies as it surrounds and cushions our organs, providing protection and support. Think of it as a protective layer that not only helps to prevent injuries but also supplies energy when needed. VAT can even be further divided into more specific locations, such as retroperitoneal fat located behind the kidneys.
On the other hand, subcutaneous adipose tissue (SAT) refers to the fat layer just beneath your skin. This layer not only provides insulation but also serves as an energy reservoir. Interestingly, SAT may contain a higher number of mitochondria, the powerhouses of cells responsible for energy production, compared to VAT.
However, dysregulation of WAT can lead to health issues. When dysregulated, WAT contributes to increased systemic inflammation, often associated with conditions like obesity and diabetes but it may play a role in other health conditions too. For example, researchers are exploring the communication between cartilage and adipose tissue. Their results suggest that proinflammatory WAT signals that are abundant in obesity, like leptin, may play a significant role in osteoarthritis progression. However, much remains unknown about the role adipose tissue plays in our health whether it is healthy or not.
Brown adipose tissue (BAT)
BAT is another better-known type but it makes up a smaller amount of adipose tissue in the body. The small amounts of BAT are located in specific areas of the body, which are behind the kidneys, on the upper back, and on the upper chest near the heart. As the name might suggest, it takes on a darker color than other types of adipose tissue. BAT is also called multilocular adipose tissue. This category of adipose tissue comes from the fact that these cells have many small lipid droplets of various sizes. Larger mitochondria are spread between these droplets, which are connected to the main function. The main purpose of BAT is to act as a heating plant to help regulate your body temperature.
Beige adipose tissue
This was first determined to be separate from WAT or BAT in 2012. Since it is relatively new and also begins with the letter 'B', this adipose tissue does not have an agreed-on shorthand. As the name suggests, it is an intermediate between WAT and BAT. You may see this transition of WAT to beige referred to as 'browning'. The purpose of beige adipose tissue is to be a short-term solution for body temperature regulation. When it gets cold out for extended amounts of time during winter, for example, our body suddenly needs more help staying warm. So it converts some of the extra WAT we have into something closer to BAT. Since it is converted WAT cells, it is also found in the same areas as WAT.
Pink adipose tissue (PAT)
PAT is found during pregnancy in the breasts and is pink-colored. Evidence suggests they are also converted from WAT cells through the process of 'pinking'. Their main purpose is to produce and secrete milk in the mammary gland, which is reflected by their more prominent rough endoplasmic reticulum and Golgi apparatus. When lactation is no longer required, these can turn back into WAT. There is also evidence that PAT can be converted to and from BAT in smaller quantities as well.
Bone Marrow adipose tissue (bMAT)
bMAT is found in the bone marrow cavity and looks much like WAT. Like WAT it serves as energy storage and is also a regulator of bone health. There are two sub-types of bMAT:
Constitutive bone marrow adipose tissue (cMAT) is at the distal ends of the bones. Distal means the end further from the heart. These adipocytes are larger, occur in yellow marrow, and do not change with whole-body disturbances, such as excess blood sugar in diabetes. Experiments suggest cMAT stores more unsaturated lipids, which we generally view as the healthier fats in food.
Regulated bone marrow adipose tissue (rMAT) is at the ends of bones nearer to the heart. rMAT seems to store more saturated lipids, which we view as unhealthy fats in food. These adipocytes are also near active hematopoiesis. Hematopoiesis is the differentiation of stem cells into red and white blood cells from the hematopoietic stem cell line. However, mesenchymal stem cells (MSCs) are another type of stem cell that exist in the same space and can differentiate into a type of bone cell called osteoblasts and bMAT cells. The balance between bMAT and osteoblast differentiation is important to bone health. For example, load-bearing exercises are a common tip to prevent age-related bone loss. The mechanical loading of these exercises persuades MSC to differentiate into osteoblasts that build bone instead of rMAT. This is one example of the incredibly close relationship between fat and bone, which I will dive into later.
The study of adipose tissue reveals a fascinating complexity within our bodies. From the well-known WAT and BAT to emerging players such as beige adipose tissue, PAT, and bMAT, each type possesses unique characteristics and functions. By delving into the spectrum of adipose tissue, we gain a deeper appreciation for the remarkable ways in which different types of fat tissue contribute to our overall well-being. As we begin to understand the dynamics of fat, we may find more potential for the improvement of health and medicine.
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