Adipose tissue is best described as a form of connective tissue in which fat cells (also called Adipocytes) predominate. These large fat cells are found isolated or in small groups within loose or dense irregular connective tissue but occur in large aggregates as adipose tissue or fat in many body regions and organs. The cells of adipose tissue are called adipocytes and are specialized to store fat in microscopic droplets; aside storage, adipose tissue is a key regulator of the overall energy metabolism in the body. In some areas of the body, fat cells are relatively common in loose connective tissue.

Excess nutrients in the body have calories that are not wasted but are converted to fat to be stored for use when food intake decreases. Any form of excess calories in the body, be it in the form of fats, carbohydrates, or amino acids from protein, may be changed to triglycerides and stored; with increasing incidence of obesity and its associated health problems such as diabetes and heart diseases worldwide; adipose tissue now constitute a major area of medical research.

Adipose Tissue Structure and Histology

Adipose tissue arises from pluripotent mesenchymal cells (stem cells) and later originates from cells of the reticular connective tissue, which are capable of producing grape-like fatty tissue lobes also. The reticulum cells store fat droplets that later coalesce to form one large drop. In the process, the cells become rounder. In the usual histological routine preparations (paraffin sections), alcohol and xylene dissolve the fat and remove it from the fat tissue, thereby creating empty spaces without stain that are usually seen as fat vacuoles with a light Microscope.

Labeled diagram showing the development of adipose tissue where undifferentiated mesenchymal cells are transformed into lipoblasts that accumulate fat and thus give rise to mature fat cells. When a large amount of lipid is mobilized by the body, mature unilocular fat cells return to the lipoblast stage. Undifferentiated mesenchymal cells also give rise to a variety of other cell types, including fibroblasts.

How Adipose tissue is formed (Histogenesis of Adipose tissues)

Adipose tissue if formed from mesenchymally derived cells called lipoblasts (also called adipoblast or preadipocytes). These cells have the appearance of fibroblasts but have the ability to store fat in their cytoplasm. Lipid accumulations are isolated from one another at first but soon fuse to form the single larger droplet that is characteristic of cells of unilocular adipose tissue.

Multilocular adipose tissue develops differently from unilocular tissue. The mesenchymal cells that constitute this tissue resemble epithelium (thus suggesting an endocrine gland) before they store fat. Apparently, there is no formation of multilocular adipose tissue after birth, and one type of adipose tissue is not transformed into another.

Differences between Adipose Tissue and other Connective Tissues

1. Adipose tissue is a form of connective tissue but it is different from other types of connective tissues in the sense that the fat cells are predominant than the intercellular substances;
2. Also each fat cell of adipose tissue is surrounded by its own basal lamina this is not found in other connective tissues.
3. The reticular and collagenous fibers of adipose tissue extend around each fat cell, thus helping to provide a supporting framework that is delicate and also contains numerous capillaries that provide blood supply to it.

Classification of Adipose Tissue

There are two types of Adipose tissue White fat(also called Unilocular Fat) and Brown fat (also known as Multilocular fat).

Diagram of White fat (Unilocular adipose tissue); the arrowheads show nuclei of adipocytes (fat cells) compressed against the cell membrane

White Fat or Unilocular Fat

White fat is the most abundant type of adipose tissue in the body and is found mainly in the subcutaneous tissue of the skin forming a layer of fat known as the panniculus adiposus; most white fat is stored subcutaneously in the areolar connective tissue between the dermis and the muscles. This layer varies in thickness among individuals; the more excess calories consumed, the thicker the layer.

Location of White Fat in the body

1. Subcutaneous tissue
2. White fat is also found in the omentum
3. In the mesenteries
4. In Pararenal tissue (perinephric fat)
5. In the bone marrow

Structure of Unilocular Adipose Tissue

White fat is a highly vascularized tissue and also conatins many nerve fibers from the autonomic nervous system. It is called Unilocular fat because the cells are filled by a single large droplet of lipid that almost fills the cytoplasm and pushes the nucleus to the periphery and giving a signet-ring appearance when viewed with light microscope; this lipid droplet is made up of glycerol esters and fatty acids. White fat usually appears yellow because it contains the lipid-soluble pigment called Carotene. The fat cells (Adipocytes) express receptors for various substances that help to regulate the uptake and release of lipids in the body; these regulators include insulin, norepinephrine, thyroid hormone and glucocorticoids.

Functions of White Adipose Tissue in the body

1. White fat is store of calories or energy in the body whenever we take food in excess of what the body needs, the excess is stored as white fat for use when needed.
2. White fate is a source of building materials as in steroid hormone synthesis
3. The panniculus adiposus of the subcutaneous layer serves as an insulating layer to conserve body heat and therefore helps regulate body temperature.
4. The white adipose tissue acts mechanically as a packing or padding material for other organs and structures of the body
5. White adipose tissue serves as a shock-absorbing pad around the eyeballs, in the palms of the hands and the soles of the feet
6. White fat can secrete Leptin a hormone that decreases appetite by acting on the satiety centers in the hypothalamic region of the brain where its receptors are found, thus adipose tissue serves as an endocrine organ.
7. White adipose tissue is also involved in inflammation, which is the bodys first response to injury; the white fat does this by producing cytokines (chemical substances) that activate white blood cells.

Diagram of brown fat (multilocular adipose tissue) showing the characteristic cells containing central spherical nuclei and multiple lipid droplets

Brown Fat or Multilocular Adipose Tissue

Brown adipose tissue or multilocular fat is found mainly in newborns (neonates of humans) and also present in many other species especially prominent in hibernating animals such as bears, ground squirrels, lemurs, ground hedgehogs and brown frogs.

Location of Brown Adipose tissue in the human body

1. Brown adipose tissue has a restricted distribution in the body of adult humans and is found mainly in the inter-scapular and inguinal regions.
2. In newborns or neonates it is the abundant type of fat before it is depleted within days because it is use for providing heat for the baby (most of it is lost during childhood).
3. Brown fat is also found in the armpits
4. In the neck
5. In the mediastinum
6. In the skin on the back

Structure of Multilocular fat (Brown fat structure)

The cells of brown adipose tissue are polygonal and generally smaller than cells of white adipose tissue; they have centrally located rounded nuclei and their cytoplasm are filled with multiple small droplets of lipid hence they are called Multilocular fat (the cytoplasm and nucleus of the cells of brown fat are not pushed to the periphery). The brown or tan color of multilocular fat is due to the high content of cytochrome enzymes. The power house (mitochondria) of these cells are larger and more numerous than those of white fat cells. Each cell of the brown fat has its own direct sympathetic nerve supply.

Functions of Brown Fat in the Body

1. In a neonate of humans, the brown fat is used for generation of heat by rapid oxidation rather than for storage of energy this is because brown fat is even more vascular than white fat and thus the generated heat raises the temperature of the blood significantly. This is also seen during arousal from hibernation in animals.
2. During the production of heat, brown fat releases some substances such as glycerol that are used by other tissues of the body for other metabolisms.
3. The energy produced by the abundant mitochondria in brown fat is dissipated as heat rather than being stored as ATP.

Medical importance of Adipose tissue

Even though white adipose tissue appears histologically similar, there are differences in gene expression which are noted between visceral deposits (in the abdomen) and subcutaneous deposits of white fat. These differences are useful in assessing medical risks of obesity because it has been established that increased visceral adipose tissue raises the risk of diabetes and cardiovascular disease but increased subcutaneous fat does not. Also, Unilocular adipose tissue can generate a very common benign tumor called lipoma and can also generate the rare type of cancer called Liposarcoma.