Hypodermis is a deeper layer of skin. As the name suggests, it is the layer of the skin below dermis. Other names for this layer are subcutaneous and also the fat layer. Some sources also call it the superficial fascia while others differentiate it from the superficial fascia. It is the third and the deepest layer of the skin and lies above the muscles. This layer is the main storage of body fat. It helps to anchor the skin to the tissues below the skin. It is firmly attached to dermis above by means of fibrous bands which fuse hypodermis with dermis. So whenever there is a deficiency in strength or number of fibrous bands, wrinkles and folds can be experienced. Another function of this layer is to insulate the body against the effects of low temperatures. Finally, this layer acts as a shock absorber against forces directed at the skin. In addition to this, this layer has specialised functions that include:
Another very important function of hypodermis is being anti-ptosis. When you are standing in an upright position, connective tissue bands in hypodermis hold the skin and fat lobules from shifting downwards and hence perform anti-ptosis function. This property plays a very important role in maintaining your look and beauty.
Although the hypodermis is considered an independent layer, its boundary with the dermis is almost indistinct. The two layers are connected by fibrous bands. Some of these bands are thick and strong while others are smaller. The later divide the hypodermis tissue into individual fat lobules. The first type of bands is referred to as the fibrous bands of the first order while the later are called the bands of the second order.
Hypodermis contains a complete set of connective tissue components like connective tissue stroma, cell parts, neuroreceptor system and vascular system.
A deeper examination of the hypodermis reveals fine and delicate collagen fibres that form a mesh of basket-like formations around fat lobules. These are the fibre bands of the third order. These fibrous bands network form a framework that supports the integrity of the skin and fat lobules. Hypodermis is spongy in nature because of these fat lobules found between collagen networks. This spongy nature gives hypodermis its shock absorbing and cushioning properties.
These include adipose cells, fibroblasts and macrophages. The adipose cells (adipocytes) are the predominant cells. These are the cells that give rise to the term adipose tissue. There are two types of adipocytes – white adipocytes and brown adipocytes. The former are found in large quantity in the hypodermis while the brown type is only found between the scapulae, around the kidneys and the thyroid. This fat is brown in color because of various iron containing compounds like cytochromes. This type of fat has more abundant blood supply.
The white adipocytes are important in cosmetology because they play a role in the development of obesity. A single white adipocyte can reach up to 200 microns in size. Compare this with only 60 microns for brown microns. Fat distribution differs in both males and females with females having more fat in their bodies.
Adipocytes actually contain one large fat droplet in their cytoplasm. This fat droplet pushes all other cellular components like nucleus and cytoplasm etc. to the margins while there is all fat in the centre.
Fibroblasts are the cells which produce fibres, as described in previous sections. These are collagen fibres predominantly which form septa between adipocytes. The structure and types of these septa have been described before.
Macrophages are large cells which basically have immune function. The purpose of these cells is to provide safety against invading factors like microorganisms and foreign bodies. These cells engulf such invaders and try to destroy them with the help of enzymes, hence provide immunity.
The gross appearance of this layer reveals loosely arranged connective tissues. There are fat lobules and the blood vessels here are much larger than those found in the dermis layer.
The fat distribution in the hypodermis varies according to a person’s genetic makeup, sex and age. Men have more of the fat around the abdomen while in women it mostly accumulates in the hips, buttocks and breasts. Fat arrangement is also changed in men and women. In men, fat is always at an angle to the skin surface while in women, it is almost perpendicular to the skin. Also, stroma is denser in men when compared with women while adipocytes are lesser in number than in women’s hypodermis.
Like any other skin layer, ageing affects hypodermis as well. With increasing age the fat layer of hypodermis gradually thins out. But for some people, the hypodermis may also thicken with age.
As described in functions of hypodermis, it performs anti-ptosis function as well. As ageing occurs, there is a loss of collagen forming ability and as collagen is involved in forming fibrous bands or septa (retaining ligaments of the face), turgor reduces and the hypodermis structures can give way to gravitational effects. This contributes to facial and body changes like sagging of skin under gravity. Contour of the body also changes. Such changes may necessitate cosmetic intervention for some people. Cosmetic interventions to this deeper layer include PDO threads, Radiofrequency & Ultrsound based treatments and facelift etc.