The skin is the largest organ of the body and is also the most varied organ, since it adapts itself as the occasion requires. It is thicker on the soles of our feet, back and palms, and the thinnest on the eyelids.
At its thickest, it measures about 1/8 inch in thickness, while the skin on the eyelids only measures 1/25 inch. Another strange statistic is that the entire surface of the skin measures about 20 square feet and weighs between 7 to 9 pounds.
human skin funtion contruction dermis epidermis
The skin consists of three layers, each with its own specialized role to play:
- the hypodermis
- the dermis
- the epidermis
The skin also contains very specialized items in the form of:
- sebaceous glands
- apocrine glands
- eccrine glands
The Three Layers of a Human Skin
The Hypodermis
The hypodermis is the innermost and thickest layer of the skin. It invaginates into the dermis and is attached to the latter, immediately above it, by collagen and elastin fibres. It is essentially composed of a type of cells specialised in accumulating and storing fats, known as adipocytes. These cells are grouped together in lobules separated by connective tissue .
The hypodermis acts as an energy reserve. The fats contained in the adipocytes can be put back into circulation, via the venous route, during intense effort or when there is a lack of energy providing substances, and are then transformed into energy. When we speak of “burning up calories”, we are burning up fats in particular. The hypodermis participates, passively at least, in thermoregulation since fat is a heat insulator.
The anatomical position of the hypodermis is clearly a sexual characteristic. Whilst the hypodermis is distributed over the entire body, it has a tendency to accumulate above the belt over the abdomen and shoulders in men, and in women, below the waist around the thighs, hips and buttocks.
The Dermis
This part of the skin is sandwiched on top of the hypodermis and the epidermis (the outermost part of the skin) and is a vitally important area of the skin since it is here where the fibroblast cells form the network of fibers of the skin – very much resembling the weft and warp of fabric – and for this reason it is also referred to as the connective tissue.
The connective tissue is mainly made-up of collagen and to a far lesser degree elastin. Collagen and elastin are complex proteins responsible for the support and elasticity of the skin and enables the skin to regain its shape after being pulled, stretched or pushed.
This area of the skin is fed by blood circulating through tiny arteries, veins and capillaries to bring nutrition and oxygen to the cells whilst removing waste products. Each square inch of the dermis contains about 15 feet of small nutrient providing vessels.
Although they provide nutrition to the skin and remove waste material, their constriction and dilation is of vital importance to keep our body temperature constant.
The dermis also contains the sensitive nerve endings, sweat glands, hair follicles and sebaceous glands (oil glands).
The Epidermis
This section of the skin is the outermost part and contains no arteries, veins or capillaries but is fed from the dermis via the lymph.
The epidermis can be divided into three distinct layers, with the basal layer being the bottommost layer of this thin epidermis. The basal layer is also known as the Stratum germinativum since this is where “birth” is given to new cells.
There is a constant generation of new cells, with the younger cells found in the deepest part, and the oldest on top.
The basal layer not only contains basal cells, but also hosts the class of cells referred to as melanocytes – these are the cells that produce the melanin in your skin, which gives your skin its unique color. Every 6th cell in the basal layer is normally a melanocyte cell.
Racial difference influences the amount of melanocytes in the basal layer as well as the arrangement of such cells.
When sunlight strikes the melanocytes, it stimulates the production of melanin – which is in fact a defense mechanism of the body to prevent the harmful rays of the sun from entering the body.
The next layer in the epidermis is the Stratum spinosum – also called the prickle cell layer – which is the thickest part of the epidermis, and derives its name from the spiny, prickly looking projections that link the cells in this layer.
The cells within this layer are referred to as squamous cells, which are matured basal cells migrating up in the epidermis towards the outer surface.
These cells undergo modification when pushed to the surface and eventually contain no nucleus, are flattened and are charged with keratin – the same material which makes up human nails and hair.
These cells are not just old and charged with keratin, but are in actual fact dead cells, which eventually detach themselves and shed. These cells are arranged in an overlapping fashion and are the first line of defense against any substance trying to enter the body.
This top layer of dead cells is called the horny layer or Stratum corneum, which are somewhat acid in nature, and are also referred to as the acid mantle.
Dead cells are normally shed in a two weekly cycle, and an abnormal accumulation of these dead cells can result in flakiness and an ashen look, which is very noticeable in darker skinned people since it gives the skin a gray look.
In fair colored people, this abnormal build-up of dead cells results in the skin looking less-than-vital or healthy.
Sweat glands
Three types of sweat glands are housed in the skin – that being the sebaceous, apocrine and eccrine glands.
Sebaceous glands
The sebaceous glands are located near the hair follicle and produce sebum – the natural oil of your skin. Sebum is a mixture of different waxes and fats, and the facial skin contains nearly three thousand of these glands per square inch.
Sebaceous glands are primarily found on the scalp, face, back and chest and are present from birth, yet only mature and start secreting sebum when the person goes into puberty.
Sebum is manufactured in the glands and is secreted through a small duct leading from the gland to the hair shaft and moves up to the skin surface via the hair follicle. During this process the sebum also pushes out any accumulated debris that may be present in the follicle.
Sebum is produced to keep the skin lubricated and to prevent it from drying out, yet an over-production of sebum creates its own range of problems.
Apocrine glands
Apocrine glands are primarily found in the armpits, round the belly button, genital and anal areas of the body.
These glands are situated deep within the subcutis and produce a milky type of sweat, which causes body odor when bacteria breaks it down on the skin surface. The sweat is secreted into the upper parts of the hair follicle and exits to the skin via the follicle.
In mammals these glands produce body odors that attract sexual partners and although some popular belief subscribes that the same holds true for humans, no conclusive evidence has been produced to sustain these claims.
These glands are also present from birth, but as with the sebaceous glands, only mature at puberty and start manufacturing sweat at that time.
During periods of stress and a heightened emotional state these glands also appear to produce more sweat.
Eccrine glands
The eccrine glands are also situated deep in the subcutis, but unlike sebaceous and apocrine glands, they do not use the hair follicle to exit the sweat to the skin, but have their own dedicated pore, or duct, to move the sweat to the surface of the skin.
The skin contains between 2 – 3 million eccrine sweat glands and are found all over the body, yet are more concentrated on the palms, armpits and soles of the feet.
The production of sweat is triggered by hot weather, exercise, fever as well as emotional stress.
Emotional stress seems to trigger the production of eccrine sweat in the concentrated areas of the soles of the feet, palms, armpits as well as those found on the forehead.
The main function of eccrine sweat is to regulate body temperature, since the evaporation of the sweat helps to lower body temperature.
Over and above the regulation of body temperature, eccrine sweat is also helpful in eliminating waste salts from the body.
Since eccrine sweat is mostly composed of water, it does not contribute much to the formation of body odor.
