Although one may think of the skin as a mere covering of the body, like an overcoat, it is in fact an active participant in the body’s biochemical and physiologic functions, and not an inert 20 square feet of covering, no matter how attractive it is to look at.
But like an overcoat, it does keep out the undesirable effects of weather with its waterproof surface, the plethora of blood vessels to keep us warm, and sweat glands to cool us off, and the nerve endings warn us of danger and can discriminate the types of danger: heat, cold, sharp, blunt etc. As an active organ the skin is the site of transformation of the precursor into Vitamin D under the effects of sunlight.
The skin has layers, the outermost, epidermis, is an epithelium and continuous in many parts of the body with the mucous membranes. The layers beneath are called dermis and hypodermis.
The epidermis is stratified squamous epithelium which has an outer layer of stratum corneum or “horny layer” of material that peels away; beneath this is the layer of actively dividing cells, the stratum granulosum, and the stratum Malpighii.
The sweat glands and hairs penetrate the epidermis, but their “roots” are in the layer of dermis beneath it; here run the blood vessels and the cutaneous nerves.
The dermis is composed of fibro-fatty tissue, varying in the quantity of each component, but basically having a fairly dense collagen mat with varying quantities of elastin and fat.
Color is given to the skin by the dark brown melanin and melanoid, and by the yellow keratin, there is a congenital condition of albinism in which the skin lacks pigmentation, and other conditions in which patches of skin are not pigmented.
There are at least five different pigments that determine the color of the skin. These pigments are present at different levels and places.
Most persons are horrified when they see a scanning electron microscope picture of their skin and find it is swarming with mites and bacteria.
As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself as a person ages.
The thinning of bone with age is widely recognised because it tends to lead to fractures of the hip. What is less widely recognised is that thinning of the bone is merely a localised manifestation of thinning of all collagenous structures in the body, and the skin thins with age just as much as does bone. It also loses its elastin component and falls more readily into folds and creases. Skin of the very old persons tears easily, or can easily be pulled away from its underlying supporting tissues.
The same changes occur with “premature aging” of the skin associated with abuse of ultra-violet light (natural or artificial) or with cortisone which inhibits collagen formation.