Blood and lymph vessels, nerves and specialised cells play a vital role in triggering the skin’s inflammatory response to invading bacteria, allergies and physical injury. Blood vessels in the dermis help to regulate heat by constricting or dilating to conserve or release the body’s heat. They also aid immunity and give oxygen and nutrients to the epidermal layers. Nutrients diffuse only into the very bottom layers as blood vessels do not extend into the epidermis. Dead cells in the upper layers do not receive oxygen and nourishment.
A wave like border at the junction between the dermis and the epidermis provides an increased surface for this exchange of food and oxygen between the two sections. Along this junction are projections called dermal papillae. They tend to flatten with ageing as the flow of oxygen and nutrients decreases.
Stratified into five horizontal layers the epidermis as the topmost layer actually consists of between fifty cell layers in the thin areas to one hundred cells in thickness, on average 0.1 millimetres, similar in fineness to a leaf. Complete renewal of the cells occurs monthly in the epidermis which acts as a protection for the body. A single layer of cubed cells sits directly on top of the dermis in the deepest or first epidermal layer, the stratum basale. New skin cells called keratinocytes are formed in this layer through cell division to replace those shed continuously from the upper layers. The rate of this regenerative skin cell renewal decreases with ageing.
Melanocytes in the basement membrane produce the skin pigment melanin, which is photo protective against the skin’s exposure to ultraviolet rays in the sun. From the melanocytes, melanin transfers to nearby keratinocytes that will eventually migrate to the skin’s surface.
The second epidermal spinous or prickle-cell layer, the stratum spinosum, has polygonal, many sided keratinocytes. These lie in about eight to ten layers which begin to become rather flattened.
Keratin, a tough fibrous protein giving skin its protective property comprises the third or granular layer, the stratum granulosum. Flattened cells in these three to five layers begin to die as they are too far from the dermis to receive nourishment by diffusion. Making up the outer layer of skin, keratin is the key structural material of hair and nails and it provides the necessary strength and toughness for masticatory organs, such as the tongue, gums and hard palate.
A fourth epidermal layer of extremely flattened cells in three to five layers is called the stratum lucidum. This clear layer makes up only fingertips, palms and soles of the feet.
Dead keratinocytes are flattened in twenty five to thirty rows in the top outermost horny layer. This fifth layer, the stratum corneum is the real protective part of the skin, shedding keratinocytes continuously by friction and replacing them by cells from deeper epidermal sections.
Lipids, ceramides and fatty acids in between the keratinocytes in the horny layer act as cement around the skin cells. A combination of keratinocytes interspersed with epidermal lipids forms a waterproof barrier to keep moisture in the skin. Loss of the balance of oil and moisture in the skin to protect against bacteria causes dryness, itching, redness, stinging and skin problems.
Acidity helps maintain the hardness of keratin proteins keeping them tightly bound together. If the skin’s surface is alkaline, keratin fibres loosen and soften, losing its protective acidity. Skin becomes dehydrated, and infection, roughness, scaliness and other irritations appear. Slightly acidic moisture layers are derived from the combined secretions of sebaceous and sweat glands, inhibiting the growth of harmful bacteria and skin blemishes.
Hair, sebaceous and sweat glands are common components of both the dermis and epidermis. Pores are formed by a folding in of the epidermis into the dermis.
Dead keratinocytes, the skin cells that line the pore are constantly being shed like the epidermal cells at the top of the skin. The lining of the pore loses keratinocytes which, mixed with the sebum, clog the pore forming eruptions on the skin surface. Oil builds up inside the pores, disrupting surrounding tissue and pores begin to enlarge. When the skin loses its protective balance of oil and moisture or becomes alkaline, skin eruption results. Hair growth out of the pores is composed of dead cells filled with keratin proteins. A bulbous follicle at the base of each hair divides to produce new cells. The follicles are nourished by tiny blood vessels and glands. Hair prevents heat loss and helps protect the epidermis from exposure to the sun’s rays.
Sebaceous glands connected to hair follicles secrete sebum to help lubricate the follicle as it grows. Sebum, the oily liquid produced within the sebaceous glands is regulated by hormones and contributes to the lipids and fatty acids in the moisture balance.
Perspiration excreted by sweat glands is acted upon by bacteria producing a characteristic odour and helps cool the body temperature, hydrate the skin, eliminate some toxins, mainly salt. Long coiled hollow tubes of cells produce sweat either by ductless glands below the epithelial surface or by long portions of a duct on apocrine and exocrine glands secreting directly to the pore opening on the skin surface and by eccrine external secretion regulating body heat.
Understanding how the skin functions and is constructed can help you better care for a healthy skin.