Beneath the epidermis lies a much thicker skin layer, the dermis. The dermis can be as much as 3000 micrometres thick.
The dermis is composed largely of the protein collagen. Most of the collagen is organised in bundles running horizontally through the dermis, which are buried in a jelly-like material called the ground substance. Collagen accounts for up to 75% of the weight of the dermis, and is responsible for the resilience and elasticity of the skin.
The ground substance of the dermis, seen under the microscope; it is an almost unstructured colloidal gel.
The collagen bundles are held together by elastic fibres running through the dermis. These are made of a protein called elastin, and make up less than 5% of the weight of the dermis. Despite their name, they are not involved in the natural elasticity of the skin.
As skin gets older, it loses some of its elasticity and ability to retain water. Collagen production declines as does subcutaneous fat, and the facial muscles start to atrophy.
Massed collagen and elastin fibres in the dermis.
Both collagen and elastin fibres are made by cells called fibroblasts, which are scattered through the dermis.
Special substances in the ground substance, called glycoproteins, can hold large amounts of water, and are responsible for maintaining a mass of water in the dermis.
Hyaluronic acid is another important substance that forms part of the tissue that surrounds the collagen and elastin fibres. It has the ability to attract and bind hundreds of times its weight in water. In this way it acts as a natural moisturising ingredient responsible for the skin's plumpness and moisture reserve. As we get older the amount of hyaluronic acid produced in the skin naturally gets less. This is one reason why aging skin becomes less resilient and supple (pliable). Recently hyaluronic acid has been experimentally injected into skin, in an attempt to reduce wrinkles.
Another reason for skin aging is that collagen and elastin production declines as the years go by, particularly after the
menopause, so that some of the skin's natural properties are lost. (See also Chapter 4, 'Skin and aging'.)
Injuries to the dermis
The deeper part of the dermis contains fewer blood vessels than the upper layers do, and many thick collagen bundles. These bundles lie parallel to each other along recognisable lines which are important to understand in wound healing. If a cut is made across these lines the skin gapes, and when the cut is healed there is more scarring than with wounds made along the lines of the bundles. Surgeons follow these lines when making their incisions, to ensure the best possible healing, which is why everyone's appendix scars are practically identical.
If the skin is seriously over-stretched, whether by too much fat or by pregnancy, the deep collagen fibres may actually rupture. This results in deep scars, which are seen through the intact epidermis as 'stretch marks'. Taking high doses of steroids for too long may have a similar result, as the collagen withers away (atrophies) under the influence of these drugs.
In a graze only the epidermis is sheared off. New epidermal cells very rapidly cover over the area with unscarred skin. Interestingly, these new cells are provided by cells from the hair follicles. Where a wound has damaged both the epidermis and the dermis, both the basal cells in the epidermis and the fibroblasts in the dermis go into intense production to seal the gap.
In a very narrow wound repair is relatively simple. But in large wounds the resulting repair is never perfect. Granulation tissue forms: this is a mixture of tiny blood vessels and fibroblasts frantically making collagen. This tissue eventually forms the scar: the greater the area to be covered, the larger will be the scar.
Different kinds of damage to the dermis: (top) stretch marks due to pregnancy; a well-healed surgical scar.
Sometimes the skin cells go on working at the repair process for much longer than necessary, so that far too much scar tissue is formed.
Scars in older people tend to be cosmetically better.
Sometimes the skin cells go on working at the repair process for much longer than necessary, so that far too much scar tissue is formed. This produces a permanent raised scar called a keloid. Keloids are common with certain types of skin, particularly in young people and those from an African background. They can be injected with steroids by a doctor, which sometimes helps. Cutting them out is seldom effective, and usually makes them worse. Eventually they decrease in size.
In many regions of the body the skin is separated from the muscles by loose fatty tissue (of variable depth) and moves very easily. In other areas it is anchored to the bones. This is most obvious in the palms of the hands, where the skin is arranged so that it closely follows the movements of the fingers. Some of these creases form while a baby is still in the womb, or very soon after birth.
Folds and creases are also found on the face even in very young babies, as the skin accommodates the movements of the muscles of the face. In the face of a very fat person the subcutaneous layer of fat becomes thick and bloated, and reduces the appearance of these creases.