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.
   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