Blisters

Anatomy

The skin has three layers. The deepest layer is called subcutaneous tissue and is composed of adipose (fat), nerves, hair follicles, arteries, veins, and sweat and oil glands. The middle layer is the dermis. It has two layers and varies in thickness depending on the location in the body. The deepest [Page 181]layer of the dermis is composed of thick collagen (connective tissue) fibers, which have parallel alignment with the skin surface. This layer sits just on top of the subcutaneous tissue. The thin, more superficial (outer) layer of dermis is also made of collagen fibers, which are randomly arranged. The most superficial layer of the skin is the epidermis. The epidermis constantly replenishes keratinocytes (skin cells), which flatten, die, and eventually slough. The overall thickness of the epidermis depends on its location in the body. It is thickest in the palms and soles. The epidermis consists of five layers (from deep to superficial): stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is named for the basal cells that form the base of the epidermis. They lie on top of the dermis, are shaped like columns, and are constantly dividing to replenish the cells lost at the surface. Basal cells divide into keratin-producing cells called keratinocytes. These keratinocytes form microscopic spines that link them together; thus, this layer is called the stratum spinosum. As the cells migrate toward the skin surface and become progressively flattened, they form a new layer called stratum granulosum because the cellular cytoplasm appears granular under microscopy. The cells then die and become the outermost layer, the stratum corneum. The stratum lucidum is a transition layer between the stratum granulosum and stratum corneum. It is found only in the thick skin of the palms of the hands and soles of the feet. This layer contains an oily substance that provides protection from friction. A friction blister forms when there is breakdown of the spinous bridging in the stratum spinosum. The blister roof and floor contain the normal layers of dermis and epidermis, and the cavity becomes filled with a clear transudate (fluid). If the friction over an area is more intermittent, hyperkeratosis (callus) formation may occur. This thickened area can be somewhat protected from friction blisters. If left to grow thick, however, a friction blister may occur under a callus.

Causes

Any repetitive external force across a localized area of skin can cause a friction blister. Susceptibility is based on the magnitude of frictional forces and the duration of repetition. Athletes are unlikely to stop their activity with the earliest signs of discomfort from friction blisters, thus predisposing them to greater severity. Typical causes are poorly fitting footwear, sweating, heat, and increased level of activity preceding protective callus formation.

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