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like the rapidly dividing cells of bone marrow, the intestinal tract, and hair matrix, basal cells are very sensitive to chemicals that affect nucleic acid synthesis. Sulfur mustard is one such agent.

Thickness of the epidermis varies greatly depending principally on the body site and the number of cornified cell layers within the stratum. The stratum corneum is thinnest on the scrotum, on the flexor surfaces of the forearms, within the axillae, and around the eyes. These are body sites through which sulfur mustard penetrates best and exerts its most profound effects after acute exposure. It has been estimated that the entire epidermis renews itself every 45 to 75 days. Sulfur mustard inhibits cell replication within the basal layer of the epidermis and thus disrupts this pattern, resulting in blister formation.

Basal cells of the epidermis are attached to the dermis through the basal lamina, which is often referred to as the basement membrane zone, or epidermal-dermal junction (Figure 9-3). A variety of collagen-like fibrils within basal cells, traverse and attach to the basal lamina within the dermis. Other collagen-like filaments are thought to serve as "anchoring rods" between dermis and epidermis, and dermis and basal lamina. Injury to or destruction of one or more types of anchoring structures causes separation of cells, giving rise to the formation of vesicles and blisters. It has been postulated that proteases released by sulfur mustard, acting on attachment structures between basal cells and basal lamina, give rise to blister formation (Papirmeister et al., 1991). Destruction of the epidermis followed by "shedding" exposes underlying tissue that is devoid of pigmentation and color. When exposed, the underlying tissue, or dermis, imparts a glistening "whitish" appearance to skin even in the darkest of races.

Dermis

The dermis makes up the greatest mass of human skin. It contains cells and fibers that contribute to the skin's elasticity and resiliency (elastic fibers and collagen fibers) and serves as a major force in protecting the internal organs from injury due to external mechanical forces. It is a true supporting structure for cutaneous blood vessels, nerves, and epidermal adnexal structures. Blister fluid is made up principally from fluid released from the dermis. The depth of injury to the dermis and underlying subcutaneous tissues will determine the depth and extent of skin ulceration. Injury to the upper levels of the dermis results in superficial, rapidly healing ulcers. Injury to the entire dermis results in deep, slow-healing skin ulcers.

The predominant cell type found within the dermis, the fibrocyte, is limited in distribution in the normal active dermis; so is the metaboli-



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