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INTRODUCTION

KEY POINTS

  • Racial differences in pigmentation are due to the number, size, and aggregation of melanosomes in the melanocytes and adjacent keratinocytes.

  • Individuals of African descent have a more compact stratum corneum when compared to Caucasian skin.

  • Racial differences in distribution and secretions of apocrine glands have been observed.

  • African Americans have higher lipid content in their hair due to higher amounts of sebum production when compared to Caucasians.

  • There are racial differences in hair pattern, elastic fiber and melanosome distribution, total hair density, and ultrastructure of the hair.

  • Racial differences in dermal organization and cellularity have been observed.

NORMAL SKIN PHYSIOLOGY

Skin, the largest organ in the body, serves as a protective barrier that is integral in thermal regulation, serves as an important sensory organ, and plays an important role in immunologic function. The skin is divided into three main anatomic layers: epidermis, dermis, and subcutis [Figure 12-1]. Epidermal appendages include pilosebaceous units and apocrine and eccrine glands.

FIGURE 12-1.

Normal skin. There are three anatomic layers of the skin: epidermis, dermis, and subcutis. Epidermal appendages include pilosebaceous units and eccrine glands.

Epidermis

The epidermis derives from the ectoderm and is the most superficial layer of the skin. It is composed of several layers: stratum corneum, stratum granulosum, stratum spinosum, and stratum basale [Figure 12-2]. The stratum spinosum and stratum basale together are sometimes referred to as the malpighian layer.

FIGURE 12-2.

Normal skin. The subdivisions of the epidermis are the cornified layer, the spinous layer, and the granular cell layer. The dermis is divided into the papillary dermis and the wider, deeper reticular dermis.

Stratum Basale The stratum basale, or basal cell layer, is composed of proliferating stem cells separated from the dermis by a thin basement membrane composed of type IV collagen.1 In histologic sections, these cells are seen as a single layer above the basement membrane.1 The daughter cells differentiate and undergo keratinization as they migrate upward toward the surface of the skin. The process of regeneration takes place every 28 to 40 days.

Stratum Spinosum The stratum spinosum lies above the basal cell layer. This layer is composed of several layers of keratinocytes. These cells differentiate from basal cells and accumulate keratin as they approach the surface of the epidermis. Desmosomes, or cell adhesion molecules, are responsible for the “spiny” appearance of this layer and hence its name.

Stratum Granulosum In the granular layer, cells continue to accumulate keratin and basophilic keratohyaline granules. These granules, coupled with the desmosomes, help to form a waterproof barrier, protecting the body from excessive water loss. They also serve as promoters ...

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