Nads, like hair, are a modified stratum corneum of the that forms in a manner similar to the formation of hair, tinize from the stratum bas ale of the nail matrix.
epidermis. They contain hard keratin Cells continually proliferate and kera-
The integument consists of the skin (epidermis and dermis) and associated appendages (sweat and sebaceous glands, hairs, and nails). It is the largest organ in the body. The epidermis is devoid of blood vessels and contains a stratified squamous epithelium derived primarily from ectoderm. It is composed of six layers in thick skin: stratum basale, which is a proliferative layer of columnar/cuboidal cells, showing mitotic activity; stratum spinosum, which is a multilaminar layer of cuboidal/polygonal cells; stratum granulosum, which has more flattened polygonal cells containing basophilic granules; stratum lucidum, which is a thin, eosinophilic layer of squamous cells; and stratum corneum, which is a thick layer containing anudeate keratinized cells. All six layers contain various amounts of keratin.
The epidermis contains four cell types: keratinocytes, which produce keratin; melanocytes derived from neural crest cells that produce melanin; Langerhans cells, which are antigen-presenting cells; and Merkel cells, associated with nerve fibers. The dermis is a connective tissue layer mainly of mesodermal origin. Other layers are the dermis-epidermal junction and hypodermis. The latter contain adipocytes.
Sweat glands may be eccrine or apocrine. These are compared in Table 1-5-1.
Sebaceous glands are branched holocrine acinar glands that discharge their secretions onto hair shafts within hair follicles. They are absent in the palms and soles.
Hair is comprised of keratinized epidermal cells. Hair follicles and the associated sebaceous glands are known as pilosebaceous units.
Nails are modified stratum corneum of the epidermis and contain hard keratin. Cells continually proliferate and keratinize from the stratum basale of the nail matrix.
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