Integument 12

GENERAL FEATURES

The integument consists of the skin (epidermis and dermis) and associated ap­ pendages (sweat glands, sebaceous glands, hairs, and nails). It is considered to be the largest organ in the body. The integument constitutes approximately 16% of total body weight. The integument functions to protect the body from injury, desiccation, and infection. It also participates in sensory reception, excretion, thermoregulation, and maintenance ofwater balance.

EPIDERMIS

The epidermis is the outermost layer of the integument (Figure 1-12-1). It is a stratified squamous epithelial layer of ectodermal origin. It is devoid of blood vessels and consists of4 or 5 layers, from deep to superficial.

Layers

The layers of the epidermis are:

•Stratum basale (stratum germinativum) is a proliferative basal layer of columnar-like cells that contain the fibrous protein keratin.

•Stratum spinosum is a multilaminar layer of cuboidal-like cells that are bound together by means of numerous desmosomal junctions.

•Stratum granulosum consists of flat polygonal cells filledwith baso­ philic keratohyalin granules. Viewed at the electron microscopic level, these cells also contain numerous membrane-coating granules.

•Stratumlucidum is the transitional zone of flat eosinophilic or pale­ staining anucleated cells found only in regions with a thick stratum cor­ neum.

•Stratum corneum is the superficial stratum consisting of several layers of flat, anucleated, and cornified (keratinized) cells.

CellTypes

The epidermis contains several cell types:

•Keratinocytes are the most numerous and are responsible for the pro­ duction of the family ofkeratin proteins that provide the barrier func­ tion of the epidermis.

•Melanocytes are derivatives of neural crest ectoderm. They are found in the dermis and are also scattered among the keratinocytes in the basal layers of the epidermis. These cells produce the pigment melanin in the form of melanosomes that are transferred to keratinocytes.

Clinical Correlate

Pemphigus is an autoimmune blistering disorder caused by disruption of desmosomes linking keratinocytes.

Psoriasis results from an increase in the number of proliferating cells in stratum basale plus stratum spinosum. In addition, there is an increase in

the rate of cell turnover. This results in greater epidermal thickness and continuous turnover ofthe epidermis.

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The epidermis contains 4 cell types:

Keratinocytes, which produce keratin

Melanocytes derived from neural crest cells that produce melanin

Langerhans cells, which are antigen-presenting cells

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 contains adipocytes.

Sweat glands may be eccrine or apocrine. These are compared in Table 1-12-1.