The skin or integument is a double-layered membrane covering the exterior of the body and is continuous with the mucous membranes lining the body's orifices. It shows a marked variation in thickness, measuring from less than 1 mm (on the eyelid) to more than 4 mm (on the back).
The skin can be divided into two parts, the epidermis (outer layer) and dermis (inner layer), which rests on and is attached to the subcutaneous fat
The epidermis is comprised of four clearly defined layers, or strata Basal layer (stratum basale, stratum germinativum): mycotic activity of keratinocytes is confined to this layer, resulting in an epidermal turnover time of approximately 4 weeks. Melanocytes, which produce melanin pigment, are found in this layer.
Prickle cell layer (stratum spinosum): contains polyhedral cells with intercellular attachments and Langerhans cells (specialized antigen-presenting cells involved in contact hypersensitivity)
Granular cell layer (stratum granulosum): characterized by loss of cell nuclei and acquisition of dense keratohyalin granules
Keratin layer (stratum corneum): acellular layer containing keratin fibrils. It serves as a protective barrier. Its thickness varies with location (maximum thickness on palms and soles, minimum thickness at flexural sites).
The dermis is the fibrous part of the skin, which provides strength. It supports the epidermis and is composed of a fibrous connective tissue component (collagen and elastic fibers) in intimate association with ground substance. Within the dermis are the epidermal appendages (surrounded by a connective tissue sheath), blood vessels and nerves, and a cellular component, including mast cells, fibroblasts, myofibroblasts, and macrophages. Smooth muscle is also represented in the erector pili muscles.
