There are numerous human skin conditions. The human skin is the organ system that covers the entire surface of the body and is composed of nails, hair, and skin, and the related muscle s and glands. The primary function of this organ is to act as a barrier against the external environment. The average weight of a human’s skin is 8.8 pounds, and it covers an approximate area of 21.5 ft2, and is made of three separate layers; the subcutaneous tissue, the dermis, and the epidermis. There are two primary types of human skin; The hair bearing skin, and the glabrous skin or the non hairy skin such as on the soles and palms. The hairs in the former are in structures known as pilosebaceous units, with each having a sebaceous gland, a hair follicle, and associated arrector pili muscle. The glands, hair and epidermis form from the ectoderm while still in the embryo, which is chemically influenced by the underlying mesoderm which forms the subcutaneous tissue and the dermis.
The most superficial layer of skin is the epidermis, which is a squamous epithelium and has several strata including; the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum. Nourishment is provided to these layers by means of diffusion from the dermis, since the epidermis does not have a direct blood supply. The epidermis has four cell types; The Merkle cell, Langerhans cells, melanocytes, and keratinocytes. The keratinocytes are the primary component and constitute roughly 95% of the epidermis. Maintenance of the squamous epithelium is accomplished by cell division within the stratum basale, where the differentiating cells move slowly outward through the stratum spinosum and on to the stratum corneum, where they are continuously shed from the surface. In normal skin, the rate of production equals the rate of loss. It usually takes about 2 weeks for a cell to migrate from the basal cell layer to the top of the granular cell layer, plus an additional 2 weeks to cross the stratum corneum.
The dermis is the layer of skin between the subcutaneous tissue and the epidermis and is comprised of two sections, the reticular dermis and the papillary. Structural components of the dermis are extrafibrillar matrix, elastic fibers, and collagen. Within these components are the apocrine glands, the eccrine, the muscles of the arrector pili, and the pilosebaceous units. The dermis has two vascular networks which run parallel to the skins surface, one of which is deep plexus and one which is superficial. Vertical communicating vessels connect these two networks. The primary function of blood vessels within the dermis is twofold; To regulate temperature and to supply nutrition.
The subcutaneous tissue is a layer of fat between the underlying fascia and the dermis. This tissue may be further subdivided into two components, a deeper layer of vestigial muscle known as the panniculus carnosus and the panniculus adiposus which is the actual fatty layer. The main cellular component of this tissue is the fat cell known as the adipocyte. The structure of this tissue is composed of lobular and septal compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat serves as an energy source, absorbs trauma, and insulates the body.