The skin, or as it is often called the integumetary system is composed histologically of two layers. These are the epidermis and the dermal layer. The epidermis is the most superficial layer of the skin. Deep to it is the dermis. The epidermis or the superficial layer of the skin contains many types of cells.
Thus it is the target of many skin diseases such as diseases of keratinocytes which are a group of cells that populate the epidermis. An example of such a disease is viral warts.
The skin is in a continuous process of renewing its cells and shedding its old and dead cells. It makes so i the skin basal layer in which newly formed basal cells migrate toward the upper skin layer to become functional keratinocytes there. After its life cycle keratinocytes loose their nuclei and die.
The epidermis of the skin is composed of stratified squamous epithelium that is keratinized. This type of epithelium which has several layers of cells has a protective function that can protect the skin from infectious organisms that usually live on the skin surface. In addition the stratified epithelium protects against chemicals and trauma to the skin.
Keratinocytes are one type of cells that populate the epidermis of the skin. They contain the protein keratin. Melanocytes are other type of cells that populate the epidermis also and have the pigment melanin which imparts to the skin its characteristic color. Malignancy of these cells can lead to the typical melanoma that affect the skin. Nevi or moles are benign skin neoplasms that also affect melanocytes.
The skin contains in addition to these two cell types free nerve endingsfor touch and heat sensations. These free nerve endings transmit sensory nerve signals to the brain which in turn can initiate a reflex such as moving the hand from a heat source. Pain receptors are also present in the skin epithelium.
The skin epithelium has rich nerve supply but does not have vascular supply. Therefore an injury to the epidermis alone does not produce bleeding. The dermis, however, has rich blood supply in addition to lymphatic supply.
One of the many functions of the skin is to synthesize vitamin D which is important for calcium metabolism in the body. Therefore exposition to sunlight for a period of time is useful as it contributes to the accumulation of vitamin D in the body.
The skin also contains appendages such as hair and sebaceous glands in addition to sweat glands. Sebaceous glands are especially important in a skin clinical syndrome which is called acne. This condition occurs mainly in the 20's years old. It is caused by excessive secretion of androgens in the body which cause an increase in the amount of secreted sebum. Thus predisposing to plugging of the glands with subsequent acne development.
Medical conditions that can affect the skin are directed at either one of the cells of the epidermis such as keratinocytes and melanocytes or nerve cells. Vitiligo and albinism are two pigmentary disorders that affect melanin secreting cells in the epidermis which are melanocytes. Tumours of nerve cells can also be possible in the epidermis such as merkel cells carcinoma.