Structurally, the epidermis is a thick keratinued stratified squamous epithelium consisting of four distinct cell types and five distinct layers.
Cells of the Epidermis
Cells populating the epidermis include: Keratinocytes, Melanocytes, Merkel cells, and Langerhans cells.
Keratinocytes: The most numerous cells are the keratinocytes which produce keratin, a fibrous protein responsible for protective properties of the epidermis. They arise from the deepest part of the epidermis from cells undergoing almost continuous mitosis. The keratinocytes are organized into 4-5 cell layers depending on body location. By the time the cells reach the surface of the skin, they are dead, scale-like structures. Every 35-45 days a totally new epidermis occurs. In areas of highest friction ( hands, feet) both cell production and keratin formation is accelerated.
Melanocytes: located at the base of the epidermis. Specialized cells that synthesize the pigment melanin. Melanin protects the cell nucleus from the destructive effects of UV radiation. Since all humans have the same relative number of these cells, individual and racial differences in skin coloring are probably due to differences in melanocyte activity.
Langerhans cells: arise from the bone marrow and migrate to the epidermis and other areas of the body containing stratified squamous epithelial tissue. They are macrophages and cooperate with T helper cells to assist in the immune response.
Merkel cells: present in small numbers at the epidermal-dermal junction. Associated with a disc-like ending of a sensory nerve fiber, called a Merkel disc, which functions as a sensory receptor.
Layers of the Epidermis:
In thick skin (palms, fingertips, soles of feet) the epidermis consists of five layers or strata: (from deep to superficial) stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum. Thin skin, which covers the rest of the body has only 4 layers, with the stratum lucidum absent.
Stratum basale: single layer of cuboidal to columnar shaped cells. It is separated from the dermis by the basement membrane. Some cells move toward the surface while others migrate into the dermis and gives rise to sweat and oil glands. Many mitotic cells are seen. About 25% of the cells in this layer are melanocytes.
Stratum spinosum: contains 5-10 rows of cells fitted closely together. The surface of the cells display minute spiney projections. Mitosis occurs here but not as frequently. Langerhans cells are scattered among the keratinocytes. Because cells superficial to this layer do not receive adequate nutrients, they become less viable and finally begin to die.
Stratum granulosum: thin zone consisting of 3-5 layers of flattened cells. Keratinization begins in this third epidermal layer. The plasma membranes of these cells also thicken so that they become more resistant to destruction. Langerhans cells are also found in this layer. At the upper border of this layer, the cells die and lysosomes begin to digest their organelles.
Stratum lucidum (clear layer): translucent band just above the S. granulosum. Consists of a few rows of flattened dead keratinocytes with indistinct boundaries. Present only in thick skin.
Stratum corneum: outermost layer; a broad zone 20-30 cell layers thick. Accounts for about 3/4 of the epidermal thickness. The shingle-like dead cells are remnants, completely filled with keratin fibrils, and are referred to as cornified or horny cells. Keratin provides a durable abrasion resistant and water-repellent "overcoat" protecting deeper cells from the environment.
Dermis:
The second major skin region, is a strong but flexible connective tissue layer. The cell types found in the dermis are fibroblasts, macrophages, and occasional mast cells and white blood cells. Its gel-like matrix is heavily embedded with collagen, elastin, and reticular fibers. The dermis is your "hide" and is richly supplied with nerve fibers, blood vessels, and lymphatic vessels. The major portions of hair follicles, as well as oil and sweat glands, reside in the dermis, but are derived from epidermal tissue. The dermis varies in thickness and it has two major layers: papillary and reticular.
Nails:
Scale-like modifications of the epidermis that forms a clear protective covering on the dorsum of the distal part of a finger or toe. Each nail has a free edge, a body, and a root embedded in the skin. The thickened proximal part of the nail bed - nail matrix - is responsible for nail growth. Nail cells are heavily keratinized. Nail body slides distally over the nail bed during growth.
Sweat Glands:
More than 2.5 million sweat glands are distributed over the entire body. Two types of sweat glands: eccrine and apocrine.
Eccrine - more numerous especially on the palms, soles and forehead. They secrete a hypotonic solution (sweat) derived from blood plasma. It is 99% water with some other substances in it. Sweating is regulated by the sympathetic nervous system. Assists in thermoregulation.
Apocrine - largely confined to the axillary and anogenital regions. They are larger in size. The secretions are the same as eccrine with the addition of fatty substances and proteins which make the secretions viscous. The secretions are odorless until bacterial decomposition occurs. Precise function unknown. Activated by the sympathetic nervous system during pain or stress.
Ceruminous Glands - secrete ear wax.
Mammary Glands - secrete milk.
Sebaceous (oil) Glands: Simple glands found all over the body except on the palms and soles. They are small on the body trunk and limbs and larger on the face, neck, and upper chest. Secrete an oily secretion called sebum. The sebum is usually ducted into a hair follicle. Sebum softens and lubricates the hair and skin and it has a bactericidal action. Acne is an active inflammation of the glands accompanied by "pimples." Acne is usually caused by bacteria, especially staphylococcus.
SOURCE:
- http://www.augustatech.edu/anatomy/chapter5.html
