The epidermis forms the outer layer of skin and is composed of keratinized stratified squamous epithelium. This outer layer is a tough waterproof barrier that protects the organs interior to the epidermis from external forces and conditions. The epidermis is thin in some locations, but is much thicker in other areas, such as the palms and the soles of the feet. The surface of the epidermis is packed with dead cells and keratin. The epidermis, like many other structures in the body, lacks blood vessels to provide nutrients and dispose of waste. Because of this, the epidermis depends on the diffusion of nutrients from the connective tissue that lies beneath it. The epidermis does contain some nerve endings, however, most nerve endings are found beneath the epidermis in the dermis.
Cells Within the Epidermis
There are five different types of cells found within the epidermis.
Keratinocytes Keratinocytes are the most common type of cell found within the skin and are responsible for the production of keratin; a key element in the skin, hair and nails. When examined under a microscope, nearly all the cells seen within an epidermal specimen are keratinocytes. These keratinocytes are produced by the mitosis of stem cells within the stratum basale. In order for the mitosis to successfully complete, a supply of oxygen must be readily available; something easily accessed from the stratum basale due to the blood vessels located in the neighboring dermis.
Melanocytes
Melanocytes are are melanin producing cells found the bottom layer of the epidermis. This bottom layer is known as the stratum basale. Melanin, produced by melanocytes, is responsible for the pigmentation, or the color of the skin. The keratinocytes consume fragments of these pigments through phagocytosis, and these fragments accumulate on the upper side of the keratinocyte, acting as a shield, protecting the DNA within the cell from harmful rays.
Stem Cells Stem cells are undifferentiated cells found in the bottom layer of the epidermis, commonly referred to as the stratum basale. These stem cells later give rise to the keratinocyte cells.
Tactile (Merkel) Cells Tactile cells, also known as Merkel cells, act as receptors for touch and other sensations. Within the epidermis, these cells are spaced apart widely, and are found in the bottom layer of the epidermis; the stratum basale. These cells are usually connected with a dermal nerve fiber. The cells, when coupled with a nerve fiber, are referred to as a tactile (Merkel) disc.
Dendritic (Langerhans) Cells The dendritic cells, also known as Langerhans cells, begin in the bone marrow but make their way into the epidermis, are macrophages responsible for protecting the body from outside pathogens and infection. These cells trigger an immune response in the body when invaders or foreign pathogens are detected. These cells are commonly found within oral cavity, esophagus, and vagina.
Keratinocytes The keratinocytes are produced by the mitosis of the stem cells within the stratum basale, due to the availability of an oxygen and nutrient rich blood supply in the neighboring dermis. As the keratinocytes move away from this blood supply in the dermis, the cells cease to divide and instead form keratin fibers. These keratinocytes begin making their way to the surface of the epidermis, pushing the older keratinoctes out of the way. Within 30-45 days, the keratinocyte will make it’s way to the surface of the epidermis and will flake off. Stress on the epidermis may speed this process up and contribute to the development of calluses or corns, thick accumulations of keratinocytes in the skin.
As keratinocytes continue to move upward through the strata of the epidermis, the cells become flatter and begin to produce lipid-filled membrane-coating vesicles. When the keratinocyte has reached the stratum granulosum, the nuclei and organelles of the keratinocyte degenerate and the cell dies. The cytoskeleton of the keratin filaments are then bound together by a protein known as filaggrin, released by the keratohyalin granules. The cells are then made waterproof by a lipid mixture released by the membrane coating vesicles.
Between the stratum granulosum and the stratum spinosum, an epidermal water barrier is formed. This barrier consists of lipids, tight junctions, and insoluble proteins.This barrier is crucial to the body’s ability to retain water. The cells located above this layer die quickly due to being cut off from the nutrients and oxygen beneath this waterproof layer. The dead keratinocytes then exfoliate (fall away) from the epidermis in tiny specks known as dander. Similarly, they may fall away in the form of dandruff, which is identical to dander with the exception that dandruff contains specs of dander held together in clusters by oils.
Layers of the Epidermis The epidermis is defined by four or five different layers, known as strata. These layers, in order from the most superficial (top) layer, to the deepest (bottom) layer, are; the stratum corneum, the stratum lucidum, the stratum granulosum, the stratum spinosum and the stratum basale.
Stratum Corneum The stratum corneum is the outermost layer of the epidermis and is composed of up to thirty layers of dead, keratinized cells. The stratum corneum plays a key role in the skin’s role as a barrier from external conditions.
Stratum Lucidum
The stratum lucidum is found only in areas of thick skin, such as the palms and the soles of the feet. The cells contained within the stratum lucidum contain no nuclei or other organelles. The keratinocytes within the stratum lucidum are tightly packed together with a clear protein known as eleidin.
Stratum Granulosum The stratum granulosum is composed of two to four layers of cells bound together. There are typically more layers of the stratum granulosum found within thick skin than within thin skin. The keratinocytes within the stratum granulosum contain coarse, dark-staining keratohyalin granules, which play a role in the formation of keratin in the more superficial (upper) layers of the epidermis.
Stratum Spinosum The stratum spinosum is composed of eight to ten layers of cells connected to one another by desmosomes. The stratum spinosum is the thickest strata in most skin, but may only be the second thickest strata in thick skin. The cells located in the deepest (bottom) layer of the stratum spinosum are capable of mitosis, however, as these cells are pushed more superficially (upward) the cells cease to divide and begin to produce keratin fibers. Dendritic cells are often present in the stratum spinosum but are usually not visible in histological samples.
Stratum Basale The stratum basale is the deepest (bottom) layer of the epidermis and consists of a single layer of cells lying on the basement membrane. Melanocytes and tacticle cells can be found in small numbers scattered among these cells. Stem cells are also located on the stratum basale and develop into keratinocytes which them move into the more superficial (upper) layers of the epidermis.
References
Saladin, Kenneth S.. Anatomy & physiology: the unity of form and function. 5th ed. Dubuque: McGraw-Hill, 2010. Print.
