Histologic image of epidermis, delimited by white bar.
Histologic image detailing epidermal layers.
Stratum corneum appears more compact in this image than above because of different sample preparation. Details Latin Epidermis Identifiers Code TH H3.12.00.1.01001 Anatomical terminology
epidermis is composed of the outermost layers of cells in the skin, "epi" in [1 ] Greek meaning "over" or "upon", which together with the dermis forms the cutis.
The epidermis is a
stratified squamous epithelium, composed of [2 ] proliferating basal and differentiated suprabasal keratinocytes which acts as the body's major barrier against an inhospitable environment, by preventing pathogens from entering, making the skin a natural barrier to infection. It also regulates the amount of [3 ] water released from the body into the atmosphere through transepidermal water loss (TEWL). [4 ]
humans, it is thinnest on the eyelids at 0.05 mm (0.0020 in) and thickest on the palms and soles at 1.5 mm (0.059 in). It is [5 ] ectodermal in origin. Structure [edit ] Cellular components [edit ]
The epidermis is
avascular, nourished almost exclusively by diffused oxygen from the surrounding air. It is 95% [6 ] keratinocytes but also containing [2 ] melanocytes, Langerhans cells, Merkel cells, and inflammatory cells. [1 ] Rete ridges ("rete tips" ) are epidermal thickenings that extend downward between [7 ] dermal papillae. [8 ] Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Layers [edit ]
Schematic image showing a section of epidermis with epidermal layers labeled.
The epidermis is composed of 4 or 5 layers depending on the region of
skin being considered. Those layers in descending order are: [9 ] [3 ] Composed of 10 to 30 layers of polyhedral, anucleated corneocytes (final step of keratinocyte differentiation), with the palms and soles having the most layers. Corneocytes are surrounded by a protein envelope (cornified envelope proteins), filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids. Most of the barrier functions of the epidermis localize to this layer. [10 ] [11 ] The skin found in the palms and soles is known as "thick skin" because it has 5 epidermal layers instead of 4. Keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids. [10 ] Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes. [4 ] Langerhans cells, immunologically active cells, are located in the middle of this layer. [10 ] Composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation. [10 ]
Malpighian layer ( stratum malpighi) is usually defined as both the stratum basale and stratum spinosum. [2 ]
The epidermis is separated from the
dermis, its underlying tissue, by a basement membrane. Cellular kinetics [edit ] Cell division [edit ]
The stratified squamous
epithelium is maintained by cell division within the stratum basale. Differentiating cell delaminate from the basement membrane and are displaced outwards through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface ( desquamation). Differentiated keratinocytes secrete keratin proteins which contribute to the formation of an extracellular matrix and is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss, taking about two weeks for a [2 ] cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum. The entire epidermis is replaced by new [3 ] cell growth over a period of about 48 days. [12 ] Calcium concentration [edit ] Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers. [4 ]
extracellular calcium concentrations induces an increase in intracellular free calcium concentrations. Part of that [13 ] intracellular increase comes from calcium released from intracellular stores and another part comes from transmembrane [14 ] calcium influx, through both calcium-sensitive [15 ] chloride channels and voltage-independent cation channels permeable to [16 ] calcium. Moreover, it has been suggested that an [17 ] extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration. [18 ] Development [edit ]
organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system. In most vertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or has formed. stratum germinativum [19 ]
This inner layer is a
germinal epithelium that give rise to all epidermal cells. It divides to form the outer spinous layer ( ). The stratum spinosum cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer ( ) of the epidermis. Stratum granulosum [19 ]
cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer ( ), the outermost epidermal layer, where the stratum corneum cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 0.001 - 0.003 ounces of skin flakes every hour, or 0.024-0.072 ounces per day. [20 ]
development is a product of several growth factors, two of which are: [19 ] Function [edit ] Barrier [edit ]
The epidermis serves as a barrier to protect the
body against microbial pathogens, oxidant stress ( UV light) and chemical compounds and provides mechanical resistance. Most of that function is played by the stratum corneum. [11 ] Characteristics of the barrier [edit ] Factors that will alter the barrier [edit ] Skin hydration [edit ]
The ability of the
skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin. [23 ] Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss. [24 ] [25 ] Skin color [edit ]
The amount and distribution of
melanin pigment in the epidermis is the main reason for variation in skin color in Homo sapiens. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes varies between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and oriental skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected. [26 ] Clinical significance [edit ] Laboratory culture of keratinocytes to form a 3D structure ( artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for drug development and testing. Additional images [edit ]
Epidermis and dermis of human skin
Cross-section of all skin layers
Illustration of epidermal layers
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