Epidermis

The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water released from the body into the atmosphere through transepidermal water loss. 
The epidermis is composed of multiple layers of flattened cells that overlie a base layer (stratum basale) composed of columnar cells arranged perpendicularly. The layers of cells develop from stem cells in the basal layer. The human epidermis is a familiar example of epithelium, particularly a stratified squamous epithelium
The word epidermis is derived through Latin , itself and . Something related to or part of the epidermis is termed epidermal.
Structure.
Cellular components.
The epidermis primarily consists of keratinocytes (proliferating basal and differentiated suprabasal), which comprise 90% of its cells, but also contains melanocytes, Langerhans cells, Merkel cells, and inflammatory cells. Epidermal thickenings called Rete ridges (or rete pegs) extend downward between dermal papillae.
Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule.
The epidermis itself has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air. Cellular mechanisms for regulating water and sodium levels (ENaCs) are found in all layers of the epidermis.
Cell junctions.
Epidermal cells are tightly interconnected to serve as a tight barrier against the exterior environment. The junctions between the epidermal cells are of the adherens junction type, formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells, although the actin filaments are actually located inside the cell and run parallel to the cell membrane. Because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells.
Layers.
The epidermis is composed of 4 or 5 layers, depending on the region of skin being considered. Those layers in descending order are: 
The Malpighian layer ("stratum malpighi") is both the stratum basale and stratum spinosum.
The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.
Cellular kinetics.
Cell division.
As a stratified squamous epithelium, the epidermis is maintained by cell division within the stratum basale. Differentiating cells delaminate from the basement membrane and are displaced outward 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 that 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 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 cell growth over a period of about 48 days.
Calcium concentration.
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.
Elevation of extracellular calcium concentrations induces an increase in intracellular free calcium concentrations. Part of that intracellular increase comes from calcium released from intracellular stores and another part comes from transmembrane calcium influx, through both calcium-sensitive chloride channels and voltage-independent cation channels permeable to calcium. Moreover, it has been suggested that an extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration.
Development.
Epidermal 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 "stratum germinativum" has formed.
This inner layer is a germinal epithelium that gives rise to all epidermal cells. It divides to form the outer spinous layer ("stratum spinosum"). The cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer ("Stratum granulosum") of the epidermis.
The 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 ("stratum corneum"), the outermost epidermal layer, where the 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.
Epidermal development is a product of several growth factors, two of which are:
Function.
Barrier.
The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light), and chemical compounds, and provides mechanical resistance to minor injury. Most of this barrier role is played by the stratum corneum.
Skin hydration.
The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin. Skin hydration is quantified using corneometry. Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss.
Skin color.
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 vary 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 Asian 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.
Clinical significance.
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.
Hyperplasia.
Epidermal hyperplasia (thickening resulting from cell proliferation) has various forms:
In contract, hyperkeratosis is a thickening of the stratum corneum, and is not necessarily due to hyperplasia.