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Epidermis Skin Layer structure & Functions Anatomy

 Our skin, the largest organ in our body, serves this crucial purpose with a layer known as the epidermis as our first line of defense. In this lesson, we will take a closer look at this skin layer and gain an understanding of why it is so important to our body.

What is the Epidermis. Function of the Epidermis?

The epidermis is the outermost layer of our skin. Tough and resilient, protection is its number one job. Think of a parka you may wear in the winter. The inside is lined with soft fleece, providing a layer of warmth. But the outside is made of a strong waterproof material that lets nothing through. It is a similar situation with our own epidermis.

The protective qualities of our outer layer are vast. Our epidermis is waterproof, which is why we don't swell with liquid each time we bathe. The cellular structure of the epidermis also forms a highly effective barrier against germs. When skin is healthy and intact, it is difficult for bacteria and viruses to make an entrance. In addition, cells of the epidermis have the miraculous ability to regenerate, or grow back, unlike many other cells in the body. When we suffer from a wound, healthy skin heals and replaces damaged cells with ease.

Layers of the Epidermis

The epidermis is composed of four main strata, or layers. The outermost layer is called the stratum corneum, which is Latin for ''horny layer.'' While we aren't literally covered in horny scales, this layer is tough nonetheless. It also varies in thickness depending on the body part it covers. If you often go barefoot, the stratum corneum on the sole of your foot is probably quite thick. Compare that to the skin on your eyelids and you can see the drastic difference

If we were to take a closer look, we find that the stratum corneum is composed almost exclusively of dead cells. It may come as no surprise that we lose skin cells on a regular basis. In fact, the dead cells of the stratum corneum slough off so often that we end up with a completely new outer layer about every 35 days.

And how, you may be wondering, do we have any skin left if we are always losing cells? This is where the stratum basale comes in. Just as its name suggests, it is the base or deepest layer of the epidermis. A cell-producing factory, the basale layer contains stem cells which are constantly dividing to make new ones. These fresh new cells make their way up to the stratum corneum to replace those that have sloughed off. This cycle runs on a continual basis, keeping our epidermis healthy and strong.

Two additional layers are sandwiched between the stratum corneum and stratum basale. The stratum spinosum borders the stratum basale. Here we find spiny keratinocytes that help bond other cells together. Finally, the stratum granulosum lies beneath the stratum corneum. Cells in this layer produce a waxy material that aids in waterproofing the skin.

While most of our body's epidermis is made up of four layers, on the palms of our hands and the soles of our feet, there is one extra layer of skin. The stratum lucidum is a translucent layer that provides extra thickness to these areas of the skin.



Layers of the Epidermis

Epidermis Layers Details

There are 5 primary layers compose Epidermis

1- The stratum corneum

2- Stratum lucidum

3- Stratum granulosum

4- Stratum spinosum

5- Stratum basale

1. The stratum corneum:

The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis, consisting of dead cells (corneocytes). This layer is composed of 15–20 layers of flattened cells with no nuclei and cell organelles. Their cytoplasm shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids.

The stratum corneum functions to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress. Desquamation, the process of cell shedding from the surface of the stratum corneum, balances proliferating keratinocytes that form in the stratum basale. These cells migrate through the epidermis towards the surface in a journey that takes approximately fourteen days.

During cornification, the process whereby living keratinocytes are transformed into non-living corneocytes, the cell membrane is replaced by a layer of ceramides which become covalently linked to an envelope of structural proteins (the cornified envelope).

 This complex surrounds cells in the stratum corneum and contributes to the skin's barrier function. Corneodesmosomes (modified desmosomes) facilitate cellular adhesion by linking adjacent cells within this epidermal layer. These complexes are degraded by proteases, eventually permitting cells to be shed at the surface. Desquamation and formation of the cornified envelope are both required for the maintenance of skin homeostasis. A failure to correctly regulate these processes leads to the development of skin disorders.

Cells of the stratum corneum contain a dense network of keratin, a protein that helps keep the skin hydrated by preventing water evaporation. These cells can also absorb water, further aiding in hydration. In addition, this layer is responsible for the "spring back" or stretchy properties of skin. A weak glutenous protein bond pulls the skin back to its natural shape.

The thickness of the stratum corneum varies throughout the body. In the palms of the hands and the soles of the feet (sometimes knees, elbows, knuckles,) this layer is stabilized and built by the stratum lucidum (clear phase) which allows the cells to concentrate keratin and toughen them before they rise into a typically thicker, more cohesive SC. The mechanical stress of heavy structural strain causes this SL phase in these regions which require additional protection in order to grasp objects, resist abrasion or impact, and avoid injury. In general, the stratum corneum contains 15 to 20 layers of dead cells. The stratum corneum has a thickness between 10 and 40 μm.

In reptiles, the stratum corneum is permanent, and is replaced only during times of rapid growth, in a process called ecdysis or moulting. This is conferred by the presence of beta-keratin, which provides a much more rigid skin layer.

In the human forearm, about 1300 cells per cm2 per hour are shed.[citation needed] Stratum corneum protects the internal structures of the body from external injury and bacterial invasion.



2. Stratum lucidum

The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened. These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water.


3. Stratum Granulosum

The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells . These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails

4. Stratum Spinosum

As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale. Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer.

5- Stratum Basale

The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made.

The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.

In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.

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