The epidermis is our outermost layer of skin, and can vary in thickness from 0.05 mm (eyelids) to over 1.5 mm (soles of the feet). There are no blood vessels within the epidermis, and the openings to hair follicles, sweat and oil glands are in the epidermis (the rest of these specialized units are within the dermis).
The Layers of the Epidermis
The epidermis is separated from the dermis by a basement membrane, but this does keep the two firmly attached. Since we lose so many skin cells each day, our body must constantly create new cells as replacements. Cells start at the bottom layer and work their way up to the surface as they mature.
1. Basal Layer (stratum basale)- the bottom layer, these are the cells that are dividing to replace the cells that are lost every day.
2. Spiny Layer (stratum spinosum)-These cells begin to create fats known as Sphingolipids and proteins known as keratins (for strength). They are called spiny because while all of the cells in the epidermis are connected to each other very tightly with little bridges called desmosomes, the desmosomes are really easy to see in this layer, and under the microscope all of these cells look like they are covered in spines.
3. Granular Layer (stratum granulosum)-The sphingolipids begin to clump together, giving these cells a granular appearance under the microscope. By the time these cells reach the top (cornified layer) they will die and release all of this lipid into the surrounding area. The lipid then will work almost like mortar to keep the cornified cells together and keep water in the skin.
4. Clear Layer (stratum lucidum)- This layer is primarily dead cells, filled with the lipid that has not yet been released. This is the layer that is the biggest provider of the skin's barrier function and is waterproof.
5. Cornified Layer (stratum corneum)- The top layer, these fibrous cells are (still) dead and will shed roughly every 2 weeks. The cells are little more than a cell membrane surrounding Keratin and other structural proteins. there is a moisturizing molecule in the cell, more on that below. This layer is very tough, and provides most of the protection of the skin. Though pictured here as pretty thin, this layer is usually 15-100 cells deep and they are very tightly bound with the lipid in between them like brick and mortar on a building.
Think of the cells here as bricks, held together by the desmosomes as in the layers below, but also in part by the lipids (fats) that are between the cells. These are the lipids released by the granular layer and clear layer (#4 and 5). Because oil and water don't mix, in between the lipids there are droplets of water basically, all clumped together. They aren't able to absorb down because the lipids are in the way, they are basically stuck there.
Within the cells themselves there are molecules that absorb water from the environment (water, air, etc), and then hold them here to be used in the lipid layer. Note that these molecules are water soluble, so they can be basically sucked out of the skin by water in the environment as well (eg- when swimming), which means that with repeated exposure to water you can become more dehydrated.
The Special Cells of the Epidermis
1. Melanocytes- these cells are nestled between the cells of the Stratum Basale, where they produce melanin. Melanin helps to absorb sunlight, protecting the skin from the harm of UV radiation. The melanin is packaged up into little packets (melanosomes), and then given over to the surrounding cells. The number of melanosomes in each race and skin tone is the same, what differs really is just the size of these little melanosomes. The melanin in them is the same. Really. Melanin is increased by several hormones, and with age the melanocytes themselves die off.
2. Langerhans- These cells come from the bone marrow and are found throughout the epidermis, they have function within the immune system, particularly for contact hypersensitivity rashes.
3. Merkels- These cells are found only in certain areas (nailbed, ends of fingers, etc) and they are involved in light touch perception.