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The epidermis is a layer of skin cells. Its composition varies in different parts of the body. It is thickest on the palms and soles. The predominant cell type is keratinocytes, which produce the protective protein keratin.
Other cells in the epidermis include pigment-producing melanocytes and antigen processing Langerhans cells. Free nerve endings are also present in the epidermis, which respond to touch, pain, and light changes.
Stratum basale
Stratum basale is the deepest layer of the epidermis and is composed of cuboidal-shaped cells. It contains the stem cells called keratinocytes (ko-rah-tin-o-sites) that divide to form all the other layers of your skin. The keratinocytes produce the protein keratin, which forms hair, nails and your skin’s outer protective layer. It also contains melanocytes, which are responsible for producing the pigment that gives your skin its color.
The keratinocytes in the stratum basale eventually migrate into the stratum spinosum, which is the next layer of the epidermis. As these cells move up, they change their shape, nuclear composition and chemical makeup to better adapt to their new environment. This is why the stratum spinosum has a “spiny” appearance in a stained sample of epidermis, while un-stained epidermis samples don’t have this characteristic.
The stratum granulosum is the intermediate layer of your epidermis and contains 3-5 cell layers of keratinocytes that contain a protein known as eleidin. The eleidin protein causes the keratinocytes to become more clumped together and create bundles. These bundles are then covered by a layer of glycolipids known as lamellar granules. These granules act as a glue, keeping the bundles together. Stratum granulosum also contains melanocytes and Merkel cells, which are oval-shaped modified epidermal cells that serve a sensory function as touch receptors. These cells are especially numerous in the thicker skin found on the palms, soles and digits.
Stratum spinosum
The keratinized cells of the stratum spinosum give skin its spiny appearance. They also secrete a water-repelling glycolipid called keratohyalin, which prevents moisture loss and makes the epidermis waterproof. This layer is a tough protective barrier that can resist some physical damage, but it is not impervious to infection or injury.
The cells of the stratum spinosum are polyhedral in shape and have large pale staining nuclei. They have a tight connection with each other via a protein complex called a desmosome, which looks like a spiky membrane projection on histology. Because of this, they are often referred to as prickle cells. During tissue fixation, the keratinocytes of this layer shrink slightly but remain tightly bound to each other. This gives them a spiny appearance that is apparent when stained with special dyes.
Eventually, the keratinocytes in this layer will become crowded and start to die as they become too far from the blood vessels of the dermis for nutrients. As they die, they become flat and release a thick coating of keratin. This keratinized coating helps to protect the skin from physical damage, as well as water loss and infection.
The next layer up is the stratum spinosum, which has eight to 10 layers of keratinocytes. These cells are distinct from those in the stratum basale because they have a lot of desmosomes that appear as spiky membrane projections on histology. They also contain thick tufts of intermediate filaments called keratin.
Stratum granulosum
The cells in this layer are called keratinocytes and are constantly dividing. As they do, they move up through the other layers of the epidermis. They also clump together into tough sheets of material that protect the lower layers of the skin. This process is referred to as keratinization.
As these keratinocytes become more mature, they begin to flatten out and accumulate dense granules of protein that give the layer its grainy appearance. These granules are called keratohyalin and help the keratin filaments to adhere to one another. These granules also form a lipid layer on the cell membrane that helps prevent water loss.
Stratum granulosum is the deepest layer of the epidermis. The granules of this layer are what gives the epidermis its prickly texture. The granules are also what makes the epidermis resistant to abrasion.
Stratum granulosum is followed by the stratum spinosum and then the stratum corneum. The last of these layers is the stratum lucidum, which appears clear (hence its name) because it is composed of dead, keratin-filled cells. The epidermis acts like armor to protect the body from UV rays, pathogens (bacteria, viruses, and fungi), and chemicals. It also helps regulate the immune system by releasing cytokines. In addition, the epidermis produces melanin, a pigment that gives skin its color. In addition to this, it provides support and flexibility for the dermis and hypodermis.
Stratum lucidum
The skin is the largest organ in your body, covering a large area and protecting the interior tissues. It also has an important role in regulating your temperature and producing body heat. The epidermis is the outermost layer of your skin and contains four to five layers (depending on body location). These layers are constantly renewing themselves in a process called exfoliation and desquamation.
The first layer of the epidermis, known as stratum basale or stratum germinativum, consists of small cells that have many connections called desmosomes and thick tufts of intermediate filaments. These filaments give the cells a prickle-like appearance and have a sticky texture, which makes them cling to each other. These connections are what make the keratin that gives your hair and skin its strength, toughness, and color.
In the next layer, known as stratum spinosum or prickle cell layer, the cells have more desmosomes and thick tufts. They are shaped like prickles or spikes, and these cells contain a clear substance called eleiden. This substance allows the cells to bind to each other more tightly and helps them resist water.
The third layer, known as stratum granulosum or keratinocytes, are flat, dead cells that are densely packed with keratin proteins and lipids. The lipids help to create the waterproof barrier that protects the inner skin from water. This layer is thickest in the palms of your hands and soles of your feet, which are referred to as “thick” skin.