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The stratum spinosum is the fourth layer of human epidermis, which is the outermost portion of the skin. Human skin tends to be more complicated than it first appears, and is made up of various layers and levels. The epidermis, or outer-most portion, contains five independent layers of its own, and the stratum spinosum — which is also called the spinous or “prickle” layer because of the way its cells look — is just one. This spinous level has an important job to do, though. Its main function is to protect against foreign materials and to produce and retain moisture, which it does through a series of cellular-level interactions and interchanges. People who have a problem with this part of their skin, whether caused by an injury like a burn or a genetic disease or illness, often need to use special moisturizers and skin creams to recreate the role this important layer plays.
There are usually three separate parts of human skin when it’s viewed at the microscopic level. First is the epidermis, which is what is immediately visible; beneath that is the dermis and then the hypodermis. Each of these is usually made up of independent levels and strata, too; the epidermis has five.
The five layers of the epidermis from inside out are the stratum basale, stratum spinosum, stratum granulosum, stratum licidum, and stratum corneum. The epidermis as a whole varies in thickness from about 0.002 to 0.059 inches (about 0.05 to 1.5 mm); it tends to be thinnest along the eyelids and thickest on the hands and feet. The spinosum typically contains five to ten layers of cells, many of which have spiny “arms” that help them reach out and bind to each other. It if for this reason that this layer is sometimes also called “prickly.”
Prickle cells are typically produced in the stratum basale and pushed upward. They manufacture bipolar lipids that are organized into layers that provide a structure that prevents evaporation of water and allows the skin to retain moisture. Prickle cells also provide the superstructure of this layer of skin. The prickles, or desmosomes, radiating from each cell are the points of attachment that join cell to cell; mitosis, or cell division, occurs infrequently in this layer.
The main job of the prickle cell layer is to help the skin retain moisture and natural emollients that can keep the epidermis lubricated and resistant to cracking. The actual process through which this happens is somewhat complex. Column-shaped keratinocytes move into the stratum spinosum from the stratum basale where they are produced, and the keratinocytes then become polygon-shaped and begin to synthesize keratin. Keratin is a strong fibrous protein that forms a mesh that holds water and aids in retaining moisture within the skin. As fresh keratinocytes move in, they push older keratinocytes upward into the next epidermal layer, where the cells begin to dry out and die.
Moist skin is usually in the best position to keep out foreign bodies and substances because moisture helps it retain its elasticity. The more flexible the skin, the better it functions in most cases. Foreign particulates still sometimes manage to enter, however, even when conditions are ideal. The spinosal layer has a role here, too, and is often a key player in keeping dangerous or even just unknown particles from infiltrating deep into the body.
It is here that langerhans cells become important. Langerhans cells, which are macrophages — immune system cells that eat foreign matter — work with T helper cells to protect the skin against foreign substances. These cells contain Birbeck granules, which are the hearts of their immune detection system. It is the job of these cells to detect skin penetration by foreign matter and then to trap and transport the invaders to the lymph nodes to be destroyed. Langerhans cells are made in the bone marrow, travel to the epidermis, and intermingle with keratinocytes.
People whose prickle cells don’t function properly typically have a range of skin problems. It’s usually very rare for there to be problems with the spinous layer and not anywhere else, though, since even though the layers are distinct they’re very closely related. Most of the time issues are caused by traumatic injury, particularly burns that warp the protective layers and damage the ways in which they interact. Some genetic and degenerative conditions can also lead to problems with both moisture retention and immune protection. Protective creams and lotions can often help boost the body’s natural defenses in this area, and medical providers may also be able to provide different medical treatments and drug regimens, depending on patient needs.