The epidermis is the outermost protective layer of skin. It functions as a semi-permeable barrier against environmental pathogens and chemicals while regulating body homeostasis.
It provides protection from physical damage, as well as chemical and UV exposure. The tough, dead keratin-filled cells of the squamous cell layer resist sharp objects and rough surfaces.
Basal Layer
The innermost layer of the epidermis is called the stratum basale. It contains stem cells — called keratinocytes — that produce all the other layers of the epidermis. The keratinocytes begin life in the stratum basale as cube-shaped cells with little or no keratin. As they mature, the keratinocytes become flat, squamous cells that form the main structure of your skin, hair, and nails. They also contain pigments like melanin and serve as a barrier to harmful ultraviolet radiation. Cells in this layer are also responsible for sending touch, heat and pressure sensations to the brain.
There are no blood vessels in the epidermis, so these cells get their oxygen and nutrients directly from the air and by absorbing water and other substances from the dermis below. New cells are produced in the basal layer on a continuous basis, and as they move upwards they push older cells into the stratum spinosum. The squamous cell layer is also home to melanocytes, Langerhans cells, and Merkel cells. The cells of the squamous cell layer are glued together with a protein called keratin, which creates the tough protective barrier that prevents evaporation of water from your skin.
Stratum Spinosum
Just above the stratum basale is a layer called the stratum spinosum. It consists of flat, hard, tightly packed, dead keratinocytes that form a protective keratin barrier to keep water from easily entering or leaving the skin. The cells in this layer are less round and more flat, giving them a spiny appearance when stained by certain dyes. This layer is also referred to as the prickle cell layer.
The cells in this layer have desmosomes, which are protein complexes that firmly anchor keratinocytes to other cells of the epidermis. These structures look like thick tufts of membrane when seen on histology and are the cause of the spiny appearance in this layer.
In this layer are a few types of cells, including Merkel cells and melanocytes. Merkel cells are receptors that send signals to the brain that get translated as your sense of touch, while melanocytes make the pigment that gives you skin and hair color. Cells in this layer also produce the proteins that hold the cells in the outermost layers together.
Stratum Granulosum
The next layer, the stratum granulosum (stra-tum g-ro-un-osm), is made up of basal cells that have matured into keratinocytes (kray-tin-oh-sites). Keratinocytes produce the protein keratin that forms hair, nails, and your skin’s outer barrier. They also secrete a substance called keratohyalin, which helps to maintain the water-sealing properties of the skin.
The cells of the granulosum are held together by proteins called desmosomes and reinforced by tonofilaments (ton-oh-fi-luhs), which appear as spiky membrane projections on histological sections. The granule cells of the granulosum have accumulated a darkly staining substance called keratin. These specialized structures are what give the granulosum its characteristic appearance on histological sections.
The granule cell layer is also home to a type of immune cell known as a Langerhans cell. These cells stick to antigens that invade damaged skin and signal the immune system to react. It is this reaction that makes it difficult for pathogens to enter the body through intact epidermal layers. This is why we can often see a pathogen only after the epidermis has been breached, such as by a cut or burn.
Stratum Corneum
The outermost layer of the epidermis, the stratum corneum, is sometimes called “the brick wall of the skin.” It has layers of keratin-filled cells (corneocytes) that are arranged in layers, like bricks, mortared together with lipids, creating an effective water barrier. It also contains granules that have the ability to trap substances that come into contact with your skin. The stratum corneum also contains Langerhans cells, which prevent things from getting into the body through your skin.
This outermost cell layer is constantly shedding its old cells and generating new ones, although the process slows down as you age. It is tougher than most other skin and can withstand some damage from sharp objects.
The granule-cell layer of the stratum corneum accumulates a substance called keratohyalin, which is visible under a microscope as dark staining granules. It is a useful diagnostic tool for the presence of squamous cell cancer, an important disease of the upper layers of the epidermis.
Stratum Lucidum
The epidermis has four layers of skin cells, and a fifth layer specific to the palms and soles of your feet called the stratum lucidum. These layers provide protection against abrasions, heat and pathogens.
The keratinocytes in the stratum basale are cuboidal to columnar mitotically active stem cells that produce keratin. These cells bond to the basement membrane through hemidesmosomes and produce other cell types that give your body its characteristic appearance. The first of these other cell types is the stratum spinosum (str-ah-sin-o-măm), sometimes referred to as the prickle cell layer because of its cellular structure that resembles a bunch of prickles. This layer has eight to ten cell layers.
As keratinocytes move up through the stratum spinosum, they lose their nuclei and become flattened. They then generate large amounts of the proteins keratin and keratohyalin. These proteins are deposited as thickened, lamellar granules within the cells. This gives the keratinocytes in this layer their grainy appearance. The granules also contain the pigment melanin, which is responsible for skin color. The stratum granulosum is the middle layer of the epidermis and is found in the skin of the fingers, hands, feet and genitals.