The stratum spinosum is the second layer of your epidermis, sitting just above the bottom layer where new skin cells are born. It consists of 8 to 10 layers of living cells that give your skin much of its mechanical strength and resilience. The name comes from the “spiny” or “prickly” appearance of its cells under a microscope, though those spines are actually connection points between neighboring cells rather than true projections.
Where It Sits in the Epidermis
Your epidermis, the outermost portion of skin, is built from several distinct layers stacked on top of each other. The stratum spinosum sits between the stratum basale (the deepest layer, where new skin cells are produced) and the stratum granulosum (the layer above, where cells begin breaking down as they prepare to become the tough, dead outer surface of your skin). Cells born in the bottom layer are constantly migrating upward, or being pushed up by newer cells beneath them. By the time they reach the stratum spinosum, they’ve already begun changing shape and building the internal scaffolding that will eventually make them part of your skin’s protective barrier.
Why the Cells Look “Spiny”
When skin tissue is prepared for viewing under a microscope, the cells of this layer shrink slightly during processing, but their connection points to neighboring cells hold firm. The result is that each cell appears to have tiny spines radiating outward, giving it a star-like shape. These aren’t actual spines. They’re structural anchors called desmosomes, and they’re the reason this layer is sometimes called the “prickle cell layer.”
Desmosomes are essentially rivets between cells. Each one connects the internal scaffolding of one cell to the scaffolding of its neighbor, creating a continuous web of reinforcement that runs through the entire layer. This network is what allows your skin to absorb friction, stretching, and pressure without tearing apart. Think of it like chain mail: each individual link isn’t particularly strong, but the interlocking pattern distributes force across the whole structure.
What the Cells Are Doing
The cells in the stratum spinosum are very much alive and active. Their primary job is producing keratin, the fibrous protein that gives skin, hair, and nails their toughness. Specifically, they begin producing two forms of keratin that pair together to form long, rope-like filaments inside each cell. These filaments run from one desmosome connection to another, effectively turning each cell into a reinforced structural unit. The filaments are essential for epidermal stability, and they also help maintain the integrity of each cell’s nucleus as it continues maturing.
Cells in this layer also begin manufacturing tiny packages called lamellar bodies, small membrane-bound capsules filled with fats and other molecules. These packages are assembled here but won’t release their contents until the cells move higher in the epidermis. Once released, their fatty contents form the waterproof seal between dead skin cells at the surface, which is a critical part of your skin’s ability to keep moisture in and irritants out.
Immune Cells in the Layer
The stratum spinosum isn’t made entirely of skin cells. Scattered among the keratinocytes are specialized immune cells called Langerhans cells, which make up about 3 to 5 percent of all the nucleated cells in the epidermis. These cells act as sentinels. They sit in the stratum spinosum sampling their surroundings for signs of bacteria, viruses, or other foreign material. When they detect a threat, they capture it, process it, and migrate to nearby lymph nodes to alert the rest of the immune system. This makes them part of the body’s first line of defense against pathogens that breach the skin’s surface.
What Happens When This Layer Fails
Because the stratum spinosum depends so heavily on desmosomes to hold everything together, diseases that attack those connections can be devastating. Pemphigus vulgaris is one example. In this autoimmune condition, the body produces antibodies against the proteins that form desmosome connections. When those connections break down, cells in the stratum spinosum separate from each other, a process called acantholysis. Fluid fills the gaps, creating painful blisters on the skin and mucous membranes. A related condition, pemphigus foliaceus, targets a different desmosome protein and causes similar separation, though typically in more superficial layers. Even in people who carry these antibodies without visible symptoms, electron microscopy can reveal widening of the spaces between cells and early loss of desmosome structure.
Staphylococcal scalded skin syndrome works through a different mechanism but produces a similar result. Toxins released by certain bacteria directly cleave the adhesion molecules in desmosomes, causing sheets of skin to peel away.
Connection to Skin Cancer
The cells of the stratum spinosum are squamous cells, named for their somewhat flattened shape. Squamous cell carcinoma, the second most common form of skin cancer, originates from these cells. As new cells are produced in the basal layer and pushed upward, they become squamous cells that populate the middle and outer layers of the epidermis. When DNA damage from ultraviolet radiation or other factors causes these cells to grow uncontrollably, the result is squamous cell carcinoma. The cancer typically appears on sun-exposed areas of skin as a firm, red nodule or a flat sore with a scaly crust.