The stratum spinosum is the second layer of your epidermis, sitting just above the bottommost layer where new skin cells are born. It makes up most of the epidermis’s thickness, typically 5 to 10 cell layers deep, and plays a central role in giving your skin its strength, immune defense, and ability to produce vitamin D.
Where It Sits in the Epidermis
Your skin’s outermost portion, the epidermis, is built from five distinct layers stacked on top of each other. From deepest to most superficial, they are: the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is where new skin cells (keratinocytes) are created through cell division. As fresh cells push older ones upward, those older cells enter the stratum spinosum, where they begin maturing. From there, they continue rising through the upper layers until they eventually become the flat, dead cells on the very surface of your skin that slough off daily.
Why It’s Called the “Prickle Cell Layer”
Under a microscope, the cells of the stratum spinosum look like they have tiny spines or thorns poking out from their surfaces. That’s why it’s also called the prickle cell layer. The “spines” aren’t actually sharp projections. They’re connection points called desmosomes, protein-based rivets that lock neighboring cells tightly together. When tissue is prepared for microscopy, the cells shrink slightly but remain attached at these anchor points, creating a spiny appearance between each cell.
These desmosome connections serve an important structural purpose. With very little space between cells, the stratum spinosum forms a dense, cohesive sheet that resists tearing and mechanical stress. The cells themselves are polyhedral (many-sided) with round to oval nuclei, packed closely together in a way that gives this layer its bulk and resilience.
What Happens Inside These Cells
The stratum spinosum is where keratinocytes begin their transformation from living, active cells into the tough, protective material that eventually forms the outermost barrier of your skin. While still in the bottom layer, these cells start producing bundles of a structural protein called keratin, the same material that makes up your hair and nails. As the cells move into the stratum spinosum, they continue assembling these protein fibers into larger bundles that reinforce each cell from the inside.
The cells also begin producing involucrin, a protein that marks the start of terminal differentiation. This is the point of no return: the cell is now committed to becoming a flat, hardened surface cell rather than continuing to divide. Involucrin only appears in layers above the stratum basale, making it a useful marker for tracking how far along a cell is in its maturation journey.
Vitamin D synthesis also takes place here. The precursor molecule 7-dehydrocholesterol is found in the cytoplasm of stratum spinosum cells, where ultraviolet B light from the sun triggers its conversion into vitamin D. The newly formed vitamin D initially binds loosely to proteins inside the cell, then moves into the spaces between cells where the process is completed and the vitamin becomes firmly protein-bound. This makes the stratum spinosum one of the key sites where your body manufactures its own vitamin D supply.
Immune Cells Scattered Among the Skin Cells
Not every cell in the stratum spinosum is a keratinocyte. About 3 to 5 percent are Langerhans cells, a type of immune cell originally derived from bone marrow. These cells have long, branching arms that extend between the surrounding keratinocytes, almost like a surveillance network woven through the layer.
Langerhans cells act as the skin’s early warning system. When a foreign substance, whether a bacterium, virus, or chemical allergen, penetrates the outer skin barrier, Langerhans cells capture and process it. They then migrate to nearby lymph nodes, where they present the foreign material to T cells and trigger an immune response. This mechanism is directly responsible for allergic contact dermatitis, the itchy rash you get from poison ivy or nickel jewelry. It’s also why skin grafts from other people can be rejected: Langerhans cells identify the transplanted tissue as foreign and sound the alarm.
When the Stratum Spinosum Goes Wrong
Several skin conditions involve abnormal changes in this layer. The most straightforward is acanthosis, a thickening of the epidermal layers below the surface. In psoriasis, for example, keratinocytes in the stratum spinosum and stratum basale proliferate far faster than normal. The result is the raised, scaly plaques characteristic of the disease, driven by both unchecked cell multiplication and abnormal differentiation of those cells as they mature.
Squamous cell carcinoma, the second most common type of skin cancer, originates in the squamous cells that populate this layer. As keratinocytes move upward from the base layer, they become squamous (flat) cells. DNA changes in these cells can cause them to multiply uncontrollably and resist the normal process of programmed cell death that would ordinarily clear them away. The result is a tumor that can grow on sun-exposed areas of skin, particularly the face, ears, and hands.
Because the stratum spinosum is where cells transition from dividing to differentiating, disruptions here can ripple outward. Cells that fail to mature properly produce a weaker skin barrier. Cells that mature too quickly create thickened, flaky patches. The layer’s position in the middle of the epidermis makes it a bottleneck: everything passing through it, from new skin cells to immune signals to vitamin D, depends on its normal function.