Integumentary System
Stratum Spinosum
(Spiny layer) Each time a keratinocyte stem cell in the stratum basale divides, a daughter cell is pushed toward the external surface from the stratum basale, while the other cell remains as a stem cell in the stratum basale. Once this new cell enters the stratum spinosum, it begins to differentiate into a non-dividing, highly specialized keratinocyte.
Eleidin
Intermediate product in the process of keratin maturation.
Stratum Lucidum
Clear layer, is a thin, translucent region of about two to three cell layers that is superficial to the stratum granulosum. This stratum is found only in the thick skin within the palms of the hands and the soles of the feet. Cells occupying this layer appear pale and featureless and have indistinct boundaries. The keratinocytes within this layer are flattened and filled with the translucent protein called eleidin.
Stratum Basale
Deepest epidermal layer. (stratum germinativum, or basal layer) single layer of cuboidal to low columnar cells is tightly attached to an underlying basement membrane that separates the epidermis from the connective tissue of the adjacent dermis. Three types of cells occupy the stratum basale
Epidermis
Epithelium of the integument. Keratinized stratified squamous epithelium.
Stratum Granulosum
Granular layer, consists of three to five layers of keratinocytes superficial to the stratum spinosum. Within this stratum begins a process called keratinization, where the keratinocytes fill up with the protein keratin, and in so doing, cause both the cell's nucleus and organelles to disintegrate and the cell starts to die. Keratinization is not complete until the cells reach the more superficial epidermal layers.
Epidermal Dendritic (Langerhans) Cells
Immune cells that help fight infection in the epidermis. These immune cells are often present in the stratum spinosum and stratum granulosum, but they are not identifiable in standard histologic preparations. Their phagocytic activity initiates an immune response to protect the body against pathogens that have penetrated the superficial epidermal layers as well as epidermal cancer cells.
Melanocytes
Long, branching processes and are scattered among the keratinocytes of the stratum basale. They produce and store the pigment melanin (mel′ă-nin) in response to ultraviolet light exposure. Their cytoplasmic processes transfer pigment granules, called melanosomes, into the keratinocytes within the basal layer and sometimes in more superficial layers.
Tactile Cells
Merkel cells, are few in number and found scattered among the cells within the stratum basale. Tactile cells are sensitive to touch and, when compressed, they release chemicals that stimulate sensory nerve endings, providing information about objects touching the skin.
Keratinocytes
Most abundant cell type in the epidermis and are found throughout all epidermal strata. The stratum basale is dominated by large keratinocyte stem cells, which divide to generate new cells that replace dead keratinocytes shed from the surface.
