Epidermis

Stratum Granulosum

At about the middle of the epidermis, consists of three to five layers of flattened keratinocytes that are undergoing apoptosis (an orderly, genetically programmed cell death in which the nucleus fragments before the cells die). A distinctive feature of cells in this layer is the presence of darkly staining granules of a protein called keratohyalin, which converts the tonofilaments into keratin. Also present in the keratinocytes are membrane enclosed lamellar granules which release a lipid-rich secretion (water-repellent). Since the cells are starting to die, this layer marks the transition between the deeper, metabolically active strata and the dead cells of the superficial strata.

Stratum Corneum

Consists on average of 25 to 30 layers of flattened dead keratinocytes. These cells are continuously shed and replaced by cells from the deeper strata. the interior of the cells contains mostly keratin and between cells are lipids from lamellar granules that help make this layer an effective water-repellent barrier. It's multiple layers of dead cells also help protect deeper layers from injury and microbial invasion.

Langerhans Cells

Constitute a small fraction of the epidermal cells, arise from the red bone marrow and migrate to the epidermis. They participate in immune responses against microbes that invade the skin, and are easily damaged by UV light. Their role in the immune response is to help other cells of the immune system recognize an invading microbe and destroy it.

Stratum Basale

Deepest layer of the epidermis, composed of a single row of cuboidal or columnar keratinocytes. Some cells in this layer are stem cells that undergo cell division to continually produce new keratinocytes. Melanocytes and Merkel cells with their associated Merkel discs are scattered among the keratinocytes of the layer.

Composed of:

Keratinized stratified squamous epithelium

4 Principal types of cells:

Keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

Melanocytes

Make up about 8% of the epidermal cells, develop from the ectoderm of a developing embryo, produce the pigment melanin (a yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet (UV) light). Long slender projections extend between the keratinocytes and transfer melanin granules to them. Once inside keratinocytes, the melanin granules cluster to form a protective veil over the nucleus, on the side toward the skin surface. In this way, they shield the nuclear DNA from damage by UV light. Although their melanin granules effectively protect keratinocytes, melanocytes themselves are particularly susceptible to damage by UV light (think melanoma).

Keratinocytes

Make up about 90% of epidermal cells, produce the protein keratin (a tough fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemicals), and produce lamellar granules that release a water-repellant sealant that decreases water entry and loss and inhibits the entry of foreign materials.

Keratinization and Growth of the Epidermis

Newly formed cells in the stratum basale are slowly pushed to the surface. As the cells move from one epidermal layer to the next, they accumulate more and more keratin, a process called keratinization. Then they undergo apoptosis. Eventually the keratinized cells slough off and are replaced by underlying cells that in turn become keratinized.

Stratum Lucidum

Present only in the thick skin areas such as fingertips, palms, and soles. It consists of three to fives layers of flattened, clear dead keratinocytes that contain large amounts of keratin and thick-ended plasma membranes.

Thin Skin Layers

Stratum basale, stratum spinosum, stratum granulosum, and a thin stratum corneum.

Thick Skin Layers

Stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and a thick stratum corneum

Stratum Spinosum

Superficial to the stratum basale, arranged in 8 to 10 layers of many-sided keratinocytes fitting closely together. These keratinocytes have the same organelles as the cells of the stratum basale. The arrangement of the cells joined tightly together provides both strength and flexibility to the skin. Langerhans cells and projections of the melanocytes are also present in the layer.

Merkel Cells

The least numerous of the epidermal cells. They are located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron (nerve cell), a structure called a Merkel (tactile) disc. Merkel cells and their associated discs detect touch sensations.

Characteristics of Keratinocytes in the Basale layer

The nuclei are large, and their cytoplasm contains many ribosomes, a small Golgi complex, a few mitochondria, and some rough endoplasmic reticulum. They cytoskeleton within the keratinocytes of the stratum basale includes scattered intermediate filaments, called tonofilaments (composed of a protein that will form keratin in more superficial epidermal layers, attach to desmosomes that bind the cells to the layer and together.)