Epidermis
- The epidermis is the superficial epithelial layer of skin that is exposed to the outside environment.
- Superficial means closer to the body surface.
- Ectodermal origin means the epidermis develops mainly from embryonic ectoderm.
- Stratified epithelium has multiple cell layers, which helps the epidermis resist abrasion.
- Squamous cells are flattened cells, especially in the upper epidermal layers.
- Keratinized epithelium contains keratin-rich dead surface cells that form a tough protective layer.
- Keratinocytes are the main epidermal cells and produce keratin proteins for mechanical strength.
- Melanocytes are pigment-producing epidermal cells that make melanin.
- Langerhans cells are immune dendritic cells in the epidermis.
- Merkel cells are sensory-associated epidermal cells involved in light-touch reception.
- Regeneration means the epidermis renews itself by replacing old surface cells with cells produced below.
- Thirty-day turnover means the epidermis normally replaces itself in about a month.
- Mitosis is cell division, and epidermal mitosis mainly occurs in deeper keratinocyte layers.
- Night timing of mitosis is a normal rhythm described for keratinocyte renewal.
- Epidermal ridges are downward projections of epidermis that interlock with dermal ridges.
- Dermal ridges are upward projections from the dermis that fit between epidermal ridges.
- Secondary ridges can form when epidermal interpapillary pegs subdivide dermal ridges.
- Interpapillary pegs are epidermal downward extensions between dermal papillae.
- Fingerprints form because surface ridges over dermal ridges create unique patterns on fingertips.
- Genetic determination means fingerprint ridge patterns are strongly influenced by inherited developmental instructions.
- Individual uniqueness makes fingerprints useful for identification.
- Immune participation by keratinocytes means epidermal cells can release signaling molecules, not just form a wall.
- Interleukins are immune signaling proteins that help cells communicate during inflammation and defense.
- Interferons are signaling proteins often involved in antiviral defense and immune regulation.
- Tumor necrosis factors are inflammatory signaling proteins that can alter cell survival and immune responses.
- Colony-stimulating factors are signals that support production or activity of certain blood and immune cells.
- Epidermal immunity is important because skin is constantly exposed to microbes and environmental antigens.
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