Epidermis•Keratinized stratified squamous epithelium•Four or five distinct layers–Stratum basale–Stratum spinosum–Stratum granulosum–Stratum lucidum(only in thick skin)–Stratum corneum•Four cell types–Keratinocytes–Melanocytes–Dendritic (langerhans) cells–Tactile (merkel) cells
Figure 5.2a The main structural features of the skin epidermis.DermisStratum spinosumSeveral layers of keratinocytes unified by desmosomes.Cells contain thick bundles of intermediate filaments made ofpre-keratin.Stratum basaleDeepest epidermal layer; one row of actively mitotic stemcells; some newly formed cells become part of the moresuperficial layers. See occasional melanocytes and dendriticcells.Stratum granulosumTypically five layers of flattened cells, organelles deteriorating;cytoplasm full of lamellar granules (release lipids) andkeratohyaline granules.Stratum corneumMost superficial layer; 20–30 layers of dead cells, essentiallyflat membranous sacs filled with keratin. Glycolipids inextracellular space.
Keratinization is the formation of a layer of dead protective cells with protective protein KeratinKeratinocytesTightly connected by desmosomes
Melanocytes10–25% of cells in deepest epidermisProduce pigment melanin– packaged into melanosomes.
•Tactile (merkel) cells–Sensory touch receptors–Associated with sensory neuron.
Dendritic (langerhans) cellsMacrophages – key activators of immune system
Thin skin covers most of the body areas while thick skin is present in the plams of the hands and the soles of the feet
Cell Differentiation in Epidermis•Cells change from stratum basale to stratum corneum •Accomplished by specialized form of apoptosis–Controlled cellular suicide–Nucleus and organelles break down–Plasma membrane thickens–Allows cells to slough off as dandruff and dander–Shed ~ 50,000 cells every minute
Stratum spinosumStratum basaleMelanocyteDendriticcellKeratinocytesStratum corneumDermisMelaningranuleSensorynerveendingTactile(Merkel) cellDesmosomesStratum granulosum.Figure 5.2b The main structural features of the skin epidermis.
Dermis•Strong, flexible connective tissue•Cells–Fibroblasts, macrophages, and occasionally mast cells and white blood cells•Fibers in matrix bind body together•Contains nerve fibers; blood and lymphatic vessels•Contains epidermal hair follicles; oil and sweat glands•Two layers –Papillary–Reticular