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 5.2aThe 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 theformation of a layerof dead protectivecells with protectiveprotein KeratinKeratinocytesTightlyconnected bydesmosomes
Melanocytes10–25% of cells indeepest epidermisProduce pigmentmelanin– packagedinto melanosomes.
•Tactile (merkel) cells–Sensory touch receptors–Associated with sensory neuron.
Dendritic(langerhans) cellsMacrophages – keyactivators of immunesystem
Thin skin covers most of thebody areas while thick skinis present in the plams ofthe hands and the soles ofthe feet
Cell Differentiation in Epidermis•Cells change from stratum basale tostratum corneum•Accomplished by specialized form ofapoptosis–Controlled cellular suicide–Nucleus and organelles break down–Plasma membrane thickens–Allows cells to slough off as dandruff anddander–Shed ~ 50,000 cells every minute
Stratum spinosumStratum basaleMelanocyteDendriticcellKeratinocytesStratum corneumDermisMelaningranuleSensorynerveendingTactile(Merkel) cellDesmosomesStratum granulosum.Figure 5.2bThe main structural features of the skin epidermis.
Dermis•Strong, flexible connective tissue•Cells–Fibroblasts, macrophages, and occasionally mast cellsand white blood cells•Fibers in matrix bind body together•Contains nerve fibers; blood and lymphatic vessels•Contains epidermal hair follicles; oil and sweatglands•Two layers–Papillary–Reticular