Ch 5 5.2
- 1. 5.2 Epidermis Cells of the Epidermis • Epidermis consists mostly of keratinized stratified squamous epithelium • Four cell types found in epidermis: 1. Keratinocytes • Produce fibrous keratin (protein that gives skin its protective properties) • Major cells of epidermis • Tightly connected by desmosomes • Millions slough off every day © 2016 Pearson Education, Inc.
- 2. Cells of the Epidermis (cont.) 2. Melanocytes • Spider-shaped cells located in deepest epidermis • Produce pigment melanin, which is packaged into melanosomes – Melanosomes are transferred to keratinocytes, where they protect nucleus from UV damage 3. Dendritic (Langerhans) cells • Star-shaped macrophages that patrol deep epidermis – Are key activators of immune system 4. Tactile (Merkel) cells • Sensory receptors that sense touch © 2016 Pearson Education, Inc.
- 3. Layers of the Epidermis • Epidermis is made up of four or five distinct layers – Thick skin contains five layers (strata) and is found in high- abrasion areas (hands, feet) – Thin skin contains only four strata • Five layers of skin 1. Stratum basale 2. Stratum spinosum 3. Stratum granulosum 4. Stratum lucidum (only in thick skin) 5. Stratum corneum © 2016 Pearson Education, Inc.
- 4. Layers of the Epidermis (cont.) 1. Stratum basale (basal layer) – Deepest of all epidermal layers (base layer) – Layer that is firmly attached to dermis – Consists of a single row of stem cells that actively divide (mitotic), producing two daughter cells each time • One daughter cell journeys from basal layer to surface, taking 25–45 days to reach surface – Cell dies as it moves toward surface • Other daughter cell remains in stratum basale as stem cell – Layer also known as stratum germinativum because of active mitosis – 10–25% of layer also composed of melanocytes © 2016 Pearson Education, Inc.
- 5. Layers of the Epidermis (cont.) 2. Stratum spinosum (prickly layer) – Several cell layers thick – Cells contain weblike system of intermediate prekeratin filaments attached to desmosomes • Allows them to resist tension and pulling – Keratinocytes in this layer appear spikey, so they are called prickle cells – Scattered among keratinocytes are abundant melanosomes and dendritic cells © 2016 Pearson Education, Inc.
- 6. Layers of the Epidermis (cont.) 3. Stratum granulosum (granular layer) – Four to six cells thick, but cells are flattened, so layer is thin – Cell appearance changes • Cells flatten, nuclei and organelles disintegrate • Keratinization begins – Cells accumulate keratohyaline granules that help form keratin fibers in upper layers • Cells also accumulate lamellar granules, a water-resistant glycolipid that slows water loss – Cells above this layer die • Too far from dermal capillaries to survive © 2016 Pearson Education, Inc.
- 7. Layers of the Epidermis (cont.) 4. Stratum lucidum (clear layer) – Found only in thick skin – Consists of thin, translucent band of two to three rows of clear, flat, dead keratinocytes – Lies superficial to the stratum granulosum © 2016 Pearson Education, Inc.
- 8. Layers of the Epidermis (cont.) 5. Stratum corneum (horny layer) – 20–30 rows of flat, anucleated, keratinized dead cells – Accounts for three-quarters of epidermal thickness – Though dead, cells still function to: • Protect deeper cells from the environment • Prevent water loss • Protect from abrasion and penetration • Act as a barrier against biological, chemical, and physical assaults © 2016 Pearson Education, Inc.
- 9. Layers of the Epidermis (cont.) • Cells change by going through apoptosis (controlled cell death) – Dead cells slough off as dandruff and dander – Humans can shed ~50,000 cells every minute © 2016 Pearson Education, Inc.
- 10. Figure 5.2 Epidermal cells and layers of the epidermis. © 2016 Pearson Education, Inc. Keratinocytes Stratum corneum Most superficial layer; 20–30 layers of dead cells, essentially flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Typically one to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and dendritic cells. Dermis Melanin granule Dermis Sensory nerve ending Tactile (Merkel) cell Desmosomes Dendritic cellMelanocyte