Chapter 2 of 5 - Integumentary System Course
The epidermis is the outermost layer of skin - a thin but remarkably complex barrier that is constantly renewed every 25 to 45 days.
In thick skin (palms and soles), the epidermis has five distinct layers. In thin skin (most of the body), the stratum lucidum is absent. From deepest to most superficial:
Stratum Corneum (outermost)
20-30 layers of dead, flat, keratinized cells
Stratum Lucidum
Thin, clear layer (thick skin only)
Stratum Granulosum
Keratohyalin granules - cells begin to die
Stratum Spinosum
8-10 layers of keratinocytes + Langerhans cells
Stratum Basale (deepest)
Single layer of stem cells - constant mitosis
A helpful mnemonic to remember the layers from deep to superficial: "British Soldiers Grumble Loudly, Come!" (Basale, Spinosum, Granulosum, Lucidum, Corneum).
Quick Check
Which stratum of the epidermis is only found in thick skin (palms and soles)?
| Cell Type | Percentage | Function |
|---|---|---|
| Keratinocytes | ~90% | Produce keratin for waterproofing and protection |
| Melanocytes | ~8% | Produce melanin pigment for UV protection |
| Langerhans cells | ~2% | Dendritic cells that detect and present antigens (immune defense) |
| Merkel cells | <1% | Sensory receptors for fine touch (in stratum basale) |
Fill in the Blank
The pigment________is produced by melanocytes and transferred to surrounding keratinocytes to protect their DNA from UV damage.
Keratinocytes are born in the stratum basale through mitosis. As new cells are produced, older ones are pushed toward the surface. During this migration, they undergo a process called keratinization:
Stratum basale - active mitosis
Stratum spinosum - keratin filaments accumulate
Stratum granulosum - lipid release, nucleus degrades
Stratum corneum - dead, flat, waterproof cells shed
The entire journey from stratum basale to desquamation (shedding) at the surface takes approximately 25 to 45 days. Humans shed roughly 30,000 to 40,000 dead skin cells every hour.
Quick Check
What is the primary function of Langerhans cells in the epidermis?
Skin color is determined by the interplay of three pigments:
All humans have roughly the same number of melanocytes. Differences in skin color arise from the amount, type, and distribution of melanin produced, not the number of melanocytes.
Fill in the Blank
All humans have approximately the same number of melanocytes. Skin color differences depend on the amount, type, and________of melanin produced.
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