Epidermis Detailed Notes

Epidermis

Overview

• The epidermis is stratified squamous epithelium.
  • Stratified: Multiple layers of cells.
  • Squamous: Flat cells at the surface (apical cells).
  • Deeper cells are cuboidal, but surface cells are flat.
• The purple area in images typically represents the epidermis.
• Surface cells flake off.

Keratinocytes

• Cells of the epidermis.
• Named after keratin, a tough protein.
• Keratin provides a protective surface.
• Keratin means "hard or tough."

Avascular Nature

• Epidermis is an epithelial tissue, meaning it's avascular (no blood vessels).
• Relies on the dermis for nutrition (glucose, oxygen).

Thickness and Layers

• Over 30 cell layers thick (a couple of millimeters).
• Organized into five distinct regions called strata (plural of stratum).
• Strata/stratum means layer.

Layers of the Epidermis (Superficial to Deep)

Stratum Corneum

• Most superficial (apical) layer.
• All dead, flat, squamous keratinocytes.
• Cells are dead due to distance from nutrition.
• Cells have lost all organelles.
• Cells flake off, contributing to household dust.

Stratum Lucidum

• Not present everywhere; found in palms of hands and soles of feet (thick skin).
• Lucid means clear; light can pass through it.
• Provides extra protection.

Stratum Granulosum

• Cells appear granular or grainy under a microscope.
• Cells are starting to die off as they migrate upwards.
• Organelles are breaking apart.
• Dye sticks to organelle bits, making them look grainy.

Stratum Spinosum

• Appears spiny under a microscope.
• Desmosomes between cells are visible.
• Desmosomes are intercellular junctions that hold cells together.
• Desmosomes have a spiky appearance due to strong proteins.

Stratum Basale

• Deepest layer, appears dark under a microscope.
• Site of pigment production (melanin).
• Darkens skin in response to UV rays.
• Also site of mitosis of keratinocytes (new cell production).
• New keratinocytes are pushed up into stratum spinosum and so on.
• Skin cells are always migrating upwards.
• Stratum corneum is always flaking away.

Stratum Basale Details

• Deepest layer, sits on the dermis on a basement membrane.
• Single layer of cube-shaped (cuboidal) cells.
• Site of pigment production and mitosis.

Keratinocytes and Keratin

• Keratinocytes in this layer undergo mitosis.
• Keratinocytes contain the protein, keratin.
• As cells migrate up, they fill with more keratin.

Melanocytes and Melanin

• Melanocytes produce melanin (pigment).
• Melanin darkens the skin.
• All humans have roughly the same number of melanocytes.
• Skin color depends on the type and amount of melanin produced.
• Determined by genes inherited from parents.

Tactile Cells

• Contain touch receptors.
• Detect delicate touches (e.g., tickling, hair brushing).

Melanocyte Illustration

• Looks like an upside-down octopus.
• Red dots are pigments.
• Melanocytes export pigments to nearby keratinocytes to darken them.

Stratum Spinosum Details

• Several layers thick.
• Contains keratinocytes that originated in the basale layer.
• Spiny appearance due to visible desmosomes.

Desmosomes

• Connect keratinocytes to prevent separation.
• Spiky appearance due to protein fibers.

Dendritic Cells

• Immune cells (macrophages).
• Macro = big, phage = eat.
• Eat pathogens (bacteria, etc.) that penetrate the epidermis.

Desmosome Image

• Close-up showing spiky appearance.
• Many protein fibers contribute to the spiky look.

Stratum Granulosum Details

• Not very thick (3-5 layers of cells).
• Keratinocytes are flattening and becoming fibrous.

Glycolipids

• Cells filled with lipids (glycolipids).
• Glycolipids are a combination of fat and carbohydrate.
• Glycolipids provide waterproofing to prevent dehydration.

Granular Appearance

• Nucleus and organelles are breaking down.
• Cells are starting to die due to distance from the dermis.
• Dye sticks to organelle fragments.

Stratum Lucidum Details

• Clear layer found only in thick skin (palms and soles).
• Palmar and plantar regions.
• Lucid means clear because cells have no nuclei or organelles; they've totally degraded.

Stratum Corneum Details

• Very thick; the thickest of all the layers.
• Over 30 layers of dead, flat keratinocytes.
• Cells flake off constantly.
• Trail of keratinocytes used by sniffer dogs.

Epidermal Mitosis and Replacement

• Epidermis is very good at mitosis.
• The basale layer is very efficient at cell division.
• All five layers of the epidermis are replaced every 45 days.
• Keratinocytes made in the basale layer reach the surface and flake away within 45 days.