5-1 Epidermis layers and function

Overview of the Epidermis

  • The epidermis is a protective covering composed of several layers, each with distinct functions.

Learning Objective

  • Learning Outcome: Describe the main structural features of the epidermis and explain the functional significance of each.

Structural Features of the Epidermis

  • The epidermis is a stratified squamous epithelium which provides physical protection for the dermis, prevents water loss, and helps keep microorganisms outside the body.
  • Avascular Nature:
    • The epidermis contains no blood vessels.
    • Nutrients and oxygen are supplied through the diffusion from capillaries in the dermis.
    • Therefore, the epidermal cells with the highest metabolic demands are located close to the basement membrane, where diffusion distance is minimized.
    • Superficial cells, farther from nutritional sources, tend to be dead.

Cell Types in the Epidermis

  • Keratinocytes:
    • Keratinocytes are the predominant cells in the epidermis.
    • They form multiple layers and are abundant in keratin, a tough fibrous protein that is the major component of hair and nails.

Types of Skin

  • Two categories based on the number of keratinocyte layers:
    • Thin Skin:
    • Contains four layers of keratinocytes.
    • Covers most body surfaces.
    • Thick Skin:
    • Contains five layers of keratinocytes.
    • Found in palms of hands and soles of feet.
    • Note: The terms thin and thick refer to the relative thickness of the epidermis, not the entire cutaneous membrane.
  • Layers of the Epidermis (Latin names starting with 'stratum'):
    • stratum basale
    • stratum spinosum
    • stratum granulosum
    • stratum lucidum
    • stratum corneum

Layers of the Epidermis Explained

Stratum Basale (Basal Layer)

  • Description:
    • The deepest layer of the epidermis, also called stratum germinativum.
  • Connection to Basement Membrane:
    • Hemidesmosomes attach the basal cells to the basement membrane separating the epidermis from the dermis.
  • Function:
    • Basal cells (germinative cells) are stem cells that divide to replace keratinocytes shed from the surface.
    • Specialized epithelial cells known as Merkel cells are present and function in touch sensation.
    • Pigmentation:
    • Contains melanocytes that synthesize pigment, impacting skin tone.
Stratum Spinosum (Spiny Layer)
  • Description:
    • Composed of 8-10 layers of keratinocytes.
  • Appearance:
    • Cells appear spiny in histological sections due to cytoplasm shrinkage and intact desmosomes.
  • Function:
    • Thickness increases as some cells here continue to divide.
    • Contains dendritic cells (Langerhans cells) that are part of the immune response against invading microorganisms and skin cancers.
Stratum Granulosum (Granular Layer)
  • Description:
    • Contains 3-5 layers of keratinocytes from the spinosum.
  • Process of Keratinization:
    • Keratin fibers accumulate here, cells become thinner, flatter, and thicker plasma membranes are formed.
    • Cells produce keratohyalin, leading to cell dehydration and the eventual death of the cells.
Stratum Lucidum (Clear Layer)
  • Description:
    • Present only in thick skin regions (palms and soles).
  • Characteristics:
    • Composed of flattened, densely packed cells that lack organelles and contain keratin.
Stratum Corneum (Horny Layer)
  • Description:
    • At the surface of thick and thin skin, varies from 15 to 30 layers of keratinized cells.
  • Keratinization Process:
    • The formation of protective layers filled with keratin.
    • Normally, keratinized cells are shed in large groups due to strong desmosomal connections.
    • Skin cell shedding rates are approximately 30,000 to 40,000 cells per minute, totaling about nine pounds a year.
    • Typically takes 7-10 days for a cell to travel from stratum basale to stratum corneum, where dead cells remain for an additional two weeks.

Function and Protective Mechanisms

  • Barrier Role:
    • The dead, keratin-rich cells in the stratum corneum create a barrier, protecting underlying tissues.
  • Water Loss Prevention:
    • The surface is coated by lipid secretions, reducing water loss and preventing gain.
  • Skin Texture:
    • The epidermal ridges formed by the stratum basale and dermal papillae increase grip through greater surface area and friction.
  • Fingerprints:
    • Unique ridge patterns determined by genetic factors and intrauterine environment.
    • Fingerprint patterns do not change throughout life.

Factors Influencing Skin Health

  • Hydration of Skin:
    • Insensible perspiration leads to a daily loss of about 500 milliliters of water, which is not perceptible.
    • Increased loss can occur from epidermal damage, resulting in blisters and compromised barrier function.
  • Response to Environmental Factors:
    • Wrinkled fingers in water result from blood vessel constriction, enhancing grip on wet objects.

Epidermal Growth Factor (EGF)

  • Overview:
    • EGF is produced by salivary glands and the duodenum.
    • Promotes:
    • Division of basal cells in stratum basale and spinosum.
    • Keratin production and keratinization in differentiating keratinocytes.
    • Epidermal development and repair after injury.
    • Synthesizes and secretes products by epithelial glands.
  • Clinical Application:
    • Human epidermal growth factor (HEGF) is produced using recombinant DNA technology for tissue growth.
    • Used to regenerate skin for burn victims by growing epidermal sheets from a small sample of intact skin.

Quiz Checkpoint

  • Identify the layers of the epidermis:
    • Stratum Basale
    • Stratum Spinosum
    • Stratum Granulosum
    • Stratum Lucidum
    • Stratum Corneum