Skin Biology Quiz

Skin Biology Refresher

• The skin is the largest organ of the human body, covering approximately 1.7-2.0 m² and accounting for about 10% of body mass.
• It's smooth, punctuated by hair and sweat pores.
• It's flexible but robust.
• The skin provides a barrier function, protecting against radiation, viruses, bacteria, and other foreign substances.
• It's waterproof, preventing dehydration.
• Touch is one of the five senses.
• The skin plays a key role in regulating body temperature (traps/releases heat).

Skin Layers

• Epidermis: The outermost layer.
• Dermis: A layer under the epidermis, containing the papillary and reticular layers.
• Subcutaneous Tissue (Hypodermis): The deepest layer, which helps separate dermis from underlying body constituents. It's mostly made of adipocytes (fat cells) in lobules separated by connective tissue. The number and size of these adipocytes can vary across the body and depend on nutritional state. Also acts as insulation, energy reserve, and padding.

Dermal Structure

• The dermis is about 1-4mm thick and has papillary and reticular regions.
• Main cell type within the dermis is fibroblasts, which provide structure, strength, and flexibility.
• The dermis has a rich blood and lymphatic supply.
• Contains sensory nerve endings and appendages like hair follicles and glands.

Papillary Dermis

• A thin layer directly contacting the epidermis.
• Composed of loose areolar connective tissue.
• Upper layer has projections called dermal papillae, which are part of the dermal-epidermal junction.
• Contains less collagen and more ECM (extracellular matrix) compared to reticular dermis.
• Fibers are randomly arranged mainly perpendicular to skin surface.
• Supports/contains blood and lymphatic vessels, involved in nutrition, temperature regulation, and waste removal.

Reticular Dermis

• Forms the bulk (about 80%) of the dermis.
• Dense connective tissue with thick collagen and elastic fibers running parallel to skin surface.
• Contains blood and lymphatic vessels, nerves, and appendages.

Elastin

• Provides flexibility to the skin.
• Makes up approximately 2-4% of dermis (dry weight).
• Has a similar half-life to human lifespan.
• Difficult to regenerate or replace.

Collagens

• Provide strength and support to the extracellular matrix (ECM).
• 80% of dermis is collagen (dry weight) and consists of related proteins (at least 29 types).
• Type 1 collagen is the predominant type within the dermis.
• Can aggregate into filaments, fibrils, and meshes.

Extracellular Matrix (ECM)

• Primarily consists of fibrillar proteins surrounded by glycosaminoglycans (GAGs).
• GAGs are large polysaccharides that retain water in the dermis.
• GAGs and proteins form proteoglycans.
• Four groups of GAGs include hyaluronic acid, chondroitin sulphate, dermatan sulphate, heparan sulphate, and keratan sulphate.

Epidermis

• Epithelial tissue, between 80-200 μm thick.
• Multiple layers (4-5): stratum corneum, (stratum lucidum), stratum granulosum, stratum spinosum, and stratum basale (descending).
• Contains keratinocytes (main cells), along with melanocytes, Langerhans cells, and Merkel cells.
• Keratinocytes are responsible for producing a strong barrier.

Stratum Corneum

• Consists of 15-20 layers of flattened, non-nucleated, and keratinized cells called corneocytes.
• Continuously shed (desquamated).
• Proteases hydrolyse desmosomes to regulate stratum corneum thickness.
• Constantly regenerated.
• Turnover time depends on age (14-28 days).
• Rate-limiting barrier allowing entry of only small and moderately lipophilic molecules.

Stratum Corneum Lipids

• Account for roughly 20% of stratum corneum volume.
• Form lamellar sheets in intercellular spaces.
• Major components include ceramides (50%), cholesterol (25%) and free fatty acids (10-20%).
• Skin generates roughly 100-150 mg of lipids daily to replace those lost through desquamation.

Ceramides

• A crucial component of stratum corneum lipids for maintaining lamellar structure.
• Various subclasses ('free' ceramides) with differences in head group hydroxylation exist in the epidermis.

Lipid Lamellar Bilayers

• Stratum corneum is organised into lipid lamellar bilayers.

Skin Surface: Hydrolipid Film/Acid Mantle

• An emulsion of water and lipids covering the epidermis.
• Contains lactic acid and free fatty acids (from sweat and sebum).
• Contains amino acids and natural moisturizing factors (NMFs), by products of keratinisation.

Sebum

• Lipid secretion from sebaceous glands, composed of glycerides, free fatty acids, wax esters, squalene, cholesterol esters, and cholesterol.
• Holocrine secretion (mature sebocytes disintegrating).
• Activity is hormonally regulated.

Skin Functions

• Protection: from physical, chemical, and microbial assaults.
• Lubrication: Maintaining skin softness and flexibility.
• Waterproofing: Preventing excessive water loss.
• Photoprotection: Providing some degree of protection against UV radiation.
• Antimicrobial activity: Inhibiting the growth of microbes.
• Delivering and containing fat-soluble antioxidants to the skin surface and showing pro- and anti-inflammatory action (in relation to lipids).
• The understanding of these functions is not yet complete in some respects.

Hydration of the Skin

• Water content ranges from 10-30%.
• Essential for enzymatic activity for processes such as desquamation and skin regeneration.
• Water originates in deeper layers and moves upward hydrating cells in stratum corneum, eventually evaporating. Factors affecting hydration of stratum corneum include pH gradients, gradients of calcium, magnesium, etc from basement membrane to surface, adequate hydration of corneocytes, rate of water transport from dermis to stratum corneum, rate of surface water loss, binding ability, and adequate maturation time of stratum corneum.

Natural Moisturizing Factor (NMF)

• A complex mixture of low molecular weight compounds.
• Includes amino acids (AA), pyrrolidone carboxylic acid (PCA), urocanic acid (UCA), lactic acid, sugars, urea, glycerol, and various ions.
• Derived from the hydrolysis of filaggrin in corneocytes but also from sweat, sebaceous glands, and triglyeride turnover.

Filaggrin

• Derived from profilaggrin.
• Major component of keratohyalin granules.
• Monomers bind to keratin 1 and 10 forming tight bundles.
• Allows the collapse and flattening of cells into squames.

Key Terms

• Corneocytes: Dead, keratinized cells in the stratum corneum.
• ECM: Extracellular matrix.
• GAGs: Glycosaminoglycans
• NMF: Natural moisturizing factor
• SG: Sebaceous gland