Skin Biology Overview

Skin Biology Refresher

• Skin is the largest organ of the human body, covering approximately 1.7-2.0 m² and making up about 10% of body mass.
• It's a smooth surface punctuated by hair and sweat pores.
• Skin is both flexible and robust.
• It provides a barrier function, protecting against radiation, viruses, bacteria, and other external substances.
• It also prevents dehydration and is one of the five senses (touch).
• Skin regulates body temperature through sweat and heat retention.

Skin Layers

• Epidermis: Outermost layer.
• Dermis: Lies beneath the epidermis, with two sublayers.
  • Papillary layer: Contains areolar connective tissue, blood vessels, and nerves.
  • Reticular layer: Contains dense irregular connective tissue, collagen, and elastin fibers for strength and flexibility.
• Subcutaneous tissue (hypodermis): Contains adipocytes (fat cells) in lobules separated by connective tissue. The number and size of adipocytes vary by body area, and some regions, such as eyelids, may not have as many. This layer acts as padding, insulation, and energy storage.

Functions of Skin Areas

• Epidermis (outside): Protects against xenobiotics, radiation, microorganisms, and physical trauma.
• Dermis (middle): Provides elasticity and strength, and plays a role in sensory, biochemical, and immunological support for the epidermis.
• Hypodermis (inside): Provides insulation, energy metabolism, padding, and lubrication.

Dermal Appendages

• Hair follicles: Structures containing hair shafts.
• Hair root: Base of the hair follicle.
• Hair follicle receptor (root hair plexus): Sensory nerves surrounding the hair root.
• Arrector pili muscle: Muscle that causes hair to stand on end.
• Sebaceous glands: Produce sebum (oil) for lubrication and waterproofing.
• Eccrine sweat glands: Produce sweat for thermoregulation.

Dermis (corium)

• Approximately 1-4 mm thick.
• Provides structure, strength, flexibility and functions in nutrient supply, waste removal, and houses appendages and plays a role in temperature regulation.
• Regions: Papillary dermis and Reticular dermis.
• Main cell type: Fibroblasts.

Dermal Structure

• Epidermis: Outer layer, with several sublayers.
• Dermis: Middle layer, consisting of papillary and reticular regions, with various connective tissues and structures.
• Subcutaneous tissue: Inner layer, made of adipose (fat) tissue, providing insulation.

Papillary Dermis

• Thin layer directly contacting the epidermis.
• Composed of loose areolar connective tissue.
• Contains dermal papillae (projections) that attach to the epidermis.
• Contains less collagen and elastin but has more extracellular matrix than the reticular dermis.
• Contains blood vessels for nourishment and lymphatic vessels for waste removal.
• Fibrous arrangements are mostly perpendicular to the skin surface.

Reticular Dermis

• Forms the majority (80%) of the dermis.
• Dense connective tissue made up of broad bands of collagen with elastic fibres, running parallel to the skin surface.
• Contains blood vessels, lymphatic vessels, and nerves, along with skin appendages.

Elastin

• Provides flexibility to the skin through stretch and rebound properties.
• Comprises about 2-4% of the dermis' dry weight.
• Has a lifespan similar to that of the human body's.
• Difficult to regenerate/replace.

Collagens

• Provide strength and support within the extracellular matrix (ECM).
• Make up around 80% of the dermis' dry weight.
• Are a family of related proteins, including at least 29 types.
• Type 1 predominant collagen type in dermis.
• Can aggregate into filaments, fibrils, or meshes.

Extracellular Matrix

• Often viewed as a gel-like substance providing a barrier to small molecules.
• Composed of fibrillar proteins and glycosaminoglycans (GAGs).
• GAGs are large polysaccharides that help retain water in the dermis.
• GAGs and proteins combine to form proteoglycans.

Epidermis (80-200 μm)

• Contains multiple layers of cells, with a distinct order.
• Layers: Stratum corneum (outermost), stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale (innermost). Living or dead status depends on the layer.
• Main cell type: Keratinocytes.
• Other cells: Melanocytes (pigment), Langerhans cells (immune response), and Merkel cells (nerve endings).
• Main function of the epidermal layers is to develop a protective barrier.

Stratum Corneum

• Outermost epidermal layer (10-30 μm).
• Composed of flattened, non-nucleated keratinocytes (corneocytes).
• Continuously sheds (desquamates).
• Regulates skin thickness through protease enzymes.
• Constantly regenerated. Turnover time: 14-28 days, depends on age.
• Acts as a rate-limiting barrier, allowing entry of only small and moderately lipophilic molecules.

Stratum Corneum Lipids

• 20% of stratum corneum volume.
• Appear as lamellar sheets in intercellular spaces.
• Major lipid types include ceramides (50%), cholesterol(25%), and free fatty acids(10-20%).
• Skin regenerates approximately 100-150mg of lipids per day to replace those lost.

NMF (Natural Moisturizing Factor)

• Exclusive to the stratum corneum.
• Represents 10% of corneocyte dry weight.
• Forms from filaggrin breakdown in corneocytes and from components of sweat and sebaceous glands products.
• Complex blend of low molecular weight compounds (amino acids, pyrrolidone carboxylic acid, urocanic acid, urea, glycerol lactic acid, sugars, and ions).

Keratinization

• Process of keratinocyte transformation, in which proteins are produced to develop rigid structures.
• Keratinization culminates in formation of corneocytes by the creation of corneocyte envelope (cell membrane) composed of lipids and proteins.
• This results in the development of a protective, waterproof surface.

Sebum

• A mixture of lipids secreted by sebaceous glands.
• Includes glycerides, free fatty acids, wax esters, squalene, cholesterol esters, and cholesterol.
• Produced when mature sebocytes disintegrate (holocrine secretion).
• Secretion is hormonally regulated.

Skin Surface (Hydrolipid Film)

• An emulsion of lipids (largely sebum and epidermal lipids) and water on the skin surface.
• Also referred to as the acid mantle, Contains lactic acid from sweat and various amino acids, free fatty acids from sebum, and natural moisturizing factors (NMFs) from the keratinization process.

Hydration of the Skin

• Stratum corneum water content typically ranges from 10-30%, with variations among different skin areas.
• Water originates from deeper epidermal layers, gradually hydrating stratum corneum cells and eventually evaporating.
• Hydration is essential for appropriate skin feel and enzyme activity for processes such as desquamation and skin regeneration. Factors that affect skin hydration include humidity, water exposure, ion gradients, and time needed for skin maturation.

Functions of the Skin Surface

• Protection: From physical, chemical, and microbial assaults.
• Lubrication: Through sebum production.
• Waterproofing: Preventing water loss.
• Photoprotection: Skin acts as a photofilter for UV rays.
• Antimicrobial activity: The presence of certain molecules and substances inhibit the growth of microbial agents.
• Delivery of fat-soluble antioxidants to the skin surface.
• Pro- and anti-inflammatory activity.

Explore the fascinating structure and function of skin, the largest organ of the human body. This quiz covers key details about skin layers, including the epidermis, dermis, and subcutaneous tissue. Test your knowledge on how skin protects and regulates the body.