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Skin Physiology - Notes for CBDC 1st Year BHMS

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1. INTRODUCTION

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2. STRUCTURE OF THE SKIN

1. Epidermis (outer)
2. Dermis (inner)

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2.1 EPIDERMIS

• Constitutes ~5% of the skin.
• Composed of keratinized, stratified squamous epithelium.
• Avascular - no blood vessels; nourishment by diffusion from dermis.
• Main cell type: Keratinocytes

Layers of the Epidermis (from deep to superficial):

Mnemonic: "Californians Like Girls in Bikinis Showing Confidence" - Corneum, Lucidum, Granulosum, Spinosum, Basalis

Cells of the Epidermis:

• Arise from stratum basalis, migrate upward, differentiate, and eventually form the stratum corneum.
• Turnover time: approximately 14-21 days from basal layer to stratum corneum.

• Dendritic cells of neural crest origin, located in stratum basalis.
• Ratio: ~1 melanocyte to 36 keratinocytes.
• Synthesize melanin (brown-black pigment) in organelles called melanosomes.
• Melanin is transferred via dendritic processes to surrounding keratinocytes.
• Function: Protection against ultraviolet radiation (UVR).
• Skin color differences among ethnicities are due to quantity and distribution of melanin, NOT the number of melanocytes.

• Found in stratum spinosum.
• Dendritic cells - act as antigen-presenting cells (immune function of skin).
• Derived from bone marrow.

• Located in stratum basalis.
• Function as mechanoreceptors (light touch sensation).

Epidermal pH:

• Surface is coated with a film of sebum + sweat = "acid mantle" (pH ~4.5-5.5).
• Protective against harmful microorganisms.

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2.2 DERMIS

• Constitutes ~95% of the skin.
• Composed of dense irregular connective tissue.
• Contains: collagen fibers, elastin fibers, ground substance (glycosaminoglycans).
• Well vascularized - supplies nutrition to epidermis by diffusion.

Two layers of dermis:

• Delicate collagen and elastin fibers.
• Contains capillary loops and lymphatics projecting into epidermal rete ridges.
• Forms fingerprints/dermatoglyphics.

• Coarse, thick collagen bundles arranged parallel to skin surface.
• Provides tensile strength and elasticity.
• Contains adnexal structures: hair follicles, sweat glands, sebaceous glands.

Dermoepidermal Junction (DEJ):

• Wave-like arrangement: epidermal rete ridges project downward, interlocking with upward dermal papillae.
• This increases surface area for adhesion and nutrient diffusion.

Cells of the Dermis:

• Fibroblasts - produce collagen and elastin
• Mast cells - contain histamine; involved in allergic reactions
• Macrophages (histiocytes) - phagocytosis
• Dermal dendritic cells - antigen presentation
• T lymphocytes (tissue-resident memory T cells, T-rm) - immune surveillance

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2.3 HYPODERMIS (Subcutaneous Layer)

• Also called superficial fascia or panniculus adiposus.
• Composed of loose connective tissue and adipose tissue.
• Functions:
  • Energy storage (fat depot)
  • Thermal insulation
  • Cushioning/mechanical protection
• Contains arrector pili muscles (smooth muscle connecting hair follicles to dermis).

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3. APPENDAGES OF THE SKIN

3.1 Hair and Hair Follicles

• Hair follicles are derived from epidermis extending into the dermis.
• Each follicle consists of:
  • Hair shaft (visible part)
  • Hair root (within follicle)
  • Hair bulb (enlarged base containing matrix cells)
  • Dermal papilla (vascular connective tissue at base)
• Arrector Pili Muscle: Smooth muscle attached to the follicle.
  • Innervated by sympathetic adrenergic fibers.
  • Contraction causes "gooseflesh" (cutis anserina) and hair erection.
  • Also assists in expressing sebum from sebaceous glands.
• Follicular Bulge: Located at insertion of arrector pili muscle; contains epidermal stem cells important for follicle regeneration and wound healing.

3.2 Sebaceous Glands

• Holocrine glands - secrete by complete cell disintegration.
• Open into the hair follicle (pilosebaceous unit).
• Secrete sebum: a mixture of fats, wax esters, cholesterol, and cell remnants.
• Functions of sebum:
  • Lubricates skin and hair
  • Waterproofing
  • Antibacterial/antifungal (acid mantle)
  • Prevents excessive water loss
• Absent on palms and soles.
• Androgen-dependent: stimulated by androgens (especially testosterone).

3.3 Sweat Glands

A. Eccrine Sweat Glands

• Most numerous type (~2-4 million); present all over the body.
• Most dense on palms, soles, and forehead.
• Structure: coiled secretory gland (in deep dermis) + a long duct opening at skin surface as sweat pore.
• Innervation: Postganglionic sympathetic cholinergic fibers (acetylcholine stimulates muscarinic receptors).
• Secretion: Isotonic primary secretion (similar to protein-free plasma); duct reabsorbs NaCl → final sweat is hypotonic.
• Composition of sweat: Water, NaCl, KCl, urea, lactic acid, ammonia.
• Functions:
  • Thermoregulation (most important) - evaporative cooling.
  • Excretion of waste products (urea, NaCl, water).
  • Maintains electrolyte balance.

1. Hypothalamus detects rise in core temperature.
2. Sympathetic cholinergic impulses reach gland.
3. ACh binds muscarinic receptors → activates phospholipase C → increases intracellular Ca²+.
4. Cl⁻ is secreted; Na⁺ and water follow (primary secretion).
5. Duct reabsorbs NaCl (aldosterone-sensitive) → hypotonic final sweat.

B. Apocrine Sweat Glands

• Located in axillae, groin, areolae, perianal region.
• Open into hair follicles (not directly to skin surface).
• Larger than eccrine glands.
• Innervation: Postganglionic sympathetic adrenergic fibers (adrenaline/noradrenaline stimulates).
• Become active at puberty (under androgen influence).
• Secrete viscous, milky, odorless fluid; odor is produced by bacterial decomposition on skin surface.
• Function: Pheromone-like role (vestigial in humans); involved in emotional sweating.

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4. FUNCTIONS OF THE SKIN

4.1 Protection (Barrier Function)

• Mechanical barrier: Stratum corneum prevents entry of microorganisms, chemicals, and trauma.
• Chemical barrier: Acid mantle (pH 4.5-5.5) is bacteriostatic.
• UV protection: Melanin absorbs and scatters UV radiation.
• Immunological barrier: Langerhans cells and dermal immune cells.

4.2 Thermoregulation

• Skin regulates body temperature through:
  • Radiation, conduction, convection (at ambient temperatures < 30°C)
  • Evaporation of sweat (dominant at ambient temperatures > 30°C and during exercise)
• Cutaneous blood flow is regulated by sympathetic vasomotor tone:
  • Vasodilation in heat → increases heat loss.
  • Vasoconstriction in cold → conserves heat.
• Skin has a blood supply 20-100 times greater than its own metabolic needs, allowing for wide range of thermoregulatory adjustment.
• Subcutaneous fat (hypodermis) provides thermal insulation in cold.

4.3 Sensory Function

• Free nerve endings are the most numerous sensory receptors.
• Hair follicle nerve plexuses are sensitive to hair movement (mechanoreceptors).

4.4 Excretory Function

• Sweat excretes: water, NaCl, KCl, urea, creatinine, lactic acid, sulphur-containing metabolites (e.g., from drugs, garlic, cumin).
• Minor compared to kidneys, but significant during heavy sweating.

4.5 Metabolic / Endocrine Function

• Vitamin D synthesis: Skin produces cholecalciferol (Vitamin D₃) when UV-B radiation (290-315 nm) acts on 7-dehydrocholesterol in the epidermis.
• Skin cells contain receptors for and respond to:
  • Peptide hormones
  • Steroid sex hormones (androgens)
  • Thyroid hormones
  • Neurotransmitters
• Sebaceous glands metabolize androgens locally.

4.6 Absorption

• Skin is relatively impermeable, but some absorption does occur:
  • Lipid-soluble substances (e.g., steroids, organic solvents, nicotine patches).
  • Basis of transdermal drug delivery.
• Absorption is enhanced by: occlusion, broken skin, increased temperature, thin skin.

4.7 Immunological Function

• Langerhans cells: Capture and present antigens to T lymphocytes.
• Dermal T-cells (T-resident memory cells): Maintain immune surveillance; express CD69, CD103, CD49a.
• Mast cells: Release histamine in immediate hypersensitivity.
• Keratinocytes: Produce cytokines (IL-1, TNF-α) - part of innate immunity.

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5. BLOOD SUPPLY OF THE SKIN

• Skin is supplied by cutaneous arteries (direct branches of segmental arteries).
• Blood anastomoses in multiple plexi:
  • Subfascial → Fascial → Subdermal → Dermal → Subepidermal plexi
• Epidermis is avascular - cells receive nutrition by diffusion from dermal capillaries.
• Venous drainage: via both valved (to deep veins) and unvalved (oscillating flow in subdermal plexus) veins.
• Angiosomes: 3D segments of tissue each supplied by a source artery; neighbouring angiosomes connected by "choke vessels."

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6. COLOUR OF SKIN

1. Melanin (brown-black) - most important; produced by melanocytes
2. Carotene (yellow-orange) - from diet; deposited in epidermis and fat
3. Oxyhaemoglobin (pink-red) - in dermal capillaries
4. Reduced haemoglobin (blue) - gives cyanotic tint

• Ethnic variation in skin colour is due to amount, distribution, and type of melanin (eumelanin vs. pheomelanin), NOT number of melanocytes.
• UV radiation stimulates melanogenesis (tanning).

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7. WOUND HEALING (Brief)

• Skin has remarkable regenerative capacity.
• Blood supply of skin is 20-100 times its basal metabolic need, allowing rapid healing of minor injuries.
• Skin tolerates ischaemia, enabling grafting and expansion.

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8. QUICK REVISION - KEY FACTS

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9. IMPORTANT MNEMONICS

• Layers of epidermis (deep to superficial): "BSGLS Commutes" - Basalis, Spinosum, Granulosum, Lucidum, Corneum
• Functions of skin: "PTERA-S" - Protection, Thermoregulation, Excretion, Reservoir (blood), Absorption, Sensation
• Apocrine vs Eccrine: "A for Adrenergic, Axilla, Adolescence" (Apocrine); "E for Everywhere, Eccrine, cholinErgic"

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