1. Dermatovenerology: scope, relevance, common terminology, and relationship to sexually transmitted infections.
2. Gross and microscopic anatomy: epidermis, dermis, subcutis, appendages, vessels, nerves.
3. Skin cells and layers: keratinocytes, melanocytes, Langerhans cells, Merkel cells; epidermal strata.
4. Physiology: barrier function, keratinization, pigmentation, sensation, temperature control, immune defense, vitamin D synthesis, secretion and absorption.
5. Clinical correlations: how normal anatomy explains lesions, wound healing, burns, acne, and routes of topical or injected medicines.

1. Dermatovenerology: scope, relevance, common terminology, and relationship to sexually transmitted infections.
2. Gross and microscopic anatomy: epidermis, dermis, subcutis, appendages, vessels, nerves.
3. Skin cells and layers: keratinocytes, melanocytes, Langerhans cells, Merkel cells; epidermal strata.
4. Physiology: barrier function, keratinization, pigmentation, sensation, temperature control, immune defense, vitamin D synthesis, secretion and absorption.
5. Clinical correlations: how normal anatomy explains lesions, wound healing, burns, acne, and routes of topical or injected medicines.

1. Introduction to dermatovenerology
2. Anatomy of the skin
3. Physiology and functions of the skin
4. Skin appendages: hair, nails, sebaceous and sweat glands
5. Clinical correlations: burns, wounds, acne, pigmentation, and topical drug absorption

Skin Anatomy and Layers: Detailed Notes

1. Overview of the skin

1. Epidermis: outer epithelial layer
2. Dermis: supporting connective-tissue layer
3. Hypodermis (subcutaneous tissue): deeper loose connective and adipose tissue

2. Surface landmarks of the skin

A. Skin lines and ridges

• Skin tension lines follow the predominant arrangement of collagen fibers in the dermis.
• Surgical incisions made parallel to these lines generally heal with less scar widening.
• Friction ridges are present on the palms, soles, and flexor surfaces of the fingers. They form fingerprints and improve grip.
• Dermatoglyphics are the characteristic ridge patterns of fingers, palms, toes, and soles.

B. Skin thickness

• Thick skin: palms and soles
  • Thick epidermis
  • Has a stratum lucidum
  • No hair follicles or sebaceous glands
  • Numerous eccrine sweat glands
• Thin skin: most of the body surface
  • Thinner epidermis
  • Usually lacks a distinct stratum lucidum
  • Contains hair follicles, sebaceous glands, and sweat glands

3. Epidermis

Definition and general features

Main characteristics

• It is avascular. It receives oxygen and nutrients by diffusion from blood vessels in the underlying dermis.
• It is mainly made of keratinocytes.
• It has a continuous renewal process in which basal cells divide, migrate upward, differentiate, keratinize, and are eventually shed.
• Its outermost layer, the stratum corneum, is the principal permeability barrier of the skin.
• Its thickness differs by site, being greatest on the palms and soles.

A. Layers of the epidermis

1. Corneum
2. Lucidum
3. Granulosum
4. Spinosum
5. Basale

1. Stratum corneum

Structure

• Composed of flattened, dead, anucleate keratinized cells called corneocytes.
• Cells are packed with keratin.
• Corneocytes are surrounded by extracellular lipids, mainly ceramides, cholesterol, and free fatty acids.

Functions

• Provides mechanical protection.
• Limits water loss from the body.
• Limits entry of irritants, allergens, chemicals, and microorganisms.
• Contributes to the acidic skin surface, often termed the acid mantle.
• Continuously desquamates, meaning that superficial corneocytes are shed.

Clinical relevance

• Disruption of this layer causes increased transepidermal water loss and dryness, as in eczema and ichthyosis.
• Topical medications must cross the stratum corneum to reach deeper skin layers.
• A superficial burn affects the epidermis alone and typically heals without scarring.

2. Stratum lucidum

• Palms
• Soles
• Palmar and plantar surfaces of digits

Structure

• Consists of flattened, eosinophilic, anucleate cells.
• Located between stratum corneum and stratum granulosum.
• Contains eleidin, a keratin-related material.

Function

• Adds resistance to friction and shear forces in areas exposed to repeated mechanical stress.

3. Stratum granulosum

Structure

• Contains flattened keratinocytes with dark basophilic keratohyalin granules.
• These granules contain proteins important for keratin organization, including profilaggrin.
• Cells also contain lamellar bodies that release lipids into the extracellular space.

Functions

• Begins formation of the water barrier.
• Keratohyalin-associated proteins aggregate keratin filaments.
• Extracellular lipids form an effective barrier against water movement and environmental substances.
• The cells progressively lose their nuclei and organelles as they become corneocytes.

Clinical relevance

4. Stratum spinosum

Structure

• Contains several layers of polygonal keratinocytes.
• Cells appear “spiny” on light microscopy because of cytoplasmic bridges between adjacent cells.
• These bridges contain abundant desmosomes, which provide strong adhesion between keratinocytes.
• Keratin filaments, called tonofilaments, are prominent in these cells.
• Langerhans cells are commonly found here.

Functions

• Provides mechanical strength.
• Maintains cohesion of the epidermis through desmosomes.
• Participates in immune surveillance through Langerhans cells.

Clinical relevance

• Loss of desmosomal adhesion causes intraepidermal blistering, as in pemphigus vulgaris.
• The term acanthosis refers to thickening of the stratum spinosum.

5. Stratum basale

Structure

• A single layer of cuboidal to low-columnar cells resting on the basement membrane.
• Contains mitotically active keratinocyte stem and progenitor cells.
• Attached to the basement membrane by hemidesmosomes.
• Contains melanocytes and Merkel cells.

Functions

• Produces new keratinocytes for epidermal renewal.
• Maintains attachment between epidermis and dermis.
• Contains melanocytes for pigment production.
• Contains Merkel cells involved in light-touch sensation.

Clinical relevance

• Basal cell carcinoma originates from cells of this layer.
• Separation at the dermoepidermal junction produces subepidermal blisters, such as in bullous pemphigoid.
• Basal keratinocytes migrate upward and eventually become keratinized, dead corneocytes.

B. Major cells of the epidermis

1. Keratinocytes

Roles

• Produce keratin.
• Form the stratum corneum.
• Create the physical and permeability barrier.
• Produce cytokines and antimicrobial substances during inflammation.
• Participate in wound healing and immune responses.

2. Melanocytes

Roles

• Produce melanin within organelles called melanosomes.
• Transfer melanosomes to surrounding keratinocytes.
• Form a protective supranuclear cap of melanin in keratinocytes that helps protect DNA from ultraviolet radiation.

Clinical relevance

• Loss or reduced activity of melanocytes causes vitiligo.
• Defects in melanin synthesis may cause albinism.
• Proliferation of melanocytes may form nevi, and malignant transformation leads to melanoma.

3. Langerhans cells

Roles

• Capture foreign antigens entering through the epidermis.
• Process and present antigen to T lymphocytes.
• Take part in cutaneous adaptive immune responses.

4. Merkel cells

Roles

• Associate with sensory nerve endings.
• Function as slowly adapting mechanoreceptors.
• Detect light touch, pressure, and texture.

4. Dermoepidermal junction

Components

• Basal keratinocytes
• Basement membrane
• Dermal papillae
• Epidermal ridges, also called rete ridges or rete pegs

• Improves attachment between epidermis and dermis
• Resists shearing forces
• Increases surface area for diffusion of nutrients to the avascular epidermis

Clinical relevance

• Intraepidermal blister: split within epidermis, such as in pemphigus vulgaris.
• Subepidermal blister: split below epidermis at the basement membrane zone, such as in bullous pemphigoid.

5. Dermis

Definition

• Collagen fibers
• Elastic fibers
• Fibroblasts
• Blood vessels and lymphatic vessels
• Nerves and sensory receptors
• Hair follicles
• Sebaceous glands
• Sweat glands
• Arrector pili muscles
• Immune cells

A. Layers of dermis

1. Papillary dermis

Structure

• Loose connective tissue.
• Fine collagen fibers and elastic fibers.
• Numerous capillary loops.
• Contains dermal papillae that interdigitate with the epidermis.
• Contains sensory nerve endings and immune cells.

Functions

• Supports and nourishes the epidermis.
• Provides oxygen and nutrients by diffusion to epidermal cells.
• Helps regulate heat exchange through its vascular network.
• Contributes to light-touch sensation.

2. Reticular dermis

Structure

• Dense irregular connective tissue.
• Thick bundles of mainly type I collagen.
• Contains elastic fibers.
• Contains larger blood vessels, nerves, hair follicles, glands, and arrector pili muscles.

Functions

• Gives skin tensile strength.
• Provides elasticity and resistance to stretching.
• Supports adnexal structures and neurovascular networks.

Clinical relevance

• Injury confined to the epidermis or superficial papillary dermis can often heal with little scarring.
• Damage extending into the reticular dermis is more likely to produce permanent scarring. K. J. Lee's Essential Otolaryngology, p. 1073.
• Deep dermal injury, as in deep burns, may require grafting and can lead to scar contracture.

6. Hypodermis or subcutaneous tissue

Definition

Structure

• Loose connective tissue
• Variable amounts of adipose tissue
• Larger blood vessels
• Larger nerves
• Lymphatic vessels

Functions

• Cushions against mechanical trauma.
• Insulates against heat loss.
• Stores energy as fat.
• Allows movement of skin over deeper tissues.
• Contains larger vessels and nerves supplying the skin.
• Serves as the site for subcutaneous injections, for example insulin and heparin.

Clinical relevance

• Thickness varies markedly with age, sex, nutritional status, and body site.
• Edema may accumulate in subcutaneous tissue.
• Lipomas arise from adipose tissue.
• Subcutaneous injections are absorbed more slowly than intramuscular injections because the tissue has less blood flow than muscle.

7. Skin appendages

A. Hair follicles

Main parts

• Hair shaft
• Hair root
• Hair bulb
• Dermal papilla
• Matrix cells
• Outer and inner root sheaths

Associated structures

• Sebaceous gland
• Arrector pili muscle
• Sensory nerve endings

B. Sebaceous glands

Characteristics

• Secrete sebum, an oily lipid-rich material.
• Secretion is holocrine, meaning whole cells break down to release their contents.
• Usually open into the upper portion of hair follicles.

Functions

• Lubricate skin and hair.
• Contribute to barrier function.
• Have antimicrobial properties.

Clinical correlation

C. Sweat glands

1. Eccrine sweat glands

• Widely distributed, especially numerous on palms, soles, and forehead.
• Open directly onto the skin surface.
• Produce watery sweat.
• Essential for thermoregulation by evaporation.

2. Apocrine sweat glands

• Mainly located in the axillae, areolae, and anogenital region.
• Open into hair follicles.
• Become active at puberty.
• Produce a thicker secretion that can develop odor after bacterial action.

D. Arrector pili muscles

• Small bundles of smooth muscle attached to hair follicles.
• Contract in response to cold or emotional stimuli.
• Cause hair erection, producing goosebumps.
• Compression of adjacent sebaceous glands may help express sebum.

E. Nails

Important structures

• Nail plate
• Nail matrix
• Nail bed
• Lunula
• Nail folds
• Hyponychium

8. Blood, lymphatic, and nerve supply

A. Blood supply

1. Superficial plexus: near the papillary dermis
2. Deep plexus: near the dermis-hypodermis junction

• Nutrition of the skin
• Temperature regulation
• Wound healing
• Inflammation

B. Lymphatics

C. Nerve supply and sensory receptors

• Touch
• Pressure
• Vibration
• Stretch
• Pain
• Temperature
• Itch

Important receptors

9. High-yield clinical anatomy

10. Quick summary

• The epidermis is an avascular keratinized epithelial barrier.
• Epidermal layers from superficial to deep are: corneum, lucidum, granulosum, spinosum, basale.
• The dermis supplies strength, elasticity, vessels, nerves, glands, and follicles.
• The papillary dermis is superficial and loose; the reticular dermis is deeper and dense.
• The hypodermis is fatty connective tissue that cushions, insulates, anchors skin, and contains larger vessels and nerves.
• Important epidermal cells are keratinocytes, melanocytes, Langerhans cells, and Merkel cells.
• Hair follicles, sebaceous glands, sweat glands, nails, and arrector pili muscles are skin appendages.