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Skin Layers - Complete Notes

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Overview

1. Epidermis - outer epithelial layer (ectodermal origin)
2. Dermis - underlying connective tissue layer (mesodermal origin)
3. Subcutaneous tissue / Hypodermis - deepest layer of loose connective tissue and fat

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1. EPIDERMIS

• Thin skin (everywhere except palms/soles): epidermis is 75-150 μm thick
• Thick skin (palms and soles): epidermis is 400-600 μm thick
• Epidermis is avascular - cells receive nutrients by diffusion from dermis
• Turnover time: 15-30 days depending on age and body region

The 5 Layers (Strata) of the Epidermis

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Layer 1: Stratum Basale (Basal Layer)

• Single layer of cuboidal to low columnar cells on the basement membrane
• Contains stem cells - intense mitotic activity
• Cells are basophilic (dark-staining due to closely spaced nuclei)
• Contains melanin granules transferred from neighboring melanocytes
• Keratinocytes connected to each other by desmosomes; attached to basal lamina by hemidesmosomes
• Begins synthesis of keratin intermediate filaments (~10 nm diameter)
• As new keratinocytes form, they migrate upward into the next layer
• Also houses Merkel cells (mechanoreceptors) and melanocytes

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Layer 2: Stratum Spinosum (Spinous/Prickle Cell Layer)

• Several cells thick; cells are larger than basal cells
• Cells exhibit numerous cytoplasmic spines (short processes extending from cell to cell)
• Spines connected to adjacent cell spines by desmosomes
• The desmosome junction appears as a thickening called the node of Bizzozero
• Cells appear "spiny" in H&E because cells shrink during preparation, expanding intercellular spaces
• Continues synthesis of keratin filaments (tonofibrils)
• In the upper spinous layer, cells begin producing keratohyalin granules and lamellar bodies (membrane-coating granules containing glycolipids)
• Langerhans cells (antigen-presenting dendritic cells) are found here

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Layer 3: Stratum Granulosum (Granular Layer)

• 3-5 cell layers thick
• Contains numerous intensely basophilic keratohyalin granules (visible by LM) - these contain filaggrin and loricrin
• Contains lamellar bodies (Odland bodies) - discharge their lipid contents into intercellular space forming the water barrier of skin
• Cells begin to exhibit apoptotic nuclear morphology (DNA fragmentation) - terminal differentiation
• pH is approximately neutral (~7.17)
• As cells move upward, nucleus and organelles break down
• The stratum lucidum exists just above this layer in thick skin only

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Layer 4: Stratum Lucidum (Clear Layer)

• Thin, translucent, pale-staining band of flattened cells
• Limited to thick skin; considered a subdivision of the stratum corneum
• Cells lack nuclei and organelles; filled with eleidin (a transformation product of keratohyalin)
• Located between stratum granulosum and stratum corneum

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Layer 5: Stratum Corneum (Cornified/Horny Layer)

• Outermost, most superficial layer
• Composed of dead, flat, anucleate keratinocytes completely filled with keratin filaments
• Cells are called corneocytes or squames
• Plasma membrane thickened and reinforced by cross-linked proteins (cornified envelope)
• Intercellular spaces filled with lipids discharged from lamellar bodies - forms the permeability barrier against water loss
• pH acidic: 4.5-6.0 at the surface (acid mantle)
• Cells are regularly shed (desquamation) via proteolytic degradation of desmosomes
  • Controlled by kallikrein-related serine peptidases (KLK5, 7, 14) in a pH-dependent manner
  • Inhibited by LEKTI (lymphoepithelial Kazal-type inhibitor, encoded by SPINK5) at neutral pH
  • At acidic pH near surface, LEKTI releases KLKs, which degrade desmosomes, causing shedding
  • Mutation in SPINK5 causes Netherton syndrome

Clinical note: Average stratum corneum thickness does not change with age, but barrier function declines. Skin of palms/soles may form corns and calluses from chronic friction.

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Non-Keratinocyte Cells of the Epidermis

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2. DERMIS

Dermal-Epidermal Junction

• The dermis surface is very irregular, with dermal papillae that interdigitate with epidermal ridges/pegs
• Reinforces adhesion between layers, especially in skin under frequent pressure
• A basement membrane (containing type IV collagen, laminin) always lies between the basal epidermal layer and dermis
• Damage to the dermal-epidermal junction causes bullous pemphigoid (autoimmune); damage to intercellular keratinocyte junctions causes pemphigus

Two Sublayers of the Dermis

Papillary Layer (superficial)

• Thin, loose connective tissue
• Contains types I and III collagen fibers, fibroblasts, scattered mast cells, dendritic cells, and leukocytes
• Anchoring fibrils of type VII collagen insert into basal lamina to bind dermis to epidermis
• Rich in capillaries forming the subpapillary plexus just below epidermis
• Contains sensory nerve endings

Reticular Layer (deep)

• Much thicker; dense irregular connective tissue
• Mainly bundles of type I collagen (thicker and denser than papillary)
• Rich network of elastic fibers (providing skin elasticity)
• Abundant proteoglycans (rich in dermatan sulfate) among the fibers
• Fewer cells than papillary layer
• Damage to this layer results in permanent scarring

Aging changes: Thickening and cross-linking of collagen, loss of hyaluronan and GAGs, loss of elastic fibers (especially from UV exposure - "solar elastosis") causes skin fragility, loss of suppleness, and wrinkling.

Vascular Supply of the Dermis

1. Subpapillary plexus - between papillary and reticular dermis; capillary branches extend into dermal papillae
2. Deep plexus - near dermis-subcutaneous tissue interface; larger vessels
3. Arteriovenous anastomoses (shunts) - between the two plexuses; regulate thermoregulation (constrict to conserve heat; dilate to lose heat)

Nerve Supply of the Dermis

• Sensory afferent fibers network in papillary dermis and around hair follicles
• Autonomic fibers supply smooth muscle of blood vessels and arrector pili muscles, and sweat glands
• Key receptors:
  • Meissner's corpuscles - light touch (dermal papillae)
  • Pacinian corpuscles - deep pressure and vibration (deep dermis/hypodermis)
  • Ruffini endings - sustained pressure and skin stretch
  • Free nerve endings - pain, temperature

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3. SUBCUTANEOUS TISSUE (Hypodermis)

• Lies beneath the dermis
• Loose connective tissue + pads of adipocytes
• Binds skin loosely to underlying tissues (muscles, fascia)
• Corresponds to the superficial fascia of gross anatomy
• Functions:
  • Energy storage (fat)
  • Thermal insulation
  • Mechanical cushioning (shock absorption)
  • Route for blood vessels, lymphatics, and nerves entering the skin

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Summary Table: Epidermal Layers

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Functions of Skin

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• Junqueira's Basic Histology - Text and Atlas, 17e, Ch. 18 (Skin)
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology
• Fitzpatrick's Dermatology, Vol. 1 & 2
• Dermatology 2-Volume Set, 5e
• K.J. Lee's Essential Otolaryngology (Skin Anatomy)

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Vicryl (Polyglactin 910) - Complete Notes

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1. Basic Identity

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2. Chemical Composition

• Copolymer of lactide and glycolide in a ratio of 90:10
• The water-repelling properties of the lactide component delay penetration of water and slow the loss of tensile strength compared to polyglycolic acid (Dexon)
• Coating with polyglactin 370 and calcium stearate serves as a lubricant, allowing the suture to pass more easily through tissue and making it more pliable for knot tying

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3. Physical Configuration

• Structure: Braided multifilament
• Color: Undyed (white/off-white) or dyed violet (violet dye usually dissipates, but occasionally may be visible through skin - some surgeons avoid violet for subcuticular use)
• Sizes available: Full range from 10-0 to #2 (USP scale)

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4. Tensile Strength Profile

• Defined as absorbable because it loses half its tensile strength within 2 months of placement
• Stronger tensile strength retention vs. polyglycolic acid (Dexon), which retains only 20% at 3 weeks
• Weaker than PDS (polydioxanone), which retains ~70% at 2 weeks and ~50% at 4 weeks

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5. Absorption / Degradation

• Mechanism: Hydrolysis (not enzymatic degradation like catgut)
  • Water penetrates the suture and breaks down the ester bonds in the polymer chain
  • This makes absorption more predictable and less variable between patients vs. catgut
• Timeline:
  • Hydrolysis is minimal until 5-6 weeks
  • Complete absorption: 60-90 days (some sources say up to 90 days)
  • Bailey & Love: complete at 60-90 days
  • Dermatology 2-Volume Set: generally complete by 90 days
  • Contrast: Polyglycolic acid (Dexon) may still be absorbing at 120 days

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6. Tissue Reactivity

• Mild tissue reaction (much less than catgut/chromic gut)
• As an absorbable suture, Vicryl produces slightly more inflammatory response than non-absorbable sutures generally, but far less than natural sutures
• Because it is braided (multifilament), it has higher capillarity than monofilament sutures - fluid and bacteria can be trapped between strands, giving it slightly higher infection risk vs. monofilaments like Monocryl or PDS
• Good knot security due to low memory (braided structure)

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7. Handling Characteristics

• Easy handling - one of Vicryl's most praised features
• Holds knots well due to braided structure (low memory)
• Does not tear tissue
• Coated surface allows smooth passage through tissue
• More pliable than monofilament sutures like PDS - easier to tie
• Passes through tissue easily despite braided design

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8. Clinical Uses

General Surgery

• Bowel anastomoses (gastrointestinal surgery) - ureterovesical, ureteroileal, and other anastomoses
• Fascial and muscle closure
• Subcutaneous fat approximation
• Vascular ligatures (vessel ligation)
• Has become the "workhorse" suture for many general surgical applications
• Subcuticular wound closures (undyed form preferred)

Dermatology / Plastic Surgery

• Most popular buried absorbable dermal suture in cutaneous surgery
• Deep dermal layer closure (subcutaneous buried interrupted suture)
• Running cutaneous suture when used on the surface (removed in 5-14 days)
• Excellent for vermilion lip - does not irritate mucosa, causes less inflammation than silk
• Subcutaneous fat closure: 2-0 Vicryl
• Skin subcuticular closure: 4-0 Vicryl

Ophthalmology

• Ophthalmic surgery (fine sizes, e.g. 6-0)
• Eyelid procedures, periorbital closures

Urology

• Bladder, ureter, and renal pelvis closures
• Anastomoses in intra-abdominal urologic surgery

Obstetrics & Gynecology

• Anterior colporrhaphy
• Uterine closure, fascial repair

ENT / Head & Neck Surgery

• Mucosal closures
• Subcuticular skin closures (rhinoplasty - 6/0 polyglactin)

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9. Vicryl Variants

Vicryl Rapide®

• Partially hydrolyzed form of polyglactin 910 (pre-treated to speed absorption)
• Tensile strength:
  • ~50% at 5 days
  • 0% at 14 days
• Absorption: Complete in ~6 weeks (42 days)
• Key advantage: Does not need to be removed - spontaneously dislodges in 7-14 days
• Indications:
  • Wound edge approximation where only 1-2 weeks of support needed
  • Mucosa (oral, tongue, genital)
  • Pediatric skin closure (avoids painful removal)
  • Percutaneous use where suture removal is inconvenient or painful
  • Dermabrasion, skin grafts
  • Lingual/tongue lacerations
  • Subcuticular closures in ENT (rhinoplasty, parathyroid, parotid surgery)

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10. Tensile Strength Comparison with Other Absorbables

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11. Contraindications / When NOT to Use Vicryl

• Tissues requiring prolonged support (e.g., high-tension fascial repairs, hernia mesh - PDS or Maxon preferred)
• Cardiovascular prostheses or vascular graft anastomoses (non-absorbable preferred)
• Infected wounds - braided structure increases bacterial colonization risk vs. monofilament
• When prolonged approximation under stress is needed
• Fascia closure in laparotomy - fast absorbables carry risk of dehiscence, evisceration, and incisional hernia (PDS preferred for abdominal fascial mass closure)

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12. Advantages Over Predecessors

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13. Suture Gauge / Size Selection (USP Scale)

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14. Key Points Summary

1. Synthetic braided absorbable suture (copolymer glycolide 90% + L-lactide 10%)
2. Absorbed by hydrolysis - predictable, patient-independent
3. Maintains 75% tensile strength at 2 weeks, 50% at 3 weeks
4. Full absorption in 60-90 days
5. Mild tissue reactivity - significantly less than catgut
6. Excellent knot security (braided = low memory)
7. Coated with polyglactin 370 + calcium stearate for smooth tissue passage
8. The most popular buried absorbable suture in cutaneous surgery
9. Available as Vicryl Rapide (faster absorption, no removal needed, self-dissolves in 7-14 days)
10. Available dyed violet (usually dissipates) or undyed
11. Workhorse suture for general, urologic, gynecologic, and dermatologic surgery

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• Dermatology 2-Volume Set, 5e - Ch. on Suture Materials
• Bailey & Love's Short Practice of Surgery, 28th Ed. - Table 7.2
• Campbell-Walsh-Wein Urology, 3-Volume Set - Suture Materials
• Fitzpatrick's Dermatology, Vol. 1 & 2 - Suture Materials
• The Harriet Lane Handbook, 23rd Ed. - Absorbable Sutures Table
• Roberts & Hedges' Clinical Procedures in Emergency Medicine