Lecture 67: Dermatology- Response to injury

Layers of the skin

Epidermis, dermis, subcutis (hypodermis).

Major cell types in the epidermis

Keratinocytes, melanocytes, Langerhans cells, Merkel cells.

Functions of the skin

Barrier, immune defense, thermoregulation, sensation, metabolism (vitamin D), excretion.

Routes of skin injury

External (physical, chemical, infectious, UV, radiation) or internal (immune, endocrine, metabolic, nutritional).

Epidermal hyperplasia (Acanthosis)

Thickened epidermis from chronic irritation or inflammation.

Epidermal atrophy

Thinning of epidermis due to endocrine disease or malnutrition.

Dyskeratosis

Premature keratinization of individual cells; seen in zinc deficiency and actinic damage.

Acantholysis

Loss of keratinocyte adhesion; typical of pemphigus complex.

Spongiosis

Intercellular edema of epidermis; seen with allergies or eczema.

Vesicle/Bulla formation

Fluid-filled pocket in or under epidermis; seen in viral and autoimmune diseases.

Crust formation

Accumulation of dried serum, cells, and debris on the surface.

Dermal edema

Fluid accumulation due to increased vascular permeability.

Dermal fibrosis

Collagen deposition during chronic inflammation or healing.

Inflammation types in dermis

Neutrophilic (acute), lymphoplasmacytic (chronic), eosinophilic (parasites/allergy).

Pigmentary disorder: hyperpigmentation

Increased melanin due to chronic irritation or endocrine disorder.

Pigmentary disorder: hypopigmentation

Decreased melanin or melanocyte loss; seen in vitiligo and leukoderma.

Albinism

Congenital absence of tyrosinase enzyme → no melanin.

Depigmentation

Autoimmune melanocyte destruction; seen in uveodermatologic syndrome.

Primary seborrhea

Genetic keratinization defect (Cocker Spaniels).

Secondary seborrhea

Due to inflammation, endocrine disease, or nutrition issue.

Ichthyosis

Congenital defect with retained scales ("fish-scale disease").

Epidermal necrosis and ulceration causes

Burns, frostbite, trauma, toxins, vasculitis, infection.

Vasculitis

Inflammation of vessel walls; immune complex deposition or infection; causes purpura, necrosis, ulceration.

Vasculitis histology

Fibrinoid necrosis and thrombosis of vessel walls.

Ischemic necrosis/Infarction

Sharply demarcated dark lesions from loss of blood supply.

Photosensitization

UV damage due to photodynamic compounds or phylloerythrin buildup in non-pigmented skin.

Primary photosensitization

Ingestion of photodynamic compound (e.g., St. John's wort).

Hepatogenous photosensitization

Liver disease increases phylloerythrin (e.g., Lantana, blue-green algae).

Congenital photosensitization

Inherited porphyrin metabolism defect (e.g., bovine congenital erythropoietic porphyria).

Pemphigus foliaceus

Autoantibodies to desmoglein-1; superficial subcorneal pustules and crusts.

Pemphigus vulgaris

Autoantibodies to desmoglein-3; deep suprabasilar clefts and ulcers.

Bullous pemphigoid

Autoantibodies to basement membrane; subepidermal bullae.

Systemic lupus erythematosus (SLE)

Immune complex deposition at dermoepidermal junction; depigmentation, ulcers.

Discoid lupus erythematosus

Localized facial lupus; depigmentation, crusts, ulceration.

Bacterial skin disease: Pyoderma

Staphylococcus pseudintermedius; pustules, crusts, hair loss.

Bacterial skin disease: Greasy Pig Disease

Staphylococcus hyicus; exudative dermatitis in piglets.

Bacterial skin disease: Dermatophilosis

Dermatophilus congolensis; "railroad track" bacteria, crusting dermatitis in wet animals.

Viral skin disease: Poxvirus

Vesicles → pustules → scabs (sheep, cattle, horses).

Viral skin disease: Papillomavirus

Benign warts; may progress to carcinoma.

Viral skin disease: Herpesvirus

Vesiculation, ulceration, intranuclear inclusions.

Fungal: Dermatophytosis (Ringworm)

Microsporum and Trichophyton; circular alopecia and scaling.

Fungal: Malassezia dermatitis

Malassezia pachydermatis; greasy, pruritic skin, secondary infection.

Subcutaneous mycoses

Sporothrix schenckii; nodular, ulcerative lesions along lymphatics.

Parasitic: Demodicosis

Demodex canis; folliculitis, alopecia, scaling.

Parasitic: Sarcoptic mange

Sarcoptes scabiei; crusting and intense pruritus.

Parasitic: Cheyletiella

"Walking dandruff" mites on dogs and rabbits.

Parasitic: Onchocerca

Nodular dermatitis in horses and cattle.

Vasculitis vs infarction: gross difference

Vasculitis = purpura and ulcers; infarction = sharp dark necrosis.

Papilloma

Benign viral skin tumor; exophytic and cauliflower-like.

Squamous cell carcinoma (SCC)

Malignant epithelial tumor; UV-induced; invasive with keratin pearls.

Trichoblastoma

Benign hair follicle tumor in dogs.

Sebaceous adenoma

Benign, common in dogs (eyelids, tails).

Sebaceous carcinoma

Malignant sebaceous gland tumor; rare, often in cats.

Apocrine adenocarcinoma

Malignant sweat gland tumor; metastasizes to lymph nodes.

Perianal gland adenoma

Benign, hormone-dependent, in intact male dogs.

Histiocytoma

Benign "button tumor" of young dogs; self-regresses.

Mast cell tumor

Common in dogs/cats; variable malignancy; eosinophils often present.

Melanoma

Can be benign (haired skin) or malignant (oral/digit); heavily pigmented.

Features of benign neoplasms

Well circumscribed, expansile, few mitoses, no metastasis.

Features of malignant neoplasms

Invasive, ulcerated, anaplastic, high mitotic index, metastasizing.

Mnemonic for epidermal layers

Come, Let's Get Sun Burned (Corneum, Lucidum, Granulosum, Spinosum, Basale).

Mnemonic for epidermal reactions

HAS DAVS (Hyperplasia, Acanthosis, Spongiosis, Dyskeratosis, Acantholysis, Vesicle, Separation).

Mnemonic for photosensitization types

Primary-Hepatic-Congenital (St. John's wort, Lantana, Porphyria).

Mnemonic for pemphigus types

Foliaceus = Flaky (superficial); Vulgaris = Very Deep.

Mnemonic for common fungal skin diseases

Dermatophyte, Malassezia, Sporothrix (surface → yeast → subcutaneous).

Mnemonic for skin tumor origins

Epi, Follicle, Sebaceous, Mast, Melanin (common sources).