Skin, Dermatopathology, and Connective Tissue

Skin And Connective Tissue Reasoning Map

The epidermis is stratified squamous epithelium organized by differentiation. The basal layer contains mitotically active keratinocytes attached to basement membrane by hemidesmosomes, melanocytes derived from neural crest, and Merkel cells involved in light touch. Basal keratinocytes move upward into the stratum spinosum, where prominent desmosomes create intercellular bridges; this is the layer targeted in pemphigus vulgaris. The stratum granulosum contains keratohyalin granules with filaggrin and loricrin that aggregate keratin and support barrier formation.

The stratum corneum is anucleate keratin in lipid-rich matrix; its integrity prevents water loss and pathogen entry. Thick skin on palms and soles also has a stratum lucidum. Langerhans cells are dendritic antigen-presenting cells in the epidermis and are Birbeck granule positive. The dermis contains type I and III collagen, elastin, vessels, nerves, hair follicles, sebaceous glands, eccrine glands, and apocrine glands.

Melanin biology explains pigment findings. Melanocytes produce eumelanin and pheomelanin in melanosomes and transfer them to keratinocytes. Tyrosinase converts tyrosine to DOPA and then dopaquinone; albinism commonly involves impaired melanin synthesis with normal melanocyte number. Vitiligo is autoimmune melanocyte destruction, producing depigmented macules and association with autoimmune thyroid disease, type 1 diabetes, and pernicious anemia. Postinflammatory hyperpigmentation reflects increased melanin production or pigment incontinence after inflammation.

Acanthosis nigricans is velvety hyperpigmented plaques in intertriginous areas caused by insulin or growth factor stimulation of keratinocytes and dermal fibroblasts; in older patients with abrupt onset, consider malignancy-associated growth factor production.

Blistering diseases are best learned by split level and immunofluorescence. Pemphigus vulgaris is IgG against desmoglein 3, often also desmoglein 1, causing intraepidermal suprabasal acantholysis. Basal cells remain attached to basement membrane, creating a tombstone appearance. Flaccid bullae rupture easily, leaving painful erosions; oral mucosal involvement is common because desmoglein 3 is important there. Nikolsky sign can be positive because lateral epidermal cohesion is weakened. Direct immunofluorescence shows intercellular netlike IgG.

Pemphigus foliaceus targets desmoglein 1, producing superficial crusted erosions without prominent mucosal involvement because mucosa has enough desmoglein 3 compensation. Bullous pemphigoid is IgG against hemidesmosomal BP180 or BP230, causing subepidermal tense bullae, pruritus, eosinophils, and linear IgG and C3 along basement membrane. Dermatitis herpetiformis is intensely pruritic grouped vesicles on extensor surfaces from IgA deposition at dermal papillae; it is associated with celiac disease and improves with gluten avoidance.

Inflammatory dermatoses have characteristic immune patterns. Psoriasis is a chronic plaque disease driven by dendritic cell activation, IL-23, Th17 cells, IL-17, IL-22, TNF, and keratinocyte hyperproliferation. Plaques are sharply demarcated, erythematous, and silvery on extensor surfaces, scalp, and sacrum. Histology shows acanthosis, parakeratosis, thinning of suprapapillary plates, elongated rete ridges, dilated dermal vessels, and neutrophils in stratum corneum forming Munro microabscesses.

Nail pitting and psoriatic arthritis can occur; arthritis may involve DIP joints, dactylitis, enthesitis, or axial disease. Atopic dermatitis is a pruritic eczematous disease of barrier dysfunction, often filaggrin-related, with Th2 skewing, high IgE, asthma, and allergic rhinitis. Acute eczema shows spongiosis, meaning intercellular epidermal edema.

Allergic contact dermatitis is type IV delayed hypersensitivity, classically from nickel, fragrances, poison ivy urushiol, or topical drugs; Langerhans cells present hapten-modified proteins to T cells, and re-exposure produces pruritic vesicles in the exposure pattern. Seborrheic dermatitis affects sebum-rich areas and is associated with Malassezia and Parkinson disease or HIV when severe.

Common infections also localize by morphology. Impetigo is superficial bacterial infection, often S aureus or Streptococcus pyogenes, producing honey-colored crusts; bullous impetigo is caused by exfoliative toxin from S aureus cleaving desmoglein 1. Cellulitis involves deeper dermis and subcutaneous tissue with warmth, erythema, swelling, and pain; erysipelas is more superficial with raised sharply demarcated borders. Necrotizing fasciitis causes severe pain out of proportion, systemic toxicity, bullae, crepitus if gas-forming organisms are present, and fascial necrosis requiring urgent debridement.

Dermatophytes digest keratin and infect stratum corneum, hair, and nails; tinea corporis has annular scaly plaques with central clearing. Candida favors moist areas and forms satellite pustules. Verrucae are HPV-driven epidermal hyperplasia with koilocytes.

Skin cancer questions focus on cell of origin and invasion. Actinic keratosis is UV-induced dysplasia of keratinocytes on sun-exposed skin and can progress to squamous cell carcinoma. Squamous cell carcinoma arises from keratinocytes, often in sun-exposed skin, chronic wounds, scars, immunosuppression, arsenic exposure, or HPV-associated anogenital sites. Histology may show keratin pearls and intercellular bridges; it can metastasize, especially on lip, ear, genital skin, or chronic ulcer.

Basal cell carcinoma arises from basal keratinocytes, is linked to UV and patched pathway activation, and presents as pearly papules with telangiectasias, rolled border, or ulceration. It is locally invasive but rarely metastasizes. Melanoma arises from melanocytes and is the most lethal common skin cancer because it invades early and metastasizes. Risk factors include intermittent intense UV exposure, blistering sunburns, fair skin, family history, many nevi, dysplastic nevi, and immunosuppression. Evaluate asymmetry, border irregularity, color variation, diameter, and evolution.

Superficial spreading is common; nodular melanoma grows vertically early; lentigo maligna occurs on chronically sun-damaged skin; acral lentiginous occurs on palms, soles, and nail beds and is less linked to UV. Breslow depth is the most important prognostic histologic factor.

Connective tissue disease is biochemistry made clinical. Collagen synthesis begins with translation of preprocollagen, hydroxylation of proline and lysine residues in the rough endoplasmic reticulum, glycosylation, triple helix formation, secretion, cleavage to tropocollagen, and extracellular cross-linking by lysyl oxidase. Vitamin C is required for hydroxylation, so scurvy causes swollen gums, perifollicular hemorrhage, corkscrew hairs, poor wound healing, anemia, and bone pain.

Copper is needed for lysyl oxidase; Menkes disease impairs copper transport, causing kinky hair, neurodegeneration, hypotonia, and weak connective tissue. Ehlers-Danlos syndromes involve defective collagen synthesis or processing, producing hyperextensible skin, joint hypermobility, easy bruising, poor wound healing, and in vascular forms arterial or organ rupture due to type III collagen defects. Osteogenesis imperfecta is usually type I collagen abnormality with brittle bones, blue sclerae, hearing loss, dental imperfections, and recurrent fractures.

Elastin and fibrillin mechanisms explain vascular and lens findings. Marfan syndrome is caused by fibrillin-1 defects, reducing microfibril integrity and increasing TGF-beta signaling. Findings include tall habitus, long extremities, arachnodactyly, pectus deformity, scoliosis, lens subluxation upward and outward, mitral valve prolapse, and cystic medial degeneration with aortic root dilation and dissection risk.

Homocystinuria can mimic marfanoid habitus but classically causes downward and inward lens subluxation, intellectual disability, thrombosis, and osteoporosis; cystathionine beta-synthase deficiency impairs homocysteine conversion to cystathionine. Cutis laxa involves defective elastin structure or processing with loose inelastic skin and possible emphysema or vascular disease. Pseudoxanthoma elasticum is mineralization and fragmentation of elastic fibers, causing yellow papules in flexural skin, angioid streaks in retina, and premature atherosclerosis.

Wound healing follows predictable phases. Hemostasis is immediate: vasoconstriction, platelet plug, and fibrin clot. Inflammation follows as neutrophils remove bacteria and debris during the first 24-48 hours, then macrophages become dominant and release growth factors such as TGF-beta, PDGF, FGF, and VEGF. The proliferative phase builds granulation tissue with angiogenesis, fibroblast migration, type III collagen deposition, and re-epithelialization from wound edges and adnexal structures. Myofibroblasts contract wounds.

Remodeling replaces type III collagen with stronger type I collagen through matrix metalloproteinases and cross-linking. Tensile strength increases for months but never returns fully to unwounded skin. Diabetes, poor perfusion, infection, glucocorticoids, malnutrition, vitamin C deficiency, zinc deficiency, smoking, and foreign bodies delay healing. Keloids and hypertrophic scars reflect excess collagen deposition; keloids extend beyond original wound margins and are more common in darker skin. Proud flesh is exuberant granulation tissue that can impair re-epithelialization.