Chapter 10: Skin Pathology

Skin Pathology begins by distinguishing between various disorders of pigmentation, such as vitiligo, which involves the autoimmune destruction of melanocytes, and hyperpigmented conditions like melasma ("mask of pregnancy") and ephelides (freckles). A significant portion of the text is dedicated to melanocytic tumors, tracing the characteristics of benign congenital and nevocellular nevi versus dysplastic nevi, which may indicate a familial syndrome linked to chromosome 1. The summary provides a detailed analysis of malignant melanoma, emphasizing critical risk factors including UV exposure and genetic mutations in genes like CDKN2A and BRAF. It outlines the clinical presentation of melanoma subtypes—lentigo maligna, superficial spreading, acral lentiginous, and nodular—and explains that prognosis is primarily determined by the vertical depth of invasion (Breslow thickness). The chapter also explores benign epidermal and dermal lesions, such as acanthosis nigricans (associated with insulin resistance or malignancy) and seborrheic keratosis, a common benign neoplasm with a "stuck-on" appearance that can signal internal cancer if multiple lesions appear suddenly (Sign of Leser-Trelat). Furthermore, the text examines inflammatory conditions like psoriasis, characterized by autoimmune-driven keratinocyte proliferation and Munro microabscesses, and eczema, often linked to filaggrin gene mutations. A key section differentiates autoimmune blistering diseases based on their molecular targets: pemphigus vulgaris involves antibodies against desmoglein 3 causing flaccid intraepidermal blisters, while bullous pemphigoid targets hemidesmosomes resulting in tense subepidermal bullae. Additional topics include dermatitis herpetiformis (associated with celiac disease), porphyria cutanea tarda, and infectious manifestations like verrucae (HPV). Finally, the chapter contrasts the two major non-melanoma skin cancers: squamous cell carcinoma, often arising from actinic keratosis precursors and showing keratin pearls, and basal cell carcinoma, the most common skin cancer presenting as pearly papules with peripheral palisading of nuclei.