Introduction to Dermatopathology Glynis A. Scott, M.D.

The skin is a dynamic organ that responds to it's internal and external environment in many different ways. In order to understand the clinical expression of dermatologic disease and the biology of skin disease, it is essential to have a knowledge Of the way in which the skin reacts at a microscopic level (dermatopathology).

The objectives of this introductory lecture and of the gross and microscopic lectures:

• Familiarize yourself with common terms used to describe microscopic changes in the skin (the vocabulary of dermatopathology).

• Know the microscopic appearance of some common skin diseases (presented below and in big Robbins).

Terms, definitions and overview of microscopic pathology

	Acanthosis is Greek for spine or prickle and indicates a thickening of the skin through an increase in the number of prickle cells (keratinocytes that lie above the basal layer). Biology: acanthosis of the skin may result from two mechanisms increased proliferative activity of the basal cell. decreased exfoliation of terminally differentiated keratinocytes Clinical examples of "a" include psoriasis and chronic irritation (rubbing). Clinical examples of "b" include several hereditary dermatoses such as ichthyosis
	Atrophy is due to a decreased number and vertical diameter of cells and shortening or absence of the rete ridges. Biology: atrophy may result from a wide variety of physical, immunological and iatrogenic stimuli. Whatever the inciting cause, the germinative cells at the basal layer respond by decreasing their proliferative activity. Clinical examples of diseases that are characterized by atrophy include radiation dermatitis (physical), locally applied (iatrogenic) or increased circulating (Cushing's disease) corticosteroids, autoimmune disease (SLE, scleroderma), pressure atrophy (underlying tumors) and vascular insufficiency (stasis dermatitis).
	Hyperkeratosis indicates an increase in the thickness of the most superficial, fully keratinized ("cornifled") layer of the skin. Biology: Because the keratin layer is the final differentiated product of the active replicating and differentiating epidermis, abnormalities of the keratin layer can almost always be traced back to an underlying disturbance in the basal or prickle cell layer. Since a wide variety of diseases, both trivial and serious, can alter the equanimity of the epidermal keratinocytes, hyperkeratosis is a frequent and non-specific finding in the skin. Clinical examples of diseases that show hyperkeratosis include psoriasis, dermatitis, and warts.
	Spongiosis is the microscopic presence of serum within the epidermis in the extracellular space (between the keratinocytes). When a small amount of fluid is present only a subtle widening of the space between keratinocytes may be seen; with larger amounts of serum, microvesicles develop. Biology: Since keratinocytes don't make serum, the only way for serum to make it's way into the epidermis is from the underlying dermis. Serum usually follows inflammatory cells into the epidermis through simple osmotic forces. The most common disease in which spongiosis is seen is dermatitis (e.g., poison ivy).
	Exocytosis is the presence of inflammatory cells in the intercellular space in the epidermis. The presence of inflammatory cells is usually accompanied by spongiosis (see above). Biology: As in other organs, the presence of inflammatory cells in the epidermis usually indicates an immunological response of the body's immune system to a perceived or real antigenic stimulation. The Langerhans cell is a bone marrow derived cell that nn grates to the epidermis during fetal development and acts as the antigen presenting cell in the epidermis.
	Dysplasia is used to describe cells that show aberrant growth and differentiation. Dysplastic cells lie in a continuum between benign and fully malignant. Biology: Dysplasia represents a stop action photograph of one of the earliest and still reversible steps in the multistep process of malignancy. It is reflected microscopically by nuclear hyperchromasia and increased cell size (disordered differentiation) and increased number (aberrant growth). It may affect any epithelial cell in the body. In the epidermis, dysplasia arises in the basal keratinocytes and is reflected by disorganization and enlargement of the cells. Clinically and microscopically this lesion is called an actinic keratosis If the process of malignant transformation continues, the dysplastic cells become fully malignant, invade the dermis, and are now capable of spread beyond the skin (metastasis). Once the malignant cells have invaded the dermis, the lesion is termed squamous cell carcinoma (reflecting the cell of origin).
	Erosion is incomplete loss of the epidermis. This is in contrast with ulceration which is complete loss of the epidermis and frequently a portion of the superficial dermis.