Page 618 - Concise Pathology for Exam Preparation ( PDFDrive )

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The Skin

Normal skin is composed of different cell types, namely:
• Squamous epithelial cells or keratinocytes (produce keratin, defensins and cytokines
responsible for regulation of proliferation and differentiation of adjacent epidermal cells,
as well as cells in the dermis).
• Melanocytes (responsible for production of melanin).
• Langerhans cells (epidermal dendritic cells that process antigens).
• Merkel cells (reside within the basal layer and function as the neuroendocrine cells of
the skin).

Layers of Skin
1. Epidermis - Composed of stratified squamous epithelium with the following layers:
(a) Stratum basale: Contains actively dividing stem cells along the basement mem-
brane. As the basal cells divide, daughter cells migrate upwards.
(b) Stratum spinosum: Intercellular bridges called desmosomes link the cells together.
The cells are polygonal and become increasingly flattened as they move upwards.
(c) Stratum granulosum: Constituted by 1–3 layers of flat cells with keratohyalin
basophilic granules.
(d) Stratum corneum: Contains anucleate cells with keratin.
2. Dermis - Consists of two parts:
(a) Superficial papillary dermis
(b) Deep reticular dermis
Dermis contains specialized appendages called adnexal structures, eg, hair. Hair folli-
cles produce hair shafts and are closely associated with sebaceous (oil-producing) glands
and erector pilaris muscle. Sweat glands guard against the deleterious effects of tempera-
ture variations.

Definitions of Macroscopic Terms
• Macule: Circumscribed flat lesion up to 5 mm in diameter, distinguished from the sur-
rounding skin by its coloration, without any alteration in the texture of the skin
• Patch: Circumscribed flat lesion more than 5 mm in diameter, distinguished from the
surrounding skin by its coloration
• Papule: Circumcised solid, dome-shaped or flat-topped lesions, 5 mm or less in size
• Nodule: Solid, raised and bumpy lesion with spherical contour greater than 5 mm
• Plaque: Elevated, flat-topped lesion greater than 5 mm across
• Vesicle: Fluid-filled, raised lesion 5 mm or less across
• Bulla: Fluid-filled, raised lesion more than 5 mm across
• Blister: Common term used for vesicle or bulla
• Pustule: Discrete pus-filled lesion
• Wheal: Itchy, transient, elevated lesion formed as a result of dermal oedema
• Scale: Dry, horny, plate-like excrescence due to imperfect cornification
• Lichenification: Thickened and rough skin with prominent skin markings, usually a
result of frequent rubbing
• Excoriation: Raw, linear lesion due to breakage of epidermis, subsequent to trauma
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