Acantholysis is the loss of cohesion between epidermal (or epithelial) cells leading to the formation of intraepidermal clefts, vesicles, or bullae. Primary acantholysis results from the dissolution or separation of the desmosomes between unaltered cells. Secondary acantholysis occurs between damaged cells such as during a viral infection. An example of primary acantholysis in pemphigus vulgaris is shown.

Acanthosis is an increase in the thickness of the prickle cell layer (stratum spinosum) of the epidermis. Acanthosis often results in elongated projections of the epidermis into the dermis, as shown in this example.

Actinic elastosis, also referred to as solar elastosis, is characterized by lightly basophilic, irregular, thickened elastic fibers in the dermis. Individual fibers are sometimes not evident, and there may be only an amorphous mass of lightly basophilic material in the dermis. Elastic tissue stains may be used to highlight actinic elastosis.

Apoptosis is programmed cell death recognizable morphologically by chromatin condensation, cell shrinkage, and hypereosinophilia. Apoptotic cells with no remaining nucleus appear as homogenous, eosinophilic, round structures termed colloid bodies or cytoid bodies. Apoptosis requires energy, transcription of new genes, and protein synthesis.

In ballooning degeneration of the epidermis, marked intracellular edema leads to acantholysis and subsequent intraepidermal vesicle or bulla formation. Ballooning degeneration is characteristic of cutaneous viral infections.

Birefringence is the splitting of a light wave into two waves that have perpendicular polarizations and speed of travel.¹,²,³ Birefringence results from a substance having different indexes of refraction and is thus also referred to as double refraction. Birefringent objects appear as shining bodies on a dark background when viewed with polarized light. Birefringent objects are usually white or yellow in sections stained with hematoxylin and eosin. Collagen and hair are normal structures in the skin that are birefringent, while foreign bodies are the most frequent extraneous birefringent materials.¹ Amyloid is birefringent when stained with Congo red, as shown in the photomicrograph taken using polarized light.

A bulla is a fluid-filled blister greater than 0.5 or 1 cm. in diameter, depending on the author. Bullae may be subcorneal (illustrated), intraepidermal, suprabasilar, or subepidermal.

Colloid bodies are also known as cytoid bodies, Civatte bodies, hyaline bodies, and apoptotic bodies. They are apoptotic epidermal cells (keratinocytes) lacking nuclei and appear as homogeneous, eosinophilic, and round structures. Colloid bodies are not specific for any disease, but they are commonly seen in lupus erythematosus, lichen planus, and graft-versus-host disease.

Decapitation secretion is characteristic of apocrine cells. During decapitation secretion, portions of the apical eosinophilic cytoplasm of the cells are pinched off into the lumina lined by the apocrine cells.

The meaning of the term “dyskeratosis” varies depending on the disease. In acute graft-versus-host disease, lichen planus, and lupus erythematosus, dyskeratotic cells are cells undergoing apoptosis, are smaller than adjacent epidermal keratinocytes, and have brightly eosinophilic cytoplasm and shrunken hyper-basophilic nuclei (see photo under “apoptosis”). In acantholytic dermatosis, the dyskeratotic cells are also termed “corps ronds” and have a central, basophilic, pyknotic nucleus surrounded by a clear halo and enveloped within a basophilic or eosinophilic rim. Neoplastic dyskeratosis is manifest as brightly eosinophilic bodies, sometimes with remnants of nuclei, within a tumor (shown at left). These bodies represent neoplastic cells undergoing apoptosis.

Epidermotropism is the presence of atypical lymphocytes in the epidermis without spongiosis and is characteristic of mycosis fungoides. The atypical lymphocytes in the epidermis may occur singly surrounded by a clear halo, or they may form small clusters referred to as Pautrier microabscesses.

Epithelioid cells are activated macrophages that have an epithelial-like appearance. They are large cells with oval to elongated nuclei, eosinophilic cytoplasm, and indistinct cell borders. They occur singly or may form groups termed granulomas.

Exocytosis refers to the presence of inflammatory cells within the epidermis in conjunction with spongiosis. Exocytosis is characteristic of inflammatory dermatoses.