12 Introduction to crusting and scaling

The outermost layer of the skin, the epidermis, is made up of multiple layers of cells (Fig. 12.1). The predominant cell type is the keratinocyte, and the epidermis is divided into basal, spinous, granular (variably present in dogs and cats) and cornified layers depending on the morphological features keratinocytes assume as they undergo progressive differentiation to form the stratum corneum.

Under normal circumstances, the epidermis is replaced every 3–4 weeks. The process of keratinocyte migration and differentiation is complex and carefully controlled. Daughter keratinocytes produced by stem cells of the basal layer of the epidermis travel up through the various layers of the epidermis and undergo a process of maturation and differentiation. The end products of this process are fully keratinized corneocytes, the cells that make up the stratum corneum. The cornified cell is packed with filamentous proteins; it normally does not have a nucleus and is imperceptibly shed from the surface of the stratum corneum, either as an individual cell or as small clusters of cells not visible to the naked eye. This desquamation process is the result of enzymatic breakdown of both the desmosomes (the small connections that bind the cells together) and the intercellular lipid ‘glue’ (present in the stratum corneum).

SCALING

Scaling is the visible accumulation of flakes of stratum corneum on the skin surface or in the hair coat, and can occur for many different reasons. Scale varies in colour and consistency, and may be white, silver, yellow or brown to grey. Scale may be branny, fine, powdery, flaky, greasy, dry, loose, adherent or ‘nit-like’. Scaling may be focal, multifocal or diffuse in distribution.

Many diseases affect the normal maturation, differentiation and desquamation processes, and can result in scaling. The appearance and distribution of the scale varies depending on the causative disease. Diffuse dorsal scaling is seen in association with pruritus as a result of cheyletiellosis (see Chapter 5). Another common cause of scaling with a multifocal distribution is as a result of epidermal collarette formation. These are circular rims of scale that are the remains of pustules after they have ruptured (Fig. 12.2). Diseases associated with epidermal collarettes include pyoderma, demodicosis, dermatophytosis and pemphigus foliaceus. Multifocal patches of scaling evident over the dorsal trunk commonly arise from sites of pyoderma and are due to the formation of epidermal collarettes. Tightly adherent patches of scale which are difficult to remove are seen in some forms of ichthyosis, a rare, congenital disorder occurring mainly in young dogs associated with a failure of breakdown of intercorneocyte adhesion (Fig. 12.3). Follicular casts, nit-like accumulations of scale surrounding hair shafts, are representative of hair follicle pathology and are seen in follicular diseases such as sebaceous adenitis and vitamin A-responsive dermatitis (Fig. 12.4).