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Scale represents either an excessive production of the non-living surface layers of the skin, or an abnormal cornification process. It is called dandruff in human medicine.
Excessive scaling is frequently seen in association with skin surface infections, where it has been proposed that there is an attempt by the skin to "push" the infection away from the body by the only physical mechanism the epidermis has, which is to proliferate.
Cornification is the end stage of epithelial development. The keratinocytes undergo a special type of programmed cell death, whereby they use their protein and the lipids in the cells to manufacture a protective "bricks and mortar" structure at the surface of the skin which is relatively impermeable. Primary cornification abnormalities can be congenital, as is seen in the case of icthyosis, which has been recently reported in certain breeds, such as the Golden Retriever or it can be acquired, such as in horses with selenosis or dogs with zinc responsive dermatoses.

Crusts can be labeled depending on their different components.
Haemorrhagic crusts are largely made up of erythrocytes and represent a previous injury with bleeding.
Serocellular crusts are typically inflammatory. The serum and inflammatory cells will either percolate between the keratinocytes, often in association with dermal oedema, or may be associated with erosions and associated inflammatory exudate. Parakeratotic crusts are formed by an abnormal cornification process which may be associated with surface infections (dermatophytes, malassezia yeast, bacteria) or may be a direct result of abnormal keratinocyte development, such as in zinc responsive dermatitis.