The epidermis is essentially a layering of differentially mature keratinocytes, going from least differentiated (stratum basale) to most differentiated (stratum corneum). The rate and extent of differentiation may change according to physiologic demand, whether due to normal, everyday need (e.g. digital pads) or in response to a disease process (e.g. bacterial dermatitis). In this way, the skin responds to external, and sometimes internal, stimuli in an exceptionally dynamic manner.
Another important point to consider is the energy needs of the integumentary system. The constant proliferation and generation of new keratinocytes is necessary to balance the constant loss of superficial keratinocytes (desquamation). As a result, the skin has an immense nutritional requirement. In the event of suboptimal nutrition, the skin and hair coat may be altered, reflecting this systemic nutritional deficiency. In fact, dogs with chronic malnutrition often have epidermal atrophy and other integumentary changes. Additionally, vitamin and mineral deficiencies (e.g. Vitamin A, zinc, copper, et al.) manifest with integumentary changes including hyperkeratosis, defective hair coat quality, and predisposition to secondary bacterial infections of the skin. Therefore, the skin often reflects both the nutritional and health status of an animal, and is therefore a clinically important aspect of the physical exam.