Skin - Hyperplasia

Epithelial hyperplasia is the most common spontaneous, non-neoplastic lesion of the skin observed in B6C3F1 mice in NTP studies. Hyperplasia of the epithelium of the epidermis and adnexa is also a common response to dermal application of chemicals. In more severe cases, especially when accompanied by inflammation, hyperplasia of follicular epithelium, follicular units, and sebaceous glands often occurs. It is characterized by increased thickness of the nonkeratinized layers of the epidermis due to an increased number (layers) of epithelial cells (Figure 2) compared with the same site in a control animal (Figure 1). The epithelium of the hair follicles may also be affected. This increased thickness may involve all three nonkeratinized layers (basal, spinous, and granular) but may be most apparent in the spinous cell layer. In more severe cases, the follicular epithelium may also be hyperplastic (Figure 2). This lesion can result after a wide variety of injuries and conditions, including spontaneous and experimentally induced inflammatory processes, exposure to irritants or toxic materials, repeated abrasion of the superficial keratin layer, and prolonged exposure to ultraviolet light.

Sebaceous gland hyperplasia is characterized by large sebaceous glands with increased numbers of cells forming numerous lobules around central ducts (Figure 3 and Figure 4). Over the period of a 2-year study, sebaceous hyperplasia has the potential to progress to benign and malignant sebaceous cell neoplasms.

Follicular hyperplasia is characterized by increased numbers of follicular units within the dermis and subcutis (Figure 5). Follicular epithelial hyperplasia is characterized by increased numbers of follicular epithelial cells within the hair follicles; it is typically an extension of epithelial hyperplasia of the epidermis but may be seen in the absence of epidermal hyperplasia.