Pituitary Gland - Hyperplasia

Focal hyperplasia is a frequent spontaneous as well as induced change and is seen more commonly in rats than in mice. The microscopic appearance is variable but consists primarily of a single cell type that blends into the adjacent parenchyma without compression (Figure 1, Figure 2, Figure 3, and Figure 4). Hyperplasia is typically a combination of increased cell number and increased cell size and increases with animal age. On occasion there may be multiple foci of hyperplasia (Figure 4). Angiectasis may occur within a focus of hyperplasia (Figure 5) and may sometimes cause slight compression of adjacent parenchyma.

Focal hyperplasia of the pars intermedia (Figure 6 and Figure 7) is less common than hyperplasia of the pars distalis. The hyperplastic cells in the pars intermedia are similar to normal pars intermedia cells, and identification of a focal proliferative lesion may rely on alteration of growth pattern or asymmetrical enlargement of the pars intermedia since there may be no compression of the adjacent parenchyma. Diffuse hyperplasia may also occur and must be distinguished from a tangential section of the pars intermedia. Diffuse hyperplasia is usually of a specific cell type that can be confirmed by immunohistochemistry and typically represents a physiologic response.

There is a morphologic continuum between hyperplasia and pituitary neoplasia, with compression of the adjacent parenchyma being a primary diagnostic feature of neoplasia. Immunohistochemistry for pituitary hormones can be used to determine cell type.