Pituitary pars intermedia of male viscacha (Lagostomus maximus maximus): a morphometric study of seasonal and age-related changes in immunohistochemistry.

The aim of this work was to study the pituitary pars intermedia in seasonally captured adult male and immature viscachas by immunohistochemistry and image analysis. The pituitary pars intermedia exhibited a well-developed parenchyma with scarce connective tissue and vascularization. It was formed by a close association of melanotrophs and folliculostellate cells. The folliculostellate cells were stellate in shape with cytoplasmic processes, and they originated follicles with PAS-positive colloid inside. The morphometric parameters of melanotrophs, follicular colloid and folliculostellate cells (S-100-ir and GFAP-ir) varied seasonally and in relation to age. These parameters showed minimal values in the adult males captured in winter and in immature animals, and they were maximal in summer. The percentage of vimentin-positive area of the folliculostellate cells was maximal in immature animals, decreased in relation to age and did not vary seasonally in the adult animals. The greatest development of pars intermedia in the adult animals in relation to the immature ones is probably related to the adults' adaptation to the semiarid environment. The expression of the tested proteins suggests a probable neuroectodermic origin for the folliculostellate cells of the viscacha pituitary pars intermedia. In addition, the cytoplasmic processes of folliculostellate cells might originate an intercellular communication network inside the pars intermedia. The decrease in the morphometric parameters melanotrophs, follicular colloid and folliculostellate cells in winter suggests a low endocrine activity of this zone. This fact might be due to the effect of the short photoperiod and high melatonin serum levels.