The histological, immunohistochemical and ultrastructural alterations in 81 pituitary allografts from Lewis rats transplanted beneath the renal
capsule of Wistar rats were investigated. Intrasellar pituitaries of rats bearing allografts were also examined. Recipient rats were sacrificed at various time points after
transplantation. Two days after
transplantation, the central portion of the allografts demonstrated ischaemic
necrosis. A week later, massive mononuclear cell infiltrates consisting primarily of lymphocytes and to a lesser extent, macrophages, plasma cells and granulocytes became prominent. At about three to four weeks after
transplantation, the mononuclear cell infiltrate diminished; the surviving adenohypophysial cells, mainly
prolactin (PRL) cells, increased in number and
necrosis was replaced by connective tissue. No histological changes were noted in the intrasellar pituitaries of rats bearing allografts. Immunohistochemistry demonstrated that the surviving adenohypophysial cells were mainly PRL-producing cells. Electron microscopy revealed adenohypophysial cell destruction, a spectrum of inflammatory cells and, in late phase, accumulation of fibroblasts and
collagen fibres. PRL cells were the prominent cell types; they increased in number. It appears that pituitary allografts are 'foreign' and evoke an immune response, suggesting that they may be used as an experimental animal model for morphological investigation of the development and progression of adenohypophysitis, a
rare disease occurring mainly in young women often associated with pregnancy.