Morphological study of pituitary tumorigenesis in transgenic mice induced by hybrid oncogene of the thyrotropin beta-subunit and the simian virus 40 large T-antigen

We have created a transgenic mouse, TTP-1, generating anterior pituitary tumors by using the simian virus 40 (SV40) large T antigen gene and human thyrotropin beta-subunit gene. To examine characteristics of tumors, histological details were investigated using light and electron microscopies. The main tumor tissues, composed of small chromophobe cells, were located inferior to but clearly separated from the hypothalamus; however, neuron fibers probably derived from the hypothalamus were observed to invade some tumor tissues. Some differentiated endocrine cells occupied the caudal region of the tumor. Immunohistochemically, SV40 large T antigen was expressed in the cell nucleus of the undifferentiated cell area, whereas cells expressing several hormones were mainly distributed in the differentiated cell area. Electron microscopically, the undifferentiated cells were divided into 2 types; electron-dense and -lucent cells, the nuclei of which were composed of obscured nucleoli and many notable invaginations of the nuclear membrane. No intracellular microfilamentous structures were observed. Sometimes it was noted that cytoplasmic processes were connected with gap junctions. In the intercellular spaces, there were neuron fibrous and synapse-like structures. In the differentiated cell area, the cell membranes directly contacting other cells were relatively smooth, and many gap junctions were demonstrated. Secretory granules, which were round and less than 100 nm in diameter, were more electron dense in smaller cells than in larger cells. They were aligned just below the cell membrane. Immuno-electron microscopically, positive reactions for SV40 were observed in the nuclei of the undifferentiated cell area. In the differentiated cell area, most of the secretory granules were labeled by GH. TTP-1 transgenic mice should provide a valuable animal model for studying the pathogenesis of anterior pituitary tumors.