Early stages of development of rat brain tumors induced by JC virus: a sequential histological and immunohistochemical study.

In order to clarify the origin of JC virus-induced brain tumors in rats, the development of tumors was sequentially analyzed histologically and immunohistochemically. Twenty-two of 30 rats (73%), which were intracerebrally inoculated with JC virus within 24 h of birth (group 1), developed, as a group, 45 brain tumors after 12 to 26 weeks. Seventeen of 27 rats (63%), which were inoculated on the 7th day after birth (group 2), developed 37 brain tumors as a group after a time 12 to 40 weeks. The tumors were found exclusively in the cerebrum. The microtumors, which were defined as tumors less than 2 mm in diameter, were located in the subependymal plate around the ventricular system. The microtumors and most part of the macrotumors consisted of cells of undifferentiated neuroectodermal nature, showing nuclear palisades and Homer-Wright-pseudorosette-like structures. Some tumor cells of macrotumors had an astrocytic nature and were positive for glial fibrillary acidic protein, S-100, Leu 7, and vimentin. In conclusion, the target cells of JC virus in rats may be undifferentiated subependymal cells of the cerebrum. The tumor cells show partial glial differentiation as they grow.