Ultrastructural study of nuclear evagination in neoplastic cells of a cerebral primitive neuroectodermal tumor.

Evaginations of the nuclear envelope containing nuclear material were observed in neoplastic cells of a cerebral primitive neuroectodermal tumor. The possible sequential development of nuclear evaginations included: 1) formation of a segmental naked inner membrane of the nuclear envelope due to loss of the underlying marginal heterochromatin, 2) elevation of the overlying outer nuclear membrane with focal expansion of the perinuclear cistern, 3) outpouching of the inner nuclear membrane forming a small bleb, and 4) protrusion of electron-lucent nuclear material into the bleb forming a nuclear evagination. Some nuclear evaginations were isolated in the cytoplasm and some protruded into the intercellular space. The inner nuclear membrane of the nuclear evagination was devoid of the marginal heterochromatin. Nuclear pores and rupture of membranes with release of the contents of nuclear evaginations into the cytoplasm were not observed. Instead, the nuclear evaginations isolated from the cytoplasm often exhibited degenerative changes with disintegration of their contents and inner nuclear membrane. It is suggested that the nuclear evaginations of the neoplastic cells in the present study may represent a means to eliminate excess nuclear material.