Elongated nuclear sheet and intranuclear myelin figure of human medulloblastoma.

Nuclei of medulloblastoma cells revealed two types of structural changes: formations of cistern-limited nuclear sheets and intranuclear vacuolar profiles or myelin figures. The cistern-limited nuclear sheets were extremely elongated and were contained on either side by the perinuclear cisterns. They were usually composed of a central granular or beaded heterochromatin layer between less dense layers, were further limited on either side by the inner nuclear membrane, and showed an approximately constant width of between 350 and 430 A. The central granular heterochromatin layer in the cistern-limited nuclear sheet was continuous with the peripheral beaded layer of the heterochromatin granules in the nuclei. In addition, in the nuclear periphery a less dense layer was seen between the inner nuclear membrane and the peripheral heterochromatin layer. The structural association of the inner nuclear membrane and its underlying less dense layer, as well as the peripheral heterochromatin layer, was similar in the cistern-limited nuclear sheet and in the nuclear periphery and seemed to form a structural and functional unit. The intranuclear vacuolar profiles and myelin figures were irregular in form and might be derived from the nuclear membranes by a complicated, irregular invagination. The membranes of the vacuolar profiles and myelin figures demonstrated increased density and thickness and surrounded flocculent substances or islands of cytoplasm and were not covered by the heterochromatin layers.