Nuclear Sheets in Epithelial and Connective Tissue Cells

Abstract

THIN sheets of nuclear material limited on both sides by the nuclear envelope have been described in several electron microscope studies and referred to as nuclear blebs, pockets, loops or projections. The first term (nuclear sheets) seems to be the most apt to describe all the morphological features of these structures on thin sections¹. They have chiefly been recorded in association with interphase nuclei of human and other mammals (see refs. 2–6), but they have also been studied in amphibia⁷, a bird⁵ and a cyclostome⁵. The remarkable feature of the nuclear sheets is their approximately constant thickness, about 300 and 500 Å⁷. The middle osmiophilic layer² can be related to the strongly osmiophilic material⁸ or to the “line C”⁴ described by others: all these structures have to be referred to the thread-like structural unit or unit thread of heterochromatin forming either envelope-limited sheets or envelope-associated layers⁶. Structural explanations for the formation of the nuclear sheets have been suggested^4,9, but so far their function is unknown, even though it seems likely to be related to the extension of the contact surface between nucleus and cytoplasm.