THE NUCLEAR ENVELOPE ITs STRUCTURE AND RELATION TO CYTOPLASMIC MEMBRANES BY MICHAEL L. WATSON, PH.D

Interest in the nature and function of the nuclear envelope of cells has flourished sincc its existence was first suspected by early light microscopists and implied by the micromanipulation studies of Kite (I) and Chambers and Fell (2). It has long been evident that if the interphase nudeus exerts fundamental control over such complex cytoplasmic activities as protein synthesis, this control must bc transmitted through the nuclear membrane. Rather strong a priori arguments and a limited amount of experimental data support the view that such control must be mediated, at least in part, by large molecules which can pass between the nucleus and the cytoplasm (Anderson (3)). However, sparsity of unambiguous experimental data leaves the question in an unsatisfactory state (Hogeboom and Schneider (4)). The application of the electron microscope to the study of fine structure of cells has shown rather dearly that the nucleus in interphase does possess an enveloping, membranous sheath. The early studies were made on nuclear membranes teased from the cell and examined directly on specimen grids with the surface of the membranes perpendicular to the optical axis of the microscope. Callan and Tomlin (5) in 1950 reported in an electron microscope study of oocytes of Triturus and Xenopus that the isolated nuclear envelope consisted of two layers, a continuous internal one and a porous, external layer. Bairati and Lehmann (6) and Harris and James (7) found a similar situation in Amoeba proteus except that they ascribed to the two layers a reversed position. Later work utilized thin-sectioning techniques, which permitted examination of the nuclear envelope in sections normal to the nuclear surface as well as tangential to it. Hartmann (8), reporting on nerve cell nuclei, confirmed the finding of double membranes, although he failed to find pores. Bahr and Beermann (9) studying sectioned salivary glands and midgut of Chironornus found ring-shaped structures in sections nearly tangential to the nuclear surface, while in sections normal to it, they were able to demonstrate the presence of pores that penetrated both nuclear membranes. Gall (10) recently reported on isolated nuclear membranes from oocytes of Triturus dried by the critical-point method (11) and showed, in shadowed specimens, circular aggregations of spherical particles disposed as were the pores described in the previously mentioned studies. Ring-shaped structures were found in the nuclear envelopes of cells of the proximal convoluted tubules of mouse kidney by Rhodin (12) which closely resembled, in the published micrographs, the similar structures demonstrated by Bahr and Beermann (9) in tangential sections. 257