Breakdown and reformation of the nuclear envelope at cell division
In all higher animal and plant forms so far studied, the nucleus is surrounded by an envelope consisting of two membranes separated by a variable distance of the order of 20 to 40 mµ. The continuity of the membranes is interrupted by circular differentiations which have been called “pores” (1) and “annuli” (2, 3). Four lines of evidence have been presented in support of the concept that the nuclear membranes are part of, and at times continuous with the vesicular component of the cytoplasm (endoplasmic reticulum): a) the cytoplasmic surface of the envelope is occasionally covered with dense particles similar to those that cover the flattened vesicles of the endoplasmic reticulum in actively synthesizing cells, as in the pancreas (1); b) actual continuity between the outer membrane and the endoplasmic reticulum has been observed in which the cisternal cavity of the reticulum is continuous with the space between inner and outer nuclear membranes (1); c) strong resemblance has been reported between the nuclear envelope and elements of the stacks of basophilic “annulate lamellae” particularly prominent in the cytoplasm of egg and sperm-forming cells of certain invertebrates (4); d) outpocketings of the nuclear envelope in Drosophila salivary gland cells (5) and in spermatocytes and spermatids of crayfish (6) have been observed which could conceivably give rise to membranous elements of the cytoplasm.
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