(nuclear envelope) A double membrane surrounding the nucleus. Each layer has a typical membrane structure (see plasma membrane
) and is 4 nm to 6 nm in width. The outer membrane is studded with ribosomes on the cytoplasm side while the surface of the inner membrane next to the nucleus is smooth. Electron micrographs of many cells reveal connections between the outer membrane and the endoplasmic reticulum. Between the membranes there is a clear space, the perinuclear cisterna. At various points the two membranes fuse and pores are formed. The number of pores varies with the degree of nuclear activity. Extensions of the outer granular or fibrous region of the nucleus form cylindrical complexes within the pores. Their precise function is obscure but they appear to be involved in the transfer of information from nucleus to cytoplasm.