Completely surrounding the nucleus, the nuclear envelope sequesters the genomic information of the cell, probably protecting it from the various enzymes and processes that occur within the cytoplasm. It is composed of two concentric membranes, each of which has a distinct protein composition: the outer membrane, which faces the cytoplasm; and the inner membrane, facing the nuclear interior. The inner and outer membranes are separated by the perinuclear space. Both the outer membrane and the perinuclear space are continuous with the endoplasmic reticulum and studded with ribosomes. Any proteins made on the nuclear outer membrane-bound ribosomes drop into the perinuclear space and are transported through the inner membrane into the nucleus. The major transport pathway in and out of the nucleus, however, is thought to be through nuclear pores.
The inner membrane is coated with a mesh-like network of intermediate filaments called the nuclear lamina. Various nuclear structures, including the chromosomes, attach directly to the lamina, which is essential for maintaining the overall architecture and function of the nucleus. Mutations in the lamina proteins, lamin and emerin, can cause the chromosomes to dissociate from the nuclear envelope and disrupt the organization and properties of the nuclear pores, both of which result in embryonic death. In humans, other lamin mutations cause several rare, inherited diseases, including Emery-Dreifuss muscular dystrophy, an inherited form of muscular dystrophy, or Dunnigan-type lipodystrophy, a disease that results in loss of adipose tissue and late-onset, insulin-resistant diabetes beginning at puberty. How lamina protein mutations cause these two diseases is unknown.