NUCLEAR ENVELOPE
Nuclear envelope (nuclear membrane, karyolemma, karyotheca) is the structure separating the nucleus of eukaryotic cells from its cytoplasm. Nuclear envelope consists of two parallel lipoprotein membranes. Each membrane is 7 to 8 nm thick. There is a lumen (perinuclear space) between them. The nuclear envelope is perforated by pores 60 to 100 nm in diameter. On their borders the outer membrane of the nuclear envelope turns into the inner membrane. The number of pores is from several ones to 200 on a square micron of the nuclear surface. The edge of each pore is formed of a ring of a dense matter (annulus). There is usually the central element, 15 to 20 nm in diameter, in the inner width of a pore. The central element is connected with the annulus by radial fibrils. These structures form nuclear pore complexes (NPCs) that control the passage of macromolecules ( proteins, ribonucleoproteins, and others) through the pores.