<P> The nuclear envelope surrounds the nucleus, separating its contents from the cytoplasm . It has two membranes, each a lipid bilayer with associated proteins . The outer nuclear membrane is continuous with the rough endoplasmic reticulum membrane, and like that structure, features ribosomes attached to the surface . The outer membrane is also continuous with the inner nuclear membrane since the two layers are fused together at numerous tiny holes called nuclear pores that perforate the nuclear envelope . These pores are about 120 nm in diameter and regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others . Since the nuclear pores are located in an area of high traffic, they play an important role in the physiology of cells . The space between the outer and inner membranes is called the perinuclear space and is joined with the lumen of the rough ER . </P> <P> The nuclear envelope's structure is determined by a network of intermediate filaments (protein filaments). This network is organized into lining similar to mesh called the nuclear lamina, which binds to chromatin, integral membrane proteins, and other nuclear components along the inner surface of the nucleus . The nuclear lamina is thought to help materials inside the nucleus reach the nuclear pores and in the disintegration of the nuclear envelope during mitosis and its reassembly at the end of the process . </P> <P> The nuclear pores are highly efficient at selectively allowing the passage of materials to and from the nucleus, because the nuclear envelope has a considerable amount of traffic . RNA and ribosomal subunits must be continually transferred from the nucleus to the cytoplasm . Histones, gene regulatory proteins, DNA and RNA polymerases, and other substances essential for nuclear activities must be imported from the cytoplasm . The nuclear envelope of a typical mammalian cell contains 3000--4000 pore complexes . If the cell is synthesizing DNA each pore complex needs to transport about 100 histone molecules per minute . If the cell is growing rapidly, each complex also needs to transport about 6 newly assembled large and small ribosomal subunits per minute from the nucleus to the cytosol, where they are used to synthesize proteins . </P> <P> The endoplasmic reticulum (ER) is a membranous synthesis and transport organelle that is an extension of the nuclear envelope . More than half the total membrane in eukaryotic cells is accounted for by the ER . The ER is made up of flattened sacs and branching tubules that are thought to interconnect, so that the ER membrane forms a continuous sheet enclosing a single internal space . This highly convoluted space is called the ER lumen and is also referred to as the ER cisternal space . The lumen takes up about ten percent of the entire cell volume . The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the nuclear envelope, it provides a channel between the nucleus and the cytoplasm . </P>

Why is the nuclear membrane considered part of the endomembrane system