Nuclear Envelope

Electron microscopy shows that the nucleus is surrounded by two parallel membranes separated by a narrow space (40-70 nm) called the perinuclear cisterna (Figures 3-2 and 3-4). Together, the paired membranes and the intervening space make up the nuclear envelope. Closely associated with the internal membrane of the nuclear envelope is a protein structure called the fibrous lamina (Figure 3-4), which helps to stabilize the nuclear envelope. The fibrous lamina is composed of three main proteins called lamins A, B, and C. In nondividing cells, chromosomes are associated with the fibrous lamina (Figure 3-5). The pattern of association is regular from cell to cell within a tissue, supporting the conclusion that chromosomes have a definite localization within the nucleus. Polyribosomes are attached to the outer membrane, showing that the nuclear envelope is a part of the endoplasmic reticulum. Proteins synthesized in the polyribosomes attached to the nuclear envelope are temporarily segregated in the perinuclear cisterna. At sites at which the inner and outer membranes of the nuclear envelope fuse, there are gaps, the nuclear pores (Figures 3-6 and 3-7), that provide controlled pathways between the nucleus and the cytoplasm. The pores are not open but show an octagonal pore complex made of more than 100 proteins (Figure 3–8). Because the nuclear envelope is impermeable to ions and molecules of all sizes, the exchange of substances between the nucleus and the cytoplasm is made only through the nuclear pores. Ions and molecules with a diameter up to 9 nm pass freely through the nuclear pore without consuming energy. But molecules and molecular complexes larger than 9 nm are transported by an active process, mediated by receptors, which uses energy from adenosine triphosphate (ATP) and takes place in two stages. First, proteins with one or several nuclear signal locations become attached to specific cytosolic proteins, originating a complex, which is temporarily attached to the nuclear pore complex without using energy. In the second stage, proteins with nuclear signal locations are transferred to the nucleus, using energy from ATP, and the cytosolic protein remains in the cytoplasm. At least part of the ATP energy may be utilized to open the nuclear pore complex to make the passage of large molecules possible. Less is known about the transfer of molecules and molecular complexes, some as large as ribosome subunits, from the nucleus to the cytoplasm.

Figure 3-4

Electron micrograph of a nucleus, showing the heterochromatin (HC) and euchromatin (EC). Unlabeled arrows indicate the nucleolus-associated chromatin around the nucleolus (NU). Arrowheads indicate the perinuclear cisterna. Underneath the cisterna is a layer of heterochromatin, the main component of the so-called nuclear membrane seen under the light microscope. x26,000.

Figure 3-5

Illustration showing the structure, the localization, and the relationship of the nuclear lamina with chromosomes. The drawing also shows that the nuclear pore complex is composed of two protein rings in an octagonal organization. From the cytoplasmic ring, long filaments penetrate the cytosol, and from the intranuclear ring arise filaments that constitute a basketlike structure. The presence of the central cylindrical granule in the nuclear pore is not universally accepted.

Figure 3-6

Electron micrographs of nuclei showing their envelopes composed of two membranes and the nuclear pores (arrows). A, B: Transverse sections; C: A tangential section. Chromatin, frequently condensed below the nuclear envelope, is not usually seen in the pore regions. x80,000.

Figure 3-7

Electron micrograph obtained by cryofracture of a rat intestine cell, showing the two components of the nuclear envelope and the nuclear pores. (Courtesy of P Pinto da Silva.)

Figure 3-8

Simplified representation of two nuclear pore complexes. In this model, the final nuclear portion is seen to be a more continuous structure, in the shape of a ring.

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