The Nuclear Envelope

On the inner side of the nuclear envelope of many cell types is present fibrous material which has been called the fibrous lamina or internal dense lamella.

1. Nuclear membranes. Each of the two membranes of the nuclear envelope appears to have a trilaminar unit membrane structure similar to that of the endoplasmic reticulum (ER). The membranes are 7-8 nm outer nuclear membrane communicates with the endoplasmic reticulum at several points. It has ribosome on the outer sides. Some areas are without ribosomes and appear to participate in vesicle formation (blebbing).

2. Perinuclear space. The space between the two nuclear membranes is 10 – 70 nm (usually 20 nm). It is probably filled with fluid similar to that contained in the ER cisternae. Antibodies have been localized in the perinuclear space in lymphoid cells, indicating that they are produced and/or stored at the site.

3. Nuclear pores. The nuclear envelope is perforated by many apertures called nuclear pores. At the margin of each pore the outer and inner nuclear membrane are continuous. The pores are randomly distributed in many nuclei. In many plant cells they are arranged in rows and in lymphocyte nuclei.



4. The annulus. Sections through the nuclear pore show the presence of an electron-dense ring or cylinder within the pore. The electron-dense material has been called the annulus.

There are three main theories regarding the structure of the nuclear pore.
(i) The annulus as a cylinder fitting into the nuclear pore.
(ii) Annular material extending laterally beyond the pore margin.
(iii) In the model proposed by Kessel (1969) the pores are described as being octagonal.

5. The central granule. A central granule, usually 10 – 15 nm in size, has been described in the pore complex. On the basis of staining properties and sensitivity to RNase it is thought to contain rib nucleoproteins.

6. The fibrous lamia or internal dense lamella. This material forms hexagonal compartments and was, therefore, called the ‘honey comb layer’. It extends for a distance of about 280 nm into the nucleoplasm.