2. Cell nucleus

Components:

1. nucleus membrane (envelope)
   
2. nucleolus


3. chromatine

Most of the cells have 1 round-shaped, loosely-packed nucleus.

Variations:

• relatively small nucleus with tightly backed contents (e.g. cells lining the blood spaces (sinusoids) of the liver (you can't see nucleolus in these nuclei);


• extraordinarily large nucleus (megakaryocytes);


• nucleus of distinctive shape (deeply segmented nucleus of neutrophils);


• binucleated (occasionally some hepatocytes and muscle cells of the heart);


• multinucleated (osteoclasts and sceletal muscle cells);


• without nucleus (erythrocytes, platelets).

Nuclear membrane is composed of 2 parallel unit membranes that fuse with each other at certain regions to form perforations knows as nuclear pores. It can be seen in EM.

• perinuclear cisterna (a 10 - to 30-nm space between two membranes);
  
• inner nuclear membrane (about 6 nm thick, faces the nuclear contact). It's in the close contact with the nuclear lamina which is made up of three polypeptides, called lamins (lamins A, B and C). It's a meshwork of intermediate filaments, 80 to 100 nm thick. Peripheral chromatin adheres to the inner aspectof the nuclear lamina. Certain integral proteins of the inner nuclear membrane act either directly or via other nuclear matrix proteins as contact sites for the nuclear RNAs and chromosomes.
  
• outer nuclear membrane (about 6 nm thick, is studded with ribosomes and faces the cytoplasm). It's continous with rough endoplasmatic reticulum. Its cytoplasmatic surface is surrounded by a thin, loose meswork of the intermediate filaments, vimentin. Its cytoplasmatic surface usually possesses ribosomes, actively synthesizing transmembrane proteins that are destined for the outer or inner nuclear membrane.
  
• nuclear pores (there are 20 - 3000 in one nucleus). It's surrounded by nonmembranous structures embedded in its rim - nuclear polar complex (NPC). It guards selectively passage through the pore.

Nuclear pore complex is composed of nuclear pore and its associated glycoproteins.

- cytoplasmic ring

- middle ring

- nucleoplasmic ring