Robert Brown (1831) discovered nucleus in cells of an orchid. It is the principal organelle of the cell. The membrane-bound nucleus is the most prominent feature of the eucaryotic cells. In prokaryotic cells it is membraneless but performs similar function. Some cells like sieve tubes of vascular plants and RBC of mammals possess nucleus when they are young, but it degenerates when cells mature.
Generally, cells are uninucleate but binucleate condition is seen in certain protozoa like Paramecium. Other cells, like the cells of striated muscles or the latex vessels of higher plants become multinucleate. Some organisms like certain fungi such as Rhizopus and alga, Vaucheria contain several nuclei due to lack of cross walls in their mycelium or filament respectively and are called coenocytic. However, the uninucleate condition is typical for vast majority of cells
The shape and size of nucleus varies greatly. The nucleus is generally spherical but in plant cells due to pressure of a central vacuole the nucleus may be pushed against the cell wall and becomes lens shaped. In some insects and RBC, the nucleus is highly irregular.
The nucleus can easily be distinguished into 4 parts the nuclear membrane, nucleoplasm, chromatin and nucleolus.
The nuclear envelope separates the nucleus from the cytoplasm. It consists of porous double membranes separated from each other by a perinuclear space of varying width. The boundary is called nuclear membrane. The lumen of the envelope is often continuous with the lumen of the rough endoplasmic reticulum. The nuclear membrane contains a number of nuclear pores which are generally octagonal and their diameter varies from 300-1000Å.
They maintain continuity between cytoplasm and nucleoplasm. However, the pores are plugged by a protein material called annulus and regulate the passage of ions and small molecules. The interphase nucleus shows an intact nuclear membrane which disintegrates during cell division and reappears after the nuclear division is complete.
The nucleus is filled with a transparent semi fluid, granular and acidophilic ground substance called the nuclear sap, nucleoplasm, karyolymph or karyoplasm. Nucleolus and chromatin materials are found scattered in the sap. The nucleoplasm is generally made of nucleic acids, proteins, enzymes, lipids and minerals. The chromatin is a network of dark-staining fibres which organizes into definite number of condensed bodies called the chromosomes prior to nuclear division. This constitutes the hereditary part of the cell.