This article provides information about the Position and Morphology of Nucleus!
Nucleus (L., nux, nut) is the most important part of the cell situated in the cytoplasm.
All the cellular activities are controlled by it. Nucleus is a directing and organizing unit without which the cell could not exist.
It was discovered by Robert Brown (1831) in flowering plants and is now recognized as the structure that contains the hereditary material of the cell.
The study of nucleus or karyosome constitutes karyology (Gr., Karion, and nucleus).
The location of nucleus varies in the cell depending upon the species. Usually it is situated in the centre of the cell surrounded on all sides by cytoplasm. In green algae, Acetabularia, it shows various positions, though mainly present in the basal part of cell. Generally the nuclei are scattered in the cytoplasm.
The shape of nucleus is variable according to cell type. It is generally spheroid but ellipsoid or flattened nuclei may also occur in certain cells. The nuclear margins are generally smooth but they may be lobulated bearing small infoldings of nuclear membrane as in leucocytes. In certain white blood corpuscles the nucleus is dumbbell-shaped and exhibits variation during life history stages. In human neutrophil, it is trilobed.
Mostly cell contains a single nucleus, known as mononucleate cell. Cells containing two nuclei are known as binucleate cells (e.g., Paramecium), and cells of cartilage and liver. Sometimes more than two nuclei (3 to 100 nuclei) are present in a single cell. Such cells are called polynucleate cells.
Such cells in animals are called syncytial cells (e.g., osteoblast) and such plant cells are termed coenocytes (e.g., siphonal algae). Cells having distinct nucleus are called eukaryotic types, whereas cells without definite nucleus are called prokaryotic cells, (e.g., bacteria, etc.). The latter possess scattered chromatin material (DNA) in the cytoplasm.
The mammalian erythrocytes (eukaryotic cells) possess no nuclei. In certain algae, like Vaucheria and some fungi (e.g., black bread mold, Rhizopus nigricans) there occur numerous nuclei in a single cell which is called a coenocyte. In animal cells, similarly, many nuclei may occur in a single cell, as in malarial parasite, forming a syncytium. The striated muscle fibres are good example of syncytium.
Size of nucleus is not constant and is generally correlated with DNA contents. The nuclear size is a function of chromosome number, i.e., the size is variable depending upon the number of chromosomes (DNA contents). Usually, there exists a ratio between the nuclear volume (Nv) and the volume of the cytoplasm (Cv) for each cell type. It can be expressed by an equation called nucleoplasmic index (NP) given by R. Hertwig which is as follows:
NP = Nv / Cv – Nv
This index is fixed for each cell and any disturbance of equilibrium of NP acts as stimulus for cell division. Cells containing more than one set of chromosome are called diploid or polyploid cells, having larger number of chromosomes.
Thus, cells with more than the diploid number of chromosomes have usually larger nuclei. The nucleus of resting period, when it does not undergo any division, is called the metabolic nucleus, while dividing nucleus, is called kinetonucleus or arbeitonukleus in German language.