Key concept –Nucleus Discovery, Shape, and Size, Nucleocytoplasmic index, Nucleus & cytoplasmic proportion, why cell divide, Syncytium, and Coenocytic condition.
Discovery of Nucleus
Robert Brown discovered nucleus in the year 1831. He discovered nucleus in the epidermal cell of the root of the orchid plant and given the term nucleus. According to him, the nucleus is a regular cell organelle. In the animals, the Leeuwenhoek was the first who finds the nucleus in the red blood cell of fishes.
The nucleus is the essential part of a cell. It is the controlling center of the cell that controls cell and cellular activities. Karyology is the branch of the biology that deals with the study of the nucleus.
Nucleus is the essential part of the cell for the survival of the cell but there are some exceptions (Nucleus Absent):-
- Mature red blood cells of mammals.
- Mature sieve tube cells of phloem.
Shape and Size of Nucleus
Generally, the size of the nucleus depends on the metabolic state of the cell. Mostly, larger cells have a larger nucleus.
The microscopic view of the nucleus can tell the state of the cell as an active, dormant or dividing cell. In every cell have a specific size of the nucleus in its normal state.
- The active state of the cell – Nucleus is large in size in proportion to the cytoplasm.
- In Dormant cell – Nucleus is much smaller in proportion. This is known as Pycontic Nucleus.
- Dividing cell – Both the nucleus and cytoplasm are double in volume than a normal cell.
In general, Most of the cell are uninucleated but in the human body, some cells like liver cell and cartilage cells have more nucleus.
Bone marrow cells are rapidly dividing cells and they have more than one nuclei. When there is more than one nucleus in a cell the condition is called Multinucleate Condition.
For the formation of the multinucleated condition there are two way:-
By the fusion of cells ( “Syncytium Condition” seen in some animal cells)
By division of Nuclei several times in a cell. (“Coenocytic condition” seen in some Plants and Fungi”).
In the animal cell, the nucleus is present in the center of the cell. In the plant cells, the nucleus is pushed towards the periphery of the cell and become lens-shaped. Reason for this is the presence of large central vacuole.In glandular cells, the nucleus is present at the base of the cell.
The shape of nucleus depends on the shape of cells:-
- Round, Cubodial, and Polyhedral cells – Spherical Nucleus
- Cylindrical cells – Oval Nucleus
- Flat cells – Disc like Nucleus
- White blood cells – Multilobed Nucleus
- Muscle cell – Elongated Shaped Nucleus
The nucleus is the controlling center of the cell. In each cell, there is a relation between the volume of nucleus and volume of cytoplasm known as Karyoplasmic or Nucleocytoplasmic Index. R. Hertwig explains it for the first time. Strasburger was the first who tells that the nucleus arises from the pre-existing nucleus.
Nucleocytoplasmic Index = Volume of Nucleus / Volume of Cytoplasm
- Volume of Cytoplasm = Volume of cell – Volume of Nucleus
Why Cell Divide?
If the ratio of the volume of the nucleus to the volume of cytoplasm become below one the cell start growing in size and the cell divide.