This was first discovered by Fontana (1874). A spheroidal body, situated either in the central or peripheral position, the nucleolus is supposed to regulate the synthetic activity of the nucleus.
Usually two or more chromosomes are associated with the nucleolus (this can be seen during late prophase) and these are called nucleoli organizers as they play a role in the reappearance of the nucleolus after cell division. The number of nucleoli per nucleus varies from one to two or three.
Chemically the nucleolus is rich in RNA. According to Palade (1963), the nucleous of liver cells contain 3 5- % RNA, while pea embroys have 10 – 20% of nuclear RNA. Orthophosphates helping in RNA synthesis have been reported in nucleolus.
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Fine structure of the nucleolus:
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Detailed analysis of the nucleolus has revealed the presence of three regions. These are granular region, fibrillar region and amorphous region. The granular region occurring at the peripheral region of the nucleolus is composed of granules of 150-250A thick.
This region has ribonucleic acid and proteins. The fibrillar region also known as nucleolonema is composed of fibrils 50-80A in length. The amorphous region has low electron density and is the one to disappear first at the time of cell division.
Types of nucleoli:
According to Wilson, there are two kinds of nucleous viz., Plasmosomes and Karyosomes (Ocata). The plasmosomes are positive to acidic stains and have a transparent outer region with a dense interior.
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The karyosomes stain positively with basic dyes. Three kinds of karyosomes have been identified-Netknot type (like nodules attached to threads), Chromosome nucleoli (occurring only in gametocytes) and Karyospheres (spheriodal bodies with basic chromatin).
There is no limiting membrane to the nucleolus and the organisation is maintained by the calcium ions.
Functions of nucleolus:
1. It is the active site of RNA synthesis
2. It is the source of ribosoinal RNA
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3. It produces precursors of ribosomes
4. According to Maggio (1990), protein synthesis takes place in the nucleolus. But this may not be totally true.