In a eukaryotic cell, the nucleus is one of the most complex yet highly organized organelles. 

It sustains an extremely dynamic environment with the help of many sub-nuclear structures, each of which serves a unique function inside the nucleus. 

Besides the nucleolus - which is the most prominent and well-studied organelle of the nucleus, there are several other nuclear domains or nuclear bodies that exist inside the nucleus. 

Cajal bodies and Interchromatin granule clusters are two such functionally specialized regions. 

These are both membrane-less structures that can change appearance depending on the cellular environment or the cell cycle stage. 

Cajal bodies are spherical structures enriched in proteins and RNAs. They can be found freely in the nucleoplasm or physically associated with histones and snRNA genes. 

They are the primary sites for the assembly of small nuclear ribonucleoproteins or snRNPs which are responsible for pre-mRNA splicing.

It is also inside Cajal bodies that guide RNA molecules called scaRNAs or small Cajal body-associated RNAs carry out post-transcriptional modifications on some snRNAs. 

These modifications are essential for the proper assembly and function of the snRNPs in pre-mRNA splicing. 

Interchromatin granule clusters or speckles are irregular, amorphous structures that are distributed in the nucleoplasm of a vertebrate cell nucleus. 

Speckles serve as the storehouse for fully mature snRNPs and other RNA processing components. 

The high local concentration of RNA and protein components in cajal bodies and interchromatin granule clusters helps in rapid assembly and transport of ribonucleoprotein complexes to the transcription and RNA processing sites within the nucleus.