Hi there!

Heteropycnosis refers to the condition of differential condensation seen in the chromosomal segments. It has been observed that during mitosis:

• some segments of the DNA in a chromosome are tightly coiled up (or higly condensed) and take up dark stain. These regions are called heterochromatin.
• while some segments of the DNA in a chromosome are less tightly coiled and hence take up less stain. These regions are called euchromatin.

Thanks