The uneven or differential condensation of chromatin (genetic material) in cells, where some regions become densely packed while others remain loose.
From Greek 'hetero-' (different) + 'pycnosis' (from 'pyknos' meaning dense, and 'osis' meaning condition). This cytology term emerged in 20th-century microscopy.
Heteropycnosis is why your Barr body (the inactive X chromosome in female cells) looks like a dark spot under a microscope—it's densely packed DNA that's been silenced, while active DNA stays loose and open.
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