Chromatin
Chromatin is a highly ordered complex of protein (mostly histones) and DNA that functions to package DNA, regulating its transcriptional activity. The level of activity is indicated by its staining properties. Heterochromatin is densely stained and reflects regions of low activity; euchromatin is pale staining and reflects areas of high activity. These light micrographs contrast nuclei that are highly heterochromatic (left) with one that is highly euchromatic (right). Most nuclei contain a mixture of both heterochromatin and euchromatin. Thymus (l), Spinal cord (r), 400x
Heterochromatin >
Heterochromatin is dark staining, indicating chromatin that is highly condensed and thus inaccessible for transcription. The dark staining of these nuclei results from the concentrated binding of basic dyes, like hematoxylin, to the closely spaced phosphate groups of the condensed DNA. These small round cells (thymocytes) have minimal cytoplasm around the nucleus.
Euchromatin >
Euchromatin is pale staining and indicates loosely packed chromatin that allows accessibility of transcriptional machinery for gene expression. Visible on the left are active cells with euchromatic nuclei amongst the thymocytes. Their shape is difficult to discern in this section. The neuron on the right has a large cell body with ample cytoplasm surrounding the nucleus.
Nucleoli
Euchromatin is pale staining and indicates loosely packed chromatin that allows accessibility of transcriptional machinery for gene expression. Visible on the left are active cells with euchromatic nuclei amongst the thymocytes. Their shape is difficult to discern in this section. The neuron on the right has a large cell body with ample cytoplasm surrounding the nucleus.
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