Page 19 - Headlines Histology2024_Neat

Chromatin

• Two types of chromatin are distinguished: heterochromatin and
euchromatin.

Heterochromatin

• The heterochromatins consist of tightly coiled portions of chromosomes.
The genes are repressed and transcription does not occur. It predominates in
inactive cells.

• With LM, they appear as fine and coarse basophilic granules scattered
throughout the different regions of the nucleus.

• With EM, it appears as electron-dense areas tend to be clumped around the
periphery of the nucleus (peripheral chromatin), around the nucleolus
(nucleolus associated chromatin) and also forms irregular clumps
throughout the nucleus (chromatin islets).

• In females, the inactive X-chromosome forms a small mass located at the
edge of the nucleus and is called Barr body.

Euchromatin

• The euchromatin is the extended, uncoiled portions of chromosomes in
which the transcription of DNA is active.

• This type of chromatin is found in active cells. With LM, euchromatin is
invisible because they are very thin and extended. With EM, they appear as
electron-dense nuclear materials represent the parts of the DNA that are
active in RNA synthesis.

Nucleolus

• It is a conspicuous, spherical, basophilic structure that is primary concerned
with synthesis of ribosomal RNA.

• With LM, usually one, sometimes several nucleoli are seen. They are
usually basophilic mainly due to nucleolus associated chromatin.

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