Chromatin, a complex of DNA and proteins, is responsible for the characteristic basophilia of the nucleus

In most cells, chromatin does not have a homogeneous appearance; rather, clumps of densely staining chromatin are embedded in a more lightly staining background. The densely staining material is highly condensed chromatin called heterochromatin, and the lightly staining material is a dispersed form called euchromatin. It is the phosphate groups of the DNA of the chromatin that are responsible for the characteristic basophilia of chromatin (page 7). The proteins of chromatin include five basic proteins called histones and other nonhistone proteins. Heterochromatin is disposed in three locations (Fig. 2.54):

• Marginal chromatin is found at the perimeter of the nucleus (the structure light microscopists formerly referred to as the nuclear membrane actually consists largely of marginal chromatin).

• Karyosomes are discrete bodies of irregular size and shape, found throughout the nucleus.

• Nucleolar associated chromatin is chromatin found in association with the nucleolus.

Heterochromatin stains with hematoxylin and basic dyes; it is also readily displayed with the Feulgen procedure (a specific histochemical reaction for the deoxyribose of DNA, page 7) and fluorescent vital dyes such as Hoechst dyes and propidium iodide. It is the heterochromatin that accounts for the conspicuous staining of the nucleus in hematoxylin and eosin (H&E) preparations.

Euchromatin indicates active chromatin, i.e., chromatin that is stretched out so that the genetic information in the DNA can be read and transcribed. It is prominent in metabolically active cells such as neurons and liver cells. Heterochromatin predominates in metabolically inactive cells such as small circulating lymphocytes and sperm or in cells that produce one major product, such as plasma cells.

Euchromatin is not evident in the light microscope. It is present within the nucleoplasm in the "clear" areas between the heterochromatin. In routine electron micrographs, there is no sharp delineation between euchromatin and heterochromatin; both have a granular, filamentous appearance, but the euchromatin is less tightly packed.

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