The nucleus is the center for the genetically determined information in every eukaryotic cell. The nucleus also serves as a command or logistics center for the regulation of cell functions. There is a correlation between the geometry of the nucleus and the cell dimensions, which offers important diagnostic clues. The nucleus is usually round in polygonal and isoprismatic (cuboid) cells and ellipsoid in pseudostratified columnar cells; it has the form of a spindle in smooth muscle cells and is flattened in flat epithelial cells. In granulocytes, the nucleus has several segments. The fibrocyte in this figure is from subcutaneous connective tissue. Its elongated, irregularly lobed nucleus shows indentations and deep dells. The structural components of a nucleus are the nuclear membrane, the nuclear lamina, the nucleoplasm, and the chromosomes with the chromatin and the nucleolus. The chromatin is finely granular (euchromatin), but more dense near the inner nuclear membrane (heterochromatin). The small electron-dense patches are heterochromatin structures aswell. TheDNA is packaged in a much denser form in heterochromatin than in euchromatin, and heterochromatin therefore appears more heavily stained in light microscopy preparations. A nucleolus is not shown here. The cytoplasm of the fibrocyte contains mitochondria 1 , osmiophilic secretory granules 2 , vesicles, free ribosomes and fragments of the rough (granular) endoplasmic reticulum membranes. Collagen fibrils are cut longitudinally
or across their axis 3 .
Electron microscopy; magnification: × 13 000
Electron microscopy; magnification: × 8500
Rectangular nucleus in an orbital gland cell from the water agama (lizard). The nucleus contains two strikingly large nucleoli 1 , which are surrounded by a ring of electron-dense heterochromatin . This heterochromatin contains the genes for the nucleolus organizer. The heterochromatin layer along the inner nuclear membrane or the nuclear lamina shows several gaps, thus forming nuclear pores.
i Expanded intercellular space In the lower part of the picture are two cell contacts in the form of desmosomes
Electron microscopy; magnification: × 12 000
Kuehnel, Color Atlas of Cytology, Histology, and Microscopic Anatomy © 2003 Thieme