List of abbreviations
of micros-
specialist terms
explained in
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Dr. med.
H. Jastrow

of use
Overview nucleolus (Nucleolus):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
nucleolus pinealocyte
nucleolus of a
parietal cell (rat)
nucleolus of a chief
cell stomach (rat)
nucleolus of a ganglion
cell (guinea pig)
prominent nucleolus of a prolac-
tine cell adenohypophysis (rat)
2 nucleoli in a human
3 nucleoli, spine cell
human skin
nucleolus of a neuron in
brain cortex (rat)
nucleolus of a mucous
salivary gland cell (rat)
nucleolus of a
human plasma cell
nucleolus Tonsilla
palatina (human)
nucleolus skeletal mus-
cle cell nucleus (pig)
Hypophysis (rat)
nucleolus of a
hepatocyte (rat)

Nucleols (Terminologia histologica: Nucleoli) are spherical to ovoid electron-dense basophilic condensations in the nucleus of a cell with diameters of 1 - 2 µm. Nucleols are the construction sites of ribosomes from ribosomal subunits and may take up to ~25% of the nuclear volume. Nucleols often are located in the centre of cell nuclei, however, they also may be associated to the nuclear membraneThey appear next to nucleolar organizing regions (Terminologia histologica: Nucleolum operans regiones) on secondary stranglings of the short arms of the acrocentric chromosomes 13, 14, 15, 21 and 22. Since the latter are present twice in each cell, theoretically a maximum of 10 nucleols is possible in human cells, however in practice this is never the case. This is due to the fact that the synthesis of sufficient ribosomal ribonucleic acid (r-RNA) even in case of high demand is only possible at 2 to 3 nucleols, since several nucleolar organizing regions then aggregate in large common nucleols. Plenty of copies of the genetic information for formation of 5-S, 5,8-S, 18-S and 28-S subunits of ribosomes are present in such argyrophilic, i.e. stainable with sliver salts, nucleolar organizing regions of the previously mentioned chromosomes.
Resting cells may show only small or even no visible nucleols. Nerve cells, which no longer undergo mitosis, in many cases only show one very large nucleolus comprising all nucleolar organizing regions of the whole nucleus. The shorter the interphase during mitosis the higher is the number of nucleols in quickly dividing cells. Since immediately after mitosis all 10 nucleolar organizing regions form smaller nucleoli which then accumulate to few larger ones. In many cases nucleols are connected to the inner nuclear membrane via electron-dense chromatin bridges or are directly attached to the nuclear membrane.
nucleols show several
1. fibrillar centres (Terminologia histologica: Centra fibrillares) of low to slight electron density,
2. electron-dense fibrillar components (Terminologia histologica: Partes fibrillares densae) with a delicate inner structure,
3. a granular component (Terminologia histologica: Pars granulosa) of the nucleolus. Additionally the nucleolus contains
4. some small non-electron-dense regions (amorphous part of nucleolus, nucleolar interstice; Terminologia histologica: Pars amorpha nucleoli).

An English page with much more detailed information is only available in the professional version of this atlas.

--> ribosomes, nuclear membrane, nuclear pore, karyoplasm, euchromatin, heterochromatin, chromosomes, nucleus
--> Electron microscopic atlas Overview
--> Homepage of the workshop

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