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A nucleus (Terminolgia histologica: Nucleus) is the cell compartment that
contains the genetic information (desoxy ribonucei acid = DNA) bound to
chromatin and usually one or more nucleoli. It is limited by the nuclear
|segmented nucleus of a human
|characteristic horse-wheel like aggre-
gation of chromatin (plasma cell, rat)
|a similar human plasma cell nucleus
||nucleus of a smooth muscle cell
(arteriole from rat brain)
|nucleus of a hepatocyte
|nucleus of a Sertoli cell
All cells contain nuclei with two exceptions: red blood cells (erythrocytes)
and platelets (thrombocytes).
Most cells only contain one nucleus. However, two nuclei may be encountered
in the following cells: liver cells (hepatocytes),
cells of the transitorial epithelium (of the urinary system), parietal
cells (of the stomach), chondrocytes,
muscle cells. Cells with many nuclei derive from cells that melt together
during development to form a syncythium:
the syncythiotrophoblast which is the surface epithelium of the placenta,
osteoclasts (bone resorbing cells), chondroclasts (cartilage resorbing
cells) and all skeletal muscle cells. Whereas
the first 3 different cell types contain 3 to about 8 nuclei, the skeletal
muscle cells possess some hundreds to a few thousand nuclei located
close to the cell surface since some of them they are several millimetres
The nuclear membrane separates the nucleus from the surrounding cell
fluid with its organelles, the cytoplasm.
The fluid inside the nucleus called karyoplasm
or karyolymph embeds the more or less dense network of the chromatin and
the nuclear bodies, called
The average diameter of a nucleus is about 5 to 10 µm. The nuclear
volume comprises 5 - 10 % of the cellular volume with considerable variation
depending on cell type. The relation of nuclear to cell volume also depends
on cell function. In very small cells, e.g., lymphocytes
or mikroglia cells the nucleus-plasma relation
is over 50 %, in univacuolar fat cells it may
be less than 1 % in times of high lipid storage. Most cells have a simple
chromosome set, this means they are diploid (every chromosome is present
twice), some cells, e.g.
liver cells or heart
muscle cells may become polyploid containing then more than a single
set of chromosomes. The nucleus-plasma relation is of importance for judgement
of dignity of tumour cells that often develop multiple sets of chromosomes.
The nuclear volume increases with cell activity as is the active part of
the chromatin where, in the transcription process, genetic information
is copied to messenger ribonucleic acid (m-RNA) for being transported into
the cytoplasm for protein synthesis in a process called translation.
The shape of a nucleus usually adapts to that of the whole cell. Thus,
e.g., long cells like fibrocytes have long nuclei, round cells like lymphocytes
spherical nuclei as have cuboid epithelial cells. Unusual shapes of nuclei
are seen in the band-shaped and lobed granulocytes that belong to the white
blood cells. The shape of a nucleus is dynamic in smooth muscle cells that
show a straight long nucleus in rest phase and a corkscrew like shape during
contraction. In univacuolar fat cells a large
lipid droplet pushes the disc like nucleus aside from the centre of the
cell. Some cells,e.g. megacaryocytes (which are the precursors of blood
platelets = thrombocytes) have large lobed
nuclei. Nuclei of other cells show invaginations of cytoplasm, e.g. cells
of the pineal gland (pinealocytes) or transitorial
epithelial cells of the urinary bladder.
In the light microscope nuclei appear as homogenous structures with
with partial densities. The latter are the nucleoli
or chromatin granules. After fixation and staining of tissues the sour
(due to richness of nucleic acids) chromatin is basophilic, i.e. may be
stained with basic colouring agents. It is in many cases more intensely
stained than the cytoplasm. The chromocentres appear as fine fibres or
bigger lumps of the fibrils from chromosomes as brighter euchromatin
or more densely packed heterochromatin.
These two forms of chromatin are best visible in the electron microscope.
Some cells in females show a dense aggregation of heterochromatin attached
to the nuclear membrane, the X-chromatin
(Barr's corpuscule). It comprises an inactivated female sex- = X-chromosome.
During cell devision (Mitosis or Meiosis) the genetic material is doubled.
The characteristic stages of these processes can be distinguished from
the changes in the nucleus due to formation of chromosomes. The majority
of cells that s not in atate of division are in a stage called interphasis.
--> nuclear membrane, nuclear
--> Electron microscopic atlas Overview
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Three pictures were kindly provided by Prof. H. Wartenberg;
other images, page & copyright H. Jastrow.