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The Cell: Basic Structure and Function
Nuclear
membrane
Nucleoli
Euchromatin
Heterochromatin
A
Histone
DNA
Nucleosome
B
Fig. 1.2 Contents of the nucleus, DNA.
(A) A nucleus displaying nucleoli, euchromatin,
and heterochromatin. (B) Two nucleosomes
consisting of DNA coiled around histone
proteins. (C) The structure of double-stranded
DNA. Organic bases are connected to a
sugar-phosphate backbone. Complementary
bases (A-T, C-G) are held together by hydrogen
bonds.
C
designates inactivated DNA regions that usually code for pro-
teins but are not necessary in the respective cell, e.g. inacti-
vation of the second X chromosome (Barr body). Functional
heterochromatin requires DNA regions not necessary for the
respective differentiation of a cell.
Hematoxylin
In many cytological applications, the chromatin is stained with
hematoxylin. Hematoxylin is a basic dye extracted from the
heartwood of the tree
Haematoxylum campechianum.
By itself,
hematoxylin is a very weak stain. Different mordants, such as
potassium alum, are used to generate the typical dark blue or
purple staining. Hematoxylin strongly binds to acidic com-
ponents of a cell, most importantly to the phosphate groups
of nuclear DNA; the stained structures are therefore called
"basophilic" (Fig. 1.3A).
Based on the nuclear stain, a wide variation of chromatin
alterations can be observed, both alterations of structure and
staining intensity. Structural aberrations include chromatin
margination, i.e. aggregation of chromatin to the nuclear mem-
brane, which is a sign of cell degeneration. Other chromatin
alterations are coarsening and clumping that is usually accom-
panied by chromatin thinning in other regions.
Hyperchromasia, i.e. increased staining intensity, can result
from increased chromatin loads or from decreased nuclear vol-
ume, which inversely applies to hypochromasia. In addition,
chemical modifications of the chromatin (e.g. during specific
stains or cell treatments) can increase the stain uptake, simulat-
ing hyperchromasia.
Nucleoli
Nucleoli are small basophilic spherical bodies located in the
nucleus. Usually they can be found in the central nuclear region
but may also be close to the nuclear membrane. A nucleolus is
built by a nucleolus organizing region (NOR) of a specific chro-
mosome. These regions contain the genes for ribosomal RNA
subunits that build the protein synthesis machinery. Since in a
diploid human cell, in total 10 chromosomes containing NORs
exist, in principal 10 nucleoli per nucleus could be present.
Usually, only one or two nucleoli are found, since NORs from
several chromosomes build a common nucleolus. Nucleoli have
two distinctive regions, the pars fibrosa that contains the proteins
required for transcription and the pars granulosa that contains
the ribosomal precursors. During mitosis, nucleoli disappear
and are reconstituted in the daughter cells. Shortly after cell
division, a larger number of nucleoli that fuse gradually can be
observed.
Depending on the cell type, the presence of nucleoli is physi-
ological or can indicate malignant processes: liver cells that regu-
larly produce a lot of protein can frequently exhibit nucleoli. In
reactive or regenerative cells, nucleoli can become more promi-
nent. In hepatocellular carcinoma, usually more than 50% of
the cells show prominent, frequently multiple nucleoli. Intesti-
nal epithelial cells also regularly show single nucleoli. In ageing
and starving cells, a shrinking of nucleoli can be observed. In
cancer cells, nucleoli can vary substantially with regard to size
and shape.
In many malignant cells, multiple nucleoli that appear dis-
joint, odd-shaped, and spiculated can be observed. Proteins
associated with nucleolar organizer regions can be visualized
by a simple argyrophilic staining method. The structures high-
lighted by this method are called "argyrophilic nucleolar organ-
izer regions" (AgNORs). Different distributions ofAgNORs have
been described between normal, dysplastic, and malignant tis-
sues. In several cancer entities, AgNOR aberrations were found
to have independent prognostic significance with respect to
patient survival.2 Increased NOR counts have been explained
by increased metabolism with a high demand of ribosomes,
but also by aneuploidy leading to increasing numbers of NOR
regions in cancer cells.
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