PART ONE
General Cytology
Mitochondria
Endoplasmic
reticulum
Nucleus
Lysosomes
Cytoskeleton
Cell junctions
Golgi
apparatus
Cell
membrane
Fig. 1.1
Schematic presentation of an epithelial cell displaying
the most
important structures
discussed in this chapter.
Nucleus
The nucleus contains the genomic DNA, histones, and several
proteins that are responsible for DNA replication, repair, and
transcription of genetic information (Fig. 1.2A).
The assessment of a cell's nucleus is one of the most impor-
tant tasks in cytopathology. The size of the normal nucleus is
highly variable, depending on the underlying cell type. In many
malignant cells, nuclei are considerably enlarged. Apart from
nuclear size, the chromatin density, the nuclear membrane,
and the presence of nucleoli are important features of nuclear
morphology and will be described in detail.
The nucleus contains about 25% dry substance, of which 18%
is DNA plus a similar amount of histone proteins. The rest of the
dry substance contains the non-histone proteins, nucleoli, and
the nuclear membrane.
Contents of the Nucleus
DNA
The genetic information of organisms is coded in deoxyribo-
nucleic acid (DNA). DNA is a long stretch of single nucleotides
connected by a sugar-phosphate backbone (Fig. 1.2C). The
genetic information is stored in specific sequences consisting
of four different bases: adenine, guanine, thymine, and cytosine
(A, G, T, C). A triplet of bases is coding for an amino acid that
constitutes the basic component of proteins. Although in prin-
ciple the triplet code allows for 64 different variations, only 20
protein-building amino acids exist. Many amino acids are coded
by multiple base triplets. The genetic code is degenerate, thereby
tolerating errors in the base sequence to some degree. Two DNA
stretches are combined as a double helix; one complete turn
is reached after 10 bases. The DNA stretches are not covalently
bound, but attached via hydrogen bonds between complemen-
tary bases A-T and C-G.
DNA is a very robust and stable molecule, since it must pro-
tect the genetic code of an organism. The genetic information is
transferred to the ribosomes (the protein production machinery)
by ribonucleic acid (RNA) that has 3 important features differ-
ent from DNA: RNA is based on a ribose backbone, it contains
uracil instead of thymidine, and it is usually single-stranded.
Compared to DNA, RNA is a rather unstable molecule.
The total DNA of a cell is separated on chromosomes. In
total, 22 different chromosomes and two sex chromosomes
exist. The chromosomes vary in size and in the content of
coding sequences, they are numbered in decreasing order of
their size. During the metaphase of mitosis, chromosomes
are condensed and can be identified in light microscopy. In
transcriptionally active cells, DNA is decondensed and takes
up the room of the complete nucleus. When metaphase chro-
mosomes are stained according to Giemsa, a heterogenous
pattern of regions with strong staining (G-bands) and regions
without staining (R-bands) can be observed. R-bands contain
more genes than G-bands and are replicated early during cell
division. The banding pattern of chromosomes has been used
to determine chromosomal regions by indicating the chromo-
some number, the position with reference to the centrosome
(p = short arm, q = long arm), and the position of the chromo-
somal banding (e.g. 3q26). In total, the human genome con-
sists of ~3.2 billion bases, coding for approximately 25,000
genes.1
Nuclear Proteins
Histones are basic proteins that build a structural unit together
with the DNA, called the nucleosome (Fig. 1.2B). In the nucleo-
some, 146 base pairs (bp) are coiled around different histone
subunits. The main function of the nucleosome is the high-
density packing of DNA inside the nucleus, leading to a 50,000-
fold increased compactness of DNA as compared to unpacked
DNA.
Histone acetylation reduces the affinity between the DNA
molecule and histones, leading to increased accessibility of DNA
for transcription machinery components such as RNA polymer-
ase and transcription factors. In general, for gene transcription,
the DNA needs to be unpacked from the histones.
Besides histones, nuclear non-histone proteins build the
nuclear scaffold structure and are involved in DNA transcription
and replication.
Nuclear Morphology
Chromatin
Chromatin represents the complex structure of proteins and
DNA in the nucleus of non-mitotic cells. There is usually twice
as much protein as DNA in a nucleus. Since most cells in the
human body are non-mitotic, chromatin is the morphological
appearance of most cell nuclei assessed in cytology. The chro-
matin distribution and organization depends on many different
factors, such as cell type, differentiation, metabolism, prolifera-
tion status, and, most important in cytopathology, neoplastic
transformation.
Two conformations of chromatin are discriminated: euchro-
matin and heterochromatin. Euchromatin contains transcrip-
tionally active protein-coding DNA regions. Heterochromatin
represents the complex of DNA that is densely packed on
histones. DNA sections not transcribed are usually stored in
heterochromatin. Heterochromatin is further differentiated
into constitutional, facultative, and functional heterochro-
matin. Constitutional heterochromatin consists mainly of
highly repetitive DNA stretches in the centromeric region that
are supposed to have structural functions but have also been
found to express microRNAs that do not code for proteins but
are involved in gene regulation. Facultative heterochromatin
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