Cell Nucleus Biogenesis, Structure and Function
401
Fig. 4
The mammalian cell. Proliferating HeLa cells
(this is a transformed human cell line) were Fxed,
stained, and processed using standard electron
microscopy techniques. Thin sections were then
prepared and inspected. This typical cell section shows
a central nucleus (N) – typically of about 10
µ
m
diameter in an equatorial section – which is separated
from the cytoplasm (C) by a double nuclear membrane
(NM). The nuclear membrane is studded with a few
thousand nuclear pores that allow molecules to pass
between the two major cell compartments. Note that
the cytoplasm contains a large number of
membrane-bound organelles – the most conspicuous
of these, the mitochondria, are dark staining and
typically about 1
µ
mind
iameterinthisexamp
le.The
nucleus does not contain any membrane-bound
subcompartments. Even so it is highly organized. In
this type of image, the most obvious substructure is
the nucleolus (No). In this example, a single large
nucleolus occupies the center of the nucleus. The
nucleolus is densely staining and has three distinct
subcompartments: the most densely staining regions
are called the
dense fbrillar component
; these structures
are generally crescent shaped and a few hundred nm in
size. The dense Fbrillar components represent the
nascent transcripts that surround the much paler
Fbrillar centers. The processed ribosomal RNAs are
assembled into ribosomal subunits in the granular
component, which comprises the bulk of the nucleolus.
Dark staining clumps of material that lie against the
surface of the nucleolus, the nuclear face of the
membrane and are scattered throughout the center of
the nucleus are dense heterochromatin (hc). The
remainder of the nucleus has an amorphous
appearance and little obvious structure. This
nucleoplasmic region contains the transcribed genes in
euchromatin and is composed of a mixture of this open
chromatin and the interchromatin material, which is
predominantly RNA and protein. The bar is 1
µ
m.
NM
C
N
No
hc
are quite different. The nucleus does not
contain subnuclear organelles – by deFni-
tion, organelles are enclosed by a mem-
brane. Instead, the nucleus has numerous
discrete zones or compartments where
particular functions are performed. The
nucleolus provides an excellent example
of this organizational theme. Indeed, it
is not unusual for the nucleoli to be the
most conspicuous interphase structure in
a mammalian cell.
3.5.2
The Nucleolus
Nucleoli are sites of ribosomal RNA syn-
thesis and processing and also centers
of biogenesis for the ribosomal subunits.
Clearly deFned subcompartments within
nucleoli reflect the centers where these
different activities are performed. Mam-
malian cells generally contain 1 to 5
nucleoli; the number will usually reflect
the proliferative status of a cell and might
be used by histologists as an indicator
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