16
UNIT 1
Basic Principles of Cell Structure and Function
The
outer
and
inner nuclear membranes
and the
perinuclear cisternae
are readily identif
ed in electron micrographs iF there is
adequate magnif
cation and a Favorable plane oF section. In some cells, the outer nuclear membrane is studded with ribosomes, and
the perinuclear cisternae are continuous with the cisternae oF
RER
. In a cell that is actively synthesizing proteins, the
nuclear enve-
lope
has numerous
nuclear pores
that can be identif ed in electron micrographs as interruptions in the double-membrane arrange-
ment oF the nuclear envelope. Some Face-on views oF nuclear pores can be seen in the
inset
. It can be seen that the pores are not
simply openings, but rather each has a diaphragm. Chromatin that is highly condensed, or
heterochromatin
, is much more electron
dense than chromatin that is accessible to transcription, or
euchromatin
. Clumps oF heterochromatin tend to be located adjacent to
the inner nuclear membrane, with gaps that correspond to sites oF nuclear pores.
Nucleoli
appear similar to heterochromatin but can
usually be distinguished by a more complex substructure oF granular, f
brous, and nucleolar organizer components.
J. Naftel
J. Naftel
Outer nuclear
membrane
Cisterna of
nuclear envelope
Heterochromatin
Euchromatin
Nucleolus
Rough
endoplasmic
reticulum
Nuclear pore
(vertical section)
Nuclear pores
(face-on view)
Inner nuclear
membrane
Figure 2-3.
The nucleus and its components.
EM,
3
43,000; inset
3
42,000
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