The Cell: Basic Structure and Function
Nuclear Envelope and Nuclear Shape
The nuclear envelope (NE) consists of two lipid membranes.
The inner membrane is associated with the telomeres and
anchors the chromosomes, while the outer membrane is part
of the endoplasmic reticulum. The space between the two lipid
layers is called perinuclear cisterna. The nuclear envelope con-
stitutes the nucleus and separates the genomic material from the
cytosol. During cell division, the nucleus disappears; the nuclear
envelope is broken down to vesicles and is reassembled during
telophase. The nuclear envelope builds a strong barrier between
nucleus and cytosol; a number of nuclear pore complexes regu-
late the traffic between both compartments. There can be passive
diffusion or active transport; in general, proteins synthesized in
the cytoplasm require a specific nuclear signal in order to have
access to the nucleus.
Inside the nuclear envelope is a network of chromatin fibrils
and a nuclear lamina built from laminins. The nuclear envelope
can be visible in light microscopy.
The regular nuclear shape is that of a smooth sphere or sphe-
roid, based on the orderly arrangement of the chromosomes
and the nuclear lamina. Many factors can affect the shape of
the nucleus: stress, transcriptional, and synthetic activities can
disturb the arrangement of interphase chromosomes; DNA
amplifications can lead to uneven distribution of the nuclear
material and to nuclear enlargements. At the same time, aber-
rations of the nuclear envelope can lead to alterations of the
nuclear skeleton, resulting in altered chromosomal distribu-
tions. It has been assumed that changes of the nuclear envelope
occur mainly after mitosis, when the nuclear envelope is reas-
sembled. Alterations of the nuclear envelope have been directly
linked to oncogene activity: Six hours after transfection with the
ret oncogene, increasing cell counts with NE alterations were
observed in human thyroid cells, indicating that nuclear altera-
tions may occur even independent of postmitotic re-assembly.3
NE alterations and the respective nuclear shape are an impor-
tant diagnostic feature of many malignancies, especially papil-
lary thyroid cancers and different types of leukemias.
Cytoplasm and Plasmalemma
The cytoplasm consists mainly of water (80-85%). The remain-
ing constituents are proteins (10-15%), lipids (2-4%), polysac-
charides (1%), and nucleic acids (1%). The cytoplasm is confined
to the outside by the plasma membrane, a lipid bilayer, and to
the inside by the nuclear membrane. In most cytology applica-
tions, normal cells have a homogenous cytoplasm with occasional
granules or inclusions.
Eosin is the most common dye to stain the cytoplasm in histol-
ogy. It is an acidic dye that binds to basic components of a cell,
mainly proteins located in the cytoplasm. It gives a bright pink
color that contrasts the dark blue nuclear hematoxylin staining
(Fig. 1.3B). A combination of hematoxylin and eosin is the most
frequently used dye in histology. In cytology, frequently, a Pap
stain is performed. It consists of a hematoxylin-based nuclear
stain and a polychromatic cytoplasmic stain, including Orange
G and two polychromic dyes, EA36 and EA50. The cytoplasmic
stain results in highly transparent cells, making it possible to
assess superimposed cells in a Pap smear. Based on the cell type
and the differentiation status, the cytoplasm can be pink-light
red (e.g. superficial cervical cells) or light blue-green (e.g. cer-
vical parabasal and intermediate cells) with all variations in
between. The nuclei are dark brown or dark blue/violet and the
nucleolus appears bright red (Fig. 1.3C).
The endoplasmic reticulum (ER) consists of a single membrane
making up for more than half of all internal membranes of the
cell (Fig. 1.4A). The part of the membrane that faces the cytosol
is studded with ribosomes. This part of the ER is called rough
ER; the regions without ribosomes are called smooth ER.
The main function of the ER is the packaging and delivery of
newly synthesized proteins.
The Golgi apparatus is part of the membrane system that also
contains the ER. It consists of stacked membrane-coated cavities,
called dictyosomes (Fig. 1.4B). The Golgi apparatus is located
close to the nucleus and can be very large in secretory cells, where
it fills almost the complete cytoplasm. The convex side facing
the ER/nucleus is called cis-Golgi; the concave side facing the
cytoplasm is called trans-Golgi. From the Golgi apparatus, small
vesicles transport products to other cellular sites or the exterior.
Inside the structure, complex biochemical operations are being
performed, most of them resulting in post-translational modi-
fications of synthesized proteins. Several secretory mammalian
cell types are characterized by a prominent polarized Golgi
apparatus located between the nucleus and the luminal surface:
Goblet cells in the respiratory and digestive tract produce large
Rough Endoplasmic Reticulum (RER)
Fig. 1.4 Membranous organelles.
(A) The rough endoplasmic reticulum.
(B) the Golgi apparatus. (C) a mitochondrion.