No Slide Title

Nucleus
By : Hosseini,M.Sc.
Figure 3–1. Liver cells (hepatocytes). Several dark-stained nuclei are shown. Note the apparent nuclear membrane
consisting mainly of a superficial condensation of chromatin. Several nucleoli are seen inside the nuclei, suggesting
intense protein synthesis. One hepatocyte contains 2 nuclei. Pararosaniline–toluidine blue (PT) stain. Medium
magnification.
Figure 3–2. Schematic representation of a cell nucleus. The nuclear envelope is made of 2 membranes of the
endoplasmic reticulum, enclosing a perinuclear cisterna. Where the two membranes fuse, they form nuclear pores.
Ribosomes are attached to the outer nuclear membrane. Heterochromatin clumps are associated with the nuclear
lamina, whereas the euchromatin (EC) appears dispersed in the interior of the nucleus. In the nucleolus, note the
associated chromatin (arrows), heterochromatin (Hc), the pars granulosa (G), and the pars fibrosa (F).
Figure 3–3. Three-dimensional representation of a cell nucleus to show the distribution of the nuclear pores, the
heterochromatin (dark regions), the euchromatin (light regions), and a nucleolus. Note that there is no chromatin
closing the pores. The number of nuclear pores varies greatly from cell to cell.
Figure 3–4. Electron micrograph of a nucleus, showing the heterochromatin (HC) and
euchromatin (EC). Unlabeled arrows indicate the nucleolus-associated chromatin around the
nucleolus (NU). Arrowheads indicate the perinuclear cisterna. Underneath the cisterna is a layer
of heterochromatin, the main component of the so-called nuclear membrane seen under the
light microscope. x26,000.
Figure 3–5. Illustration to show the structure, the localization, and the relationship of the nuclear lamina with
chromosomes. The drawing also shows that the nuclear pore complex is made of 2 protein rings in an octagonal
organization. From the cytoplasmic ring, long filaments penetrate the cytosol, and from the intranuclear ring arise
filaments that constitute a basketlike structure. The presence of the central cylindrical granule in the nuclear pore is
not universally accepted.
Figure 3–6. Electron micrographs of nuclei showing their envelopes composed of 2 membranes and the nuclear
pores (arrows). The two upper pictures are of transverse sections; the bottom is of a tangential section. Chromatin,
frequently condensed below the nuclear envelope, is not usually seen in the pore regions. x80,000.
Figure 3–7. Electron micrograph obtained by cryofracture of a rat intestine cell, showing the two components of the
nuclear envelope and the nuclear pores. (Courtesy of P Pinto da Silva.)
Figure 3–8. Simplified representation of 2 nuclear pore complexes. In this model, the final nuclear portion is seen to
be a more continuous structure, in the shape of a ring.
Figure 3–9. Schematic representation of a nucleosome. This structure consists of a core of 4 types of histones (2
copies of each)—H2A, H2B, H3, and H4—and one molecule of H1 or H5 located outside the DNA filament.
Figure 3–10. The orders of chromatin packing believed to exist in the metaphase chromosome. Starting at the top,
the 2-nm DNA double helix is shown; next is the association of DNA with histones to form filaments of nucleosomes
of 11 nm and 30 nm. Through further condensation, filaments with diameters of 300 nm and 700 nm are formed.
Finally, the bottom drawing shows a metaphase chromosome, which exhibits the maximum packing of DNA.
Figure 3–11. Morphologic features of sex chromatin in human female oral (buccal) epithelium and in a
polymorphonuclear leukocyte. In the epithelium, sex chromatin appears as a small, dense granule adhering to the
nuclear envelope. In the leukocyte, it has a drumstick shape.
Figure 3–12. Human karyotype preparation made by means of a banding technique. Each chromosome has a
particular pattern of banding that facilitates its identification and also the relationship of the banding pattern to
genetic anomalies. The chromosomes are grouped in numbered pairs according to their morphologic characteristics.
Figure 3–13. Photomicrograph of 2 primary oocytes, each one with its pale cytoplasm and round, dark-stained
nucleus. In each nucleus the nucleolus, very darkly stained, is clearly seen. The sectioned chromosomes are also
seen, because they are condensed. These cells stopped at the first meiotic division. Meiosis will proceed just before
ovulation (extrusion of the oocyte from the ovary; see Chapter 23).
Figure 3–14. Electron micrograph of a nucleolus. The nucleolar organizer DNA (NO), pars fibrosa (PF), pars
granulosa (PG), nucleolus-associated chromatin (NAC), nuclear envelope (NE), and cytoplasm (C) are shown.
Figure 3–15. Phases of mitosis.
Figure 3–16. Photomicrograph of cultured cells to show cell division. Picrosirius-hematoxylin stain. Medium
magnification. A: Interphase nuclei. Note the chromatin and nucleoli inside each nucleus. B: Prophase. No distinct
nuclear envelope, no nucleoli. Condensed chromosomes. C: Metaphase. The chromosomes are located in a plate at
the cell equator. D: Late anaphase. The chromosomes are located in both cell poles, to distribute the DNA equally
between the daughter cells.
Figure 3–17. Images obtained with a confocal laser scanning microscope from cultured cells. An interphase
nucleus and several nuclei are in several phases of mitosis. DNA appears red, and microtubules in the cytoplasm
are blue. Medium magnification. A: Interphase. A nondividing cell. B: Prophase. The blue structure over the nucleus
is the centrosome. Note that the chromosomes are becoming visible because of their condensation. The cytoplasm
is acquiring a round shape typical of cells in mitosis. C: Metaphase. The chromosomes are organized in an
equatorial plane. D: Anaphase. The chromosomes are pulled to the cell poles through the activity of microtubules. E:
Early telophase. The two sets of chromosomes have arrived at the cell poles to originate the two daughter cells,
which will contain sets of chromosomes similar to those in the mother cell. F: Telophase. The cytoplasm is being
divided by a constriction in the cell equator. Note that the daughter cells are round and smaller than the mother cell.
Soon they will increase in size and become elongated. (Courtesy of R Manelli-Oliveira, R Cabado, and G MachadoSantelli )
Figure 3–18. Electron micrograph of a section of a rooster spermatocyte in metaphase. The figure shows the two
centrioles in each pole, the mitotic spindle formed by microtubules, and the chromosomes in the equatorial plane.
The arrows show the insertion of microtubules in the centromeres. Reduced from x19,000. (Courtesy of R McIntosh.)
Figure 3–19. Electron micrograph of the metaphase of a human lung cell in tissue culture. Note the insertion of
microtubules in the centromeres (arrows) of the densely stained chromosomes. Reduced from x50,000. (Courtesy of
R McIntosh.)
Figure 3–20. Phases of the cell cycle in bone tissue. The G1 phase (presynthesis) varies in duration, which
depends on many factors, including the rate of cell division in the tissue. In bone tissue, G1 lasts 25 h. The S phase
(DNA synthesis) lasts about 8 h. The G2-plus-mitosis phase lasts 2.5–3 h. (The times indicated are courtesy of RW
Young.)
Figure 3–21. The 4 phases of the cell cycle. In G1 the cell either continues the cycle or enters a quiescent phase
called G0. From this phase, most cells can return to the cycle, but some stay in G0 for a long time or even for their
entire lifetime. The checking or restriction point (R) in G1 stops the cycle under conditions unfavorable to the cell.
When the cell passes this restriction point, it continues the cycle through the synthetic phase (S) and the G2
phase, originating 2 daughter cells in mitosis (M) except when interrupted by another restriction point (not shown) in
G2.
Figure 3–22. Section of a malignant epithelial skin tumor (squamous cell carcinoma). An increase in the number of
cells in mitosis and diversity of nuclear morphology are signs of malignancy. PT stain. Medium magnification.
Figure 3–23. Section of a fast-growing malignant epithelial skin tumor showing an increased number of cells in
mitosis and great diversity of nuclear morphology. PT stain. Medium magnification.
Figure 3–24. Section of a mammary gland from an animal whose lactation was interrupted for 5 days. Note atrophy
of the epithelial cells and dilation of the alveolar lumen, which contains several detached cells in the process of
apoptosis, as seen from the nuclear alterations. PT stain. Medium magnification.
Figure 3–25. Electron micrograph of a cell in apoptosis showing that its cytoplasm is undergoing a process of
fragmentation in blebs that preserve their plasma membranes. These blebs are phagocytized by macrophages
without eliciting an inflammatory reaction. No cytoplasmic substances are released into the extracellular space.
THE END