Cell Division

Cell division, or mitosis (Gr. mitos, a thread), can be observed with the light microscope. During this process, the parent cell divides, and each of the daughter cells receives a chromosomal set identical to that of the parent cell. Essentially, a longitudinal duplication of the chromosomes takes place, and these chromosomes are distributed to the daughter cells. The phase between two mitoses is called interphase, during which the nucleus appears as it is normally observed in microscopic preparations. The process of mitosis is subdivided into phases to facilitate its study (Figures 3-15, 3-16, and 3-17).

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 Machado-Santelli.)

The prophase of mitosis is characterized by the gradual coiling of nuclear chromatin (uncoiled chromosomes), giving rise to several individual rod- or hairpin-shaped bodies (coiled chromosomes) that stain intensely. At the end of prophase, the nuclear envelope is broken by phosphorylation (addition of PO₄^3-) of the nuclear lamina proteins, originating vesicles that remain in the cytoplasm. The centrosomes with their centrioles separate, and a centrosome migrates to each pole of the cell. The duplication of the centrosomes and centrioles starts in the interphase, before mitosis. Simultaneously with centrosome migration, the microtubules of the mitotic spindle appear between the two centrosomes, and the nucleolus disintegrates.

During metaphase, chromosomes, due to the activity of microtubules, migrate to the equatorial plane of the cell, where each divides longitudinally to form two chromosomes called sister chromatids. The chromatids attach to the microtubules of the mitotic spindle (Figures 3-18 and 3-19) at an electron-dense, DNA protein plaque, the kinetochore (Gr. kinetos, moving, + chora, central region), located close to the centromere (Gr. kentron, center, + meros, part) of each chromatid.

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.)

In anaphase, the sister chromatids separate from each other and migrate toward the opposite poles of the cell, pulled by microtubules. Throughout this process, the centromeres move away from the center, pulling the remainder of the chromosome along. The centromere is the constricted region of a mitotic chromosome that holds the two sister chromatids together until the beginning of anaphase.

Telophase is characterized by the reappearance of nuclei in the daughter cells. The chromosomes revert to their semidispersed state, and the nucleoli, chromatin, and nuclear envelope reappear. While these nuclear alterations are taking place, a constriction develops at the equatorial plane of the parent cell and progresses until the cytoplasm and its organelles are divided in two. This constriction is produced by microfilaments of actin associated with myosin that accumulate in a beltlike shape beneath the cell membrane.

Most tissues undergo constant cell turnover because of continuous cell division and the ongoing death of cells. Nerve tissue and cardiac muscle cells are exceptions, since they do not multiply postnatally and therefore cannot regenerate. The turnover rate of cells varies greatly from one tissue to another rapid in the epithelium of the digestive tract and the epidermis and slow in the pancreas and the thyroid gland.

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