CELL DIVISION

Cell division is the process by which new cells arise. A cell that is undergoing division is called a mitotic figure. The importance of learning to recognize mitotic figures should be emphasized. For example, the number of mitotic figures in a tissue provides an idea of the rate of turnover of cells in that tissue. Recognition of mitotic figures is useful in interpreting various kinds of pathology. For example, mitotic figures are usually absent in a benign tumor whereas they are usually numerous in a malignant tumor.

Types of cell division

1. Meiosis. In this type of cell division, the number of chromosomes is halved. The diploid number (46) is reduced to the haploid number (23) in the daughter cells. Meiosis is restricted to gametes (spermatocytes and oocytes)

2. Amitosis. Amitosis is often referred to as direct cell division. It is a type of cell division where the cell breaks into two parts by constriction of the nucleus and cytoplasm. There may be unequal distribution of the chromosomes. This type of cell division occurs in the placenta and in the surface cells of transitional epithelium.

3. Mitosis. This type of cell division is often called indirect cell division. It is characterized by an exact division and distribution of the chromosomal material between the two daughter cells. There is a division of the cytoplasm (cytokinesis) and a division of the nucleus (karyokinesis). It is the most common type of cell division.

Events prior to mitosis. These are events, which occur during interphase.

1. Replication of chromosomal DNA. Replication occurs at the molecular level and is not visible microscopically.

2. Growth of the cell.

Stages of mitosis. There are four stages: prophase, metaphase, anaphase, and telophase. Each stage merges imperceptibly into the next, making it impossible in some cases to recognize the end of one stage and the beginning of the next. It should be stressed that mitosis is a continuous process, which is arbitrarily divided into four stages. See Figure 9 below.

1. Prophase. Events include:

a. Chromosomes become visible. They are delicate coiled filaments at first, but later shorten and thicken.

b. Each chromosome divides longitudinally into two chromatids. The

chromatids, two of these constituting a chromosome, remain attached at some point along their length. This zone of attachment is called the centromere and may be located anywhere along the length of the two chromatids.

c. Centrioles migrate to opposite nuclear poles.

d. Nucleolus disappears. This event occurs in late prophase.

e. Nuclear membrane disappears. This event also occurs in late prophase.

f. Mitotic spindle appears. The mitotic spindle is a system of microtubules, which develop in the area between the two centrioles.

g. Kinetochore appears. The kinetochore is a small disc-like structure, which has been shown by electron microscopy to develop on each chromatid at the site of the centromere. Some of the microtubules of the mitotic spindle become attached to the kinetochore.

Figure 9: Phases of Mitosis. Taken from: Junqueira and Carneiro, Basic Histology, Text and Atlas, p. 60, Figure 3-15.

2. Metaphase. Events include:

a. Chromosomes become fully condensed and their centromeres aligned midway in the mitotic spindle. The centromeres occupy the area referred to as the equatorial or metaphase plate of the cell.

b. Mitotic spindle becomes fully developed. Two groups of microtubules form the spindle: chromosomal tubules and continuous tubules. The latter are not attached to the centriole at the other pole. Chromosomal tubules extend from the kinetochore of a chromatid to one of the centrioles. The kinetochore of the other chromatid of the chromosome is attached by chromosomal tubules to the opposite centriole. Thus, each chromosome has tubules from the other centriole attached to its other chromatid. Colchicine has been shown to arrest mitosis at the metaphase stage. It interferes with formation of the microtubules of the mitotic spindle, leaving the cell suspended in metaphase and unable to complete mitosis.

c. Disappearance of the nuclear envelope and nucleolus.

Colchicine is sometimes added to cultures of leukocytes whose mitotic activity has been stimulated with phytohemagglutinin, an extract from the red kidney bean. The cells are then spread on glass slides and stained. The chromosomes of these cells arrested in metaphase by colchicine can be studied and photographed. The photographs can be cut up, and the pairs of identical chromosomes can be matched to form a karyotype of the chromosomes of the individual from whom the cells were taken. The karyotype can be studied for abnormalities in number and form of the chromosomes.

3. Anaphase. Events include:

a. Centromeres split. The splitting may be due to the pull exerted on them by the microtubules as they move in opposite directions. Each chromatid is now called a chromosome.

b. Chromosomes move to opposite poles of the cell. In moving toward the cell pole, each chromosome travels with the kinetochore leading the way, the remainder of the chromosome appearing to be dragged behind. Anaphase is concluded when the two chromosomal masses have moved to opposite poles of the cell.

c. Centrioles divide.

4. Telophase. Events are reverse of prophase.

a. Chromosomes lose their identity. They straighten out into long thin strands, and they are so fine that they are beyond the resolution of the light microscope.

b. Nuclear membrane appears. It develops from cisternae of endoplasmic reticulum.

c. Nucleolus appears.

d. Organelles distributed between two daughter cells. They are distributed in approximately equal numbers.

Identification of mitotic figures in tissue sections

Although you should be familiar with the process of mitosis from previous courses, you should review the textbook material on mitosis before identifying mitotic figures in the class slides. Compare mitotic figures you see in the microscope with textbook illustrations. Frequently you will be asked to refer to your textbook as an aid to understanding the laboratory work and for purposes of orienting you in regard to unfamiliar tissues. You should therefore always bring your textbook to the laboratory.

You should learn to recognize cells in the various stages of cell division (mitosis) and develop an awareness of those region(s) of organs and tissues in which “mitotic figures" are most numerous. Study mitotic figures in the following slides.

Slide A1, Vicia faba, root tip, (Feulgen). This slide was prepared from a growing squash root, stained with the Feulgen technique, which is relatively specific for DNA, to show the various stages of mitosis. Individual chromosomes can be seen. Find prophase, metaphase, anaphase, and telophase stages in this slide. Try to recognize a mitotic figure with the low power objectives for careful study.

Slide A2, Ambystoma larva, epidermis, whole mount, (Fe-Hematoxylin). Skin from an

amphibian larva. You will have to focus up and down as the piece of tissue has numerous folds. You should find good examples of the stages of mitosis. The large, brown dendritic-appearing cells are subepithelial melanophores.

Slide B60, ileum, monkey, (H&E). “Mitotic figures” typical of tissue sections can be seen in the intestinal crypts (base of the finger-like projections or villi). Some, but not all, of the mitotic stages can be found.