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Cell Division

Objectives

  1. Recognize and distinguish the various stages of mitosis
  2. Understand the changes in DNA content and chromosome number as cells progress through the cell-cycle and mitosis.

The history of all cells includes the division of pre-existing cells. Except for the gametes, the ordinary method of cell division is mitosis, a morphologically visible process. The hereditary characteristics are carried by the DNA-protein complexes which are found in equal amounts in nearly all nuclei of the body.

Mitosis is preceded by an exact duplication of the DNA and associated proteins in the nucleus. This occurs during interphase—the period between actual mitotic divisions. After this doubling, the chromosomes are organized, complete division and are then equally distributed to the two daughter cells during mitosis.

All somatic cells pass through the mitotic and interphase periods at one time or another. The duration of the mitotic period is usually about 0.5 to 2 hours, while the intermitotic period can vary from a few hours to many years. The mitotic index (proportion of cells undergoing division at any one time) varies greatly depending upon the type of tissue and the physiological needs of the organism. A high mitotic index characterizes all growing embryonic tissues, and certain adult tissues such as bone marrow and intestinal crypts as well as cancer cells.

We will study Cell Division during the G.I lab since the crypts of the small intestine are a convenient place to find mitotic cells. The student should learn to recognize and roughly estimate the number of mitotic figures normally seen in various tissues. Later, when pathological tissue is examined, the number of mitotic figures can give important clues to the type of disease. In humans and other mammals, the cells are relatively small in size and the number of chromosomes great, thus making accurate observations rather difficult. For this reason, cell division will be first studied in a plant (onion root tip) which has larger cells with fewer chromosomes. Mitotic figures will then be examined in mammalian bone marrow and intestine.

Examine the section of onion root tips, slide #1). Using the reversed 10x ocular, note that it consists of several sections mounted on the slide. Locate the growing tip (smaller, tapering, end). Under high magnification it can be seen that many of the cells are undergoing mitosis. With the aid of the textbook, pick out the various stages of mitosis and identify the components of the mitotic figures. It is important to realize that the stages of mitosis are defined only with reference to the nucleus and chromosomes. Cytokinesis, the division of the cytoplasm, may fail to occur giving rise to binucleated and/or polyploid cells. Although the actual process of mitosis is continuous, it is usually arbitrarily divided into 4 stages.

Interphase (not a stage of mitosis): The chromosomes cannot be distinguished and appear as scattered granules connected by a network of pale-staining strands within a distinct nuclear membrane. A nucleolus is usually present.

  1. In prophase, morphologically distinct chromatin threads appear and these shorten and thicken, forming distinct chromosomes. The two pairs of centrioles begin to separate; (onion root cells lack centrioles). Around the centrioles fine fibrils appear forming aster rays. The whole structure is called an amphiaster and some of the aster rays elongate to form the spindle fibers. The nucleolus and the nuclear membrane begin to disappear.

  2. At metaphase the chromosomes become arranged at the equatorial plate which is midway between the two pairs of centrioles. The spindle is now fully formed. It does not stain (achromatic) and consists of fine microtubules, some of which are attached to the chromosomes. During this period the chromosomes begin to show indications of splitting.

  3. At anaphase, each chromosome pair completes its splitting, and the two daughter chromosomes move toward opposite poles. Spindle microtubules may be seen between the retreating chromosomes.

  4. At telophase, the chromosomes have reached the spindle poles and appear as a dense, basophilic mass within which individual chromosomes cannot be defined. The nuclear membrane reappears, and the outlines of the chromosomes disappear leaving scattered chromatin granules connected by a pale-staining network. Nucleoli reappear and seem to be associated with a particular chromosome. Cytokinesis, the division of the cytoplasm, usually occurs during the telophase, but the synchrony between nuclear and cytoplasmic telophase, but the synchrony between nuclear and cytoplasmic events is not constant; cytokinesis may begin as early as late anaphase or be delayed beyond the nuclear reconstruction of telophase.

Mitosis occurs very frequently in cells of the bone marrow, gonads, epidermis, and epithelium of the intestine. However, slides of these structures will often show very few mitotic figures. This is due in part to marked diurnal variation in the rate of mitosis in different tissues; for example, sections of the skin will seldom show mitotic figures unless the biopsies are taken at night when cell division is most frequent. The methods used to obtain and prepare the cells for examination can also influence the number of mitotic figures seen. This involves not only the fixing and staining procedures, but also the treatment of the cells prior to fixing. For example, bone marrow slides are usually prepared by smearing the cells on a slide. This treatment causes most of the cells in mitosis to be disrupted, thereby greatly reducing the number actually seen. Mitosis is also affected by many factors in vivo, such as hormones, irradiation and drugs. Colchicine appears to prevent the formation of spindle fibers, thereby holding the cell in prolonged metaphase and preventing it from proceeding into anaphase.

Next, examine a section of intestine (slide #3). Look for mitotic figures in the crypts (the deepest part of the epithelial layer). The cytoplasm of the mitotic cells stains less intensely, and nuclei of mitotic cells stain more intensely than those of interphase cells, and change their position in the cell. The mitotic figures will not be as distinct as in the onion root. Cell division occurs in the crypt epithelium (called crypts of Lieberkuhn) and the cells push out over the villus where they slough off into the lumen of the intenstine. The process from division to sloughing takes from two days to seven days depending on the segment of the G.I. tract.