Control of Cell Division

How often a cell divides is strictly controlled and varies with cell type. Skin cells, blood-forming cells, and cells that line the intestine, for example, divide often and continually. In contrast, cells of the liver divide a specific number of times and then cease—they are alive and specialized, but no longer divide. If, however, injury or sur gery removes some liver cells, the remaining cells may be stimulated to divide again, regenerating the organ. Some cells, such as certain nerve cells, lose their ability to divide as they differentiate; therefore, damage to these nerve cells usually permanently impairs nerve function. Most organs include cells, called stem cells, that retain the ability to divide into adulthood, giving the body a built-in repair mechanism of sorts. In the brain, for example, neural stem cells can produce new nerve cells. Researchers are just discovering the extent of the body's reserve of stem cells.

Clinical Application 3.4 considers cloning, a technique that places the nucleus of a differentiated cell into a

Chromosomes

Centrioles

Chromosomes

Centrioles

Nuclear envelopes

Figure 3.42

Nuclear envelopes

Figure 3.42

(b)

(a) In telophase, chromosomes elongate to become chromatin threads, and the cytoplasm begins to be distributed between the two newly forming cells. The replicated chromatids have separated to form the chromosomes of the two new cells. (b) A micrograph of a cell in telophase (280x micrograph enlarged to 1,100x).

Stage

Major Events

Prophase

Chromatin condenses into chromosomes; centrioles move to opposite sides of cytoplasm; nuclear membrane and nucleolus disperse; microtubules appear and associate with centrioles and chromatids of chromosomes.

Metaphase

Spindle fibers from the centrioles attach to the centromeres of each chromosome; chromosomes align midway between the centrioles.

Anaphase

Centromeres separate, and chromatids of the chromosomes separate; spindle fibers shorten and pull these new individual chromosomes toward centrioles.

Telophase

Chromosomes elongate and form chromatin threads; nuclear membranes appear around each chromosome set; nucleoli appear; microtubules break down.

fertilized egg cell lacking a nucleus and regenerates a new individual from the altered cell. The ability to clone indicates that even a nucleus in a highly differentiated cell can be stimulated to express genes that it normally represses.

Most types of human cells divide up to about 50 times when grown in the laboratory. Adherence to this limit can be startling. A connective tissue cell from a human fetus divides 35 to 63 times, the average being about 50 times. However, a similar cell from an adult does so only 14 to 29 times, as if the cells "know" how many times they have divided.

A physical basis for this mitotic clock is the DNA at the tips of chromosomes (telomeres), where the same six-nucleotide sequence repeats many hundreds of times. Each mitosis removes up to 1,200 nucleotides. When the chromosome tips wear down to a certain point, this somehow signals the cell to cease dividing.

Other external and internal factors influence the timing and frequency of mitosis. Within cells, waxing and waning levels of proteins called kinases and cyclins control the cell cycle. Another internal influence is cell size, specifically the ratio between the surface area the cell membrane provides and the cell volume. The larger the cell, the more nutrients it requires to maintain the activities of life. However, a cell's surface area limits the amount of nutrients that can enter. Because volume increases faster than does surface area, a cell can grow too large to efficiently obtain nutrients. A cell can solve this growth problem by dividing. The resulting daughter cells are smaller than the original cell and thus have a more favorable surface area-to-volume relationship.

External controls of cell division include hormones and growth factors. Hormones are biochemicals manufactured in a gland and transported in the bloodstream to a site where they exert an effect. Hormones signal mitosis in the lining of a woman's uterus each month, building up the tissue to nurture a possible pregnancy. Similarly, a pregnant woman's hormones stimulate mitosis in her breasts when their function as milk-producing glands will soon be required.

Growth factors are like hormones in function but act closer to their sites of synthesis. Epidermal growth factor, for example, stimulates growth of new skin beneath the scab on a skinned knee. Salivary glands also

Turtle Salivary Gland
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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