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263 CHAPTER 9 Basic Cell Biology and Cancer This chapter describes basic cell biology and then looks closely at what goes wrong on the cellular level when someone develops cancer . Cancer arises when there is a disruption of the normal mechanisms that govern how frequently cells divide, how long cells survive, and where cells grow. Understanding the processes that determine the behavior of normal, healthy cells helps us understand how relatively small changes in the molecules that make up cells can lead to the abnormal behavior displayed by cancer cells. And the differences in the behavior and biochemical makeup of healthy cells and cancer cells make it possible for cancer treatments to work. What Is Cancer? Just as the different organs that make up a living organism must work together , the individual cells that make up the different tissues and organs of the body must fulfill their distinct functions and work together for the good of the whole. Although all the cells in an organism are derived from one original cell—the fertilized egg—they differentiate during development and become specialized to carry out their distinctive functions. The differ‑ entiated properties of a mature cell depend on both the inherent nature of the specific cell type and the cell’s response to cues in its environment. Both of these factors influence the specific traits any individual cell displays. Functional specializations are reflected in the appearance of the different cell types. Like so many tiny Bauhaus creations, living cells obey the “form follows function” rule. For example, nerve cells, or neurons, which are specialized to receive information and then pass it on, often have small, branching processes that gather incoming signals from other cells and one long U 264 Understanding Lymphoma “arm” to convey that information to target cells. The cells that line the intestines , on the other hand, which transport nutrients from the inside of the gut to the bloodstream, have highly folded membranes on the side facing the gut to maximize the surface area exposed to these nutrients. Cells must not only carry out their appropriate functions but must also grow in the appropriate environment. A kidney cell, for instance, would be useless growing in the middle of the liver, no matter how faithfully it attempted to carry out the functions appropriate to kidney cells. Each cell type also has a characteristic life span. Some cells, like the neurons in the brain, last for the lifetime of the organism and do not continue to divide and propagate themselves once development is complete. Others, like the cells that line the gut, have brief life spans and must be replaced. Some mature, fully differentiated cell types retain the ability to replicate themselves, but many lose this ability. These cell types need to be replenished from a population of stem cells. Stem cells are partially committed to a certain fate—the hematopoietic stem cells in the bone marrow, for example, may become different types of blood cells, but they will never differentiate into the cells that line the intestines. However, they retain the ability to divide and are not yet fully differentiated; they do not look like mature cells or carry out their functions. When something goes wrong with the mechanisms that control the total numbers of a given cell’s offspring, cancer may ultimately result. Cancer may develop either after a cell starts to divide too frequently or after a type of cell that normally propagates rapidly but does not normally survive very long begins to survive for a long time. Either of these two mechanisms will lead to an abnormal accumulation of that cell’s offspring. A cell may begin to divide too frequently because it either (1) loses the ability to respond to external cues that slow down cell division or (2) abnormally expresses some molecule that promotes cell division. Such aberrations in cell division and cell death are caused by mutations (see below). To become unquestionably cancerous, a cell needs to acquire a number of such mutations. While cancer cells often resemble the cell type from which they arise and may retain some of the behaviors characteristic of that cell type, they don’t express all the properties of the original cell. Not only do they grow and divide inappropriately, but they are also unable to carry out their proper function. Cancer cells often look abnormal, and they may not respond appropriately to cues in the environment—both those that control Basic Cell Biology and Cancer 265 V...


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