The expansion in size of a region of tissue, often called growth, is critical to embryonic development and tissue repair. Growth of a tissue most often occurs by an increase in cell number. In fact, sequential cell division—and a resulting increase in total cell number—is the most important change of early development. As development proceeds, however, the rate of increase in cell number slows but the overall size of the organism continues to increase steadily. Growth throughout life can occur by a variety of mechanisms in addition to increased cell number; for example, increases in cell volume or extracellular volume also produce growth. The overall growth of an organ or tissue can involve multiple mechanisms. For example, in the nervous system, neurons increase in size, but not number, as a juvenile grows to adulthood. By contrast, glial cells within the nervous system divide and proliferate throughout life. Overall, however, cell proliferation (which occurs by the process of sequential cell division) is the most important feature of tissue growth. Growth is only one of the changes that occurs with development. As a child grows to adulthood, her increase in size is probably less astonishing than her overall change in behavior and ability. Underlying this overall change are dramatic alterations in function and operation of individual cells; this observation is related to the discussion in Chapter 3, in which the processes of cellular differentiation and specialization were introduced. The child develops by reference to a fixed instructional program, the genome, which somehow encodes all of the molecular signals that lead to increases in size, changes in shape, and inexorable dynamics of aging. But the child is also influenced by her environment and the opportunities for change that her environment presents. One child becomes a doctor and another a cellist; the factors and forces that nudge each down her path are not programmed by the genes alone. Similarly, differentiation of a cell is influenced by its genetic composition and the environment that surrounds it. This chapter begins with a discussion of mechanisms and kinetics of cell division. Later parts of the chapter consider some of the factors that influence cell differentiation. The relationship of cell growth during development of a normal organism and cell growth in culture is introduced in the final sections.
DIBUTYRYL CYCLIC ADENOSINE 3' 5' MONOPHOSPHATE, SUGAR TRANSPORT, AND REGULATORY CONTROL OF CELL DIVISION IN NORMAL AND TRANSFORMED CELLS