16 Lymphocyte Growth

Abstract

Size is one of the most obvious manifestations of natural variation. Differences in size abound between organisms—even within a single species, such as dogs, there can be over tenfold differences. Since the discovery of the cell as the basic unit of biology, the conundrum has been how such remarkable diversity of size can be achieved. For a long time, size differences were thought to be accounted for exclusively by differences in cell numbers. However, the sizes of cells within an individual organism may vary by orders of magnitude. For example, in most vertebrates a lymphocyte is several hundred times smaller than the largest neuron. Elegant studies have recently shown that the size of biological structures is determined not only by the number of cells but also by the size of those cells (Neufeld et al. 1998; see also Chapter 2).

Cell size is not infinitely variable. Cell growth and division are linked such that most dividing cells produce offspring of approximate parental size, a process referred to as replicative cell division. The outstanding progress in the understanding of the molecular control of the cell cycle over the past quarter century has perhaps overshadowed the importance of cell growth as an independent phenomenon. Specific circumstances that uncouple cell growth from cell proliferation demonstrate that understanding the mechanisms which regulate growth—the net acquisition of biomass—is one of the fundamental problems of cell biology, along with cell division, death, and differentiation. For example, the initial divisions of a fertilized egg...