Figure 21–59 Effects of ploidy on cell size and organ size. In all organisms, from bacteria to humans, cell size is proportional to ploidy—the number of copies of the genome per cell. this is illustrated for (a–D) Arabidopsis flowers and (e) for salamanders. In each case, the upper panels show cells in a specific tissue [a petal for Arabidopsis, a pronephric (kidney) tubule for the salamander]; the lower panels show the gross anatomy—flowers for Arabidopsis, the whole body for the salamander. In the case of Arabidopsis flowers, increase in cell size increases organ size. By contrast, the salamander and its individual organs attain their normal standard size regardless of ploidy, because large cell size is compensated for by fewer cells. this indicates that the size of an organism or organ in this species is not controlled simply by counting cell divisions or cell numbers; size must somehow be regulated at the level of total cell mass. [a–D, from c. Breuer et al., Plant Cell 19:3655–3668, 2007. With permission from the american Society of plant Biologists; e, adapted from G. Fankhauser, in analysis of Development (B.h. Willier, p.a. Weiss and V. hamburger, eds), pp. 126–150. philadelphia: Saunders, 1955.] for example, are about five times the size of those in a haploid salamander, but there are only one-fifth as many cells. This scaling operates not only in the body as a whole, but in its individual organs.
The imaginal discs of Drosophila provide another striking example of homeostatic size control. These are epithelial pouches that grow by cell proliferation during the larval period and, during the pupal stage, form the organs and extremities of the adult fly (Figure 21–60). Experiments have been chiefly done on the wing imaginal disc. Mutations in components of the cell-cycle control machinery can be used to speed up or slow down the rate of cell division in the disc. Remarkably, such mutations can result in an excessive number of abnormally small cells
Figure 21–60 The imaginal discs in the Drosophila larva (below) and the structures in the adult (above) that they give rise to. [after J.W. Fristrom et al., in problems in Biology: rNa in Development (e.W. hanley, ed.), p. 382. Salt Lake city: University of Utah press, 1969.]
Figure 21–61 Pituitary dwarf and pituitary giant. the “giant” on the  right is robert Ladlow (1914–1940), the tallest recorded man at 8 feet  11 inches (2.72 m), together with his father, who was almost 6 feet tall (1.82 m). the dwarf on the left is General tom thumb, which was the stage name of charles Sherwood Stratton (1838–1883). On his 18th birthday, he was measured at 2 feet 8.5 inches (82.6 cm) tall, and at his death, he was  3 feet 4 inches (102 cm).(Images from http://en.wikipedia.org/wiki/ File:robert_Wadlow.jpg. © Bettmann/cOrBIS.) or a reduced number of abnormally large cells, respectively, leaving the size (area) and patterning of the adult wing practically unchanged. Thus, the size of the disc is not regulated so as to contain a set number of cells. Instead, there must be a regulatory mechanism that halts growth when the disc’s total cell mass reaches the appropriate value, so that the size and pattern of the adult wing that develops from the disc are normal. Remarkably, developing discs—or even disc fragments, taken out of their normal context and transplanted into the abdomen of an adult female—will grow until they reach their normal size. Clearly, the mechanisms that regulate disc size are intrinsic to the disc.