Changes in cell-cycle kinetics responsible for limiting somatic growth in mice.

In a growing organism, cell proliferation at a constant rate would be expected to produce unlimited, exponential somatic growth (1). In reality, body mass in mammals does not even increase at a linear rate (1). Instead, the growth rate decreases with age; growth is rapid in early postnatal life and then slows, approaching zero, as the animal approaches its adult body size (2, 3). In small mammals, somatic growth deceleration occurs over weeks (2) whereas in large mammals it occurs over years (3), leading to the enormous variation in body size among different mammalian species. Growth deceleration is caused in large part by a decrease in the rate of cell proliferation. This decline does not appear to result simply from a decrease in growth hormone or insulin-like growth factor-I levels; in late human adolescence, for example, when the somatic growth rate is approaching zero, circulating growth hormone and insulin-like growth factor-I levels are greater than they are in early childhood when the growth rate is more rapid (4, 5). For the growth plate, in particular, there is evidence that the decline in growth rate is due to a local, rather than a systemic, mechanism (6). We asked whether the decline in cell proliferation that occurs with age is due to an increase in the cell-cycle time or to a decrease in the growth fraction. The cell-cycle time is the average period of time required for a cell to pass through the entire cell cycle (7). The growth fraction represents the number of cells that remain in the cell cycle divided by the total number of cells. Thus, a decrease in growth fraction indicates that more cells have entered G0 or otherwise exited the cell cycle. To answer this question, we used an approach involving an initial injection of [methyl-3H]thymidine (3H-thymidine) and subsequent injections of 5’-bromo-2’deoxyuridine (BrdU), each of which labels newly synthesized DNA (7). A cell becomes labeled by both 3H-thymidine and BrdU only if it is in S-phase at the time of both injections. Therefore, double labeling occurs in a cell if the interval between the 3H-thymidine and BrdU injection is equal to the cell-cycle time. We used this method to assess changes in cell-cycle kinetics in the mouse as somatic growth decelerates postnatally. We chose to study liver and kidney as examples of organs in which proliferation slows with age. In other tissues, such as intestinal epithelium, epidermis, and hematopoietic tissue, continued proliferation is needed in adulthood because of the rapid turnover of terminally differentiated cells. Kidney was chosen as an example of a tissue in which the decline in proliferation is largely irreversible while liver was chosen as an example of a tissue in which the decline can be reversed e.g. after partial hepatectomy.