Cell size and cell division of the anterior pituitary: time course in the growing rat.

Pituitary cells increase their numbers more than 3-fold during the 1st 10 days of life while maintaining the same cell size ratios. In the 25-day-old animal, the rate of cell division slows and there is a slight increase in the number of large cells. An increase in adult weight is attributed to hyperplasia and a shift to a population of larger cells.