Evidence for independent regulators of organ maturation in fetal rabbits

The degree of organ development bears a relationship to gestational age, but in a given infant not all organs need be at the same stage of development. The range of variation of organ maturation can be best studied in an animal with a large litter, such as the rabbit, in which gestation can be precisely timed. Body weight, ossification centers, and lung "maturity," defined as distensibility and stability on deflation, were all measured. Marked variations within a litter allowed study of the relationships between them. Body weight and the number of ossification centers were closely correlated at the same gestational age (R = 0.88, p 001). Ossification centers were less well correlated with gestational age. Lung "age," on the other hand, was more closely related to gestational age than body weight. A single injection of the fetus with 9-fluoroprednisolone, a glucocorticoid, at 24 days resulted in an acceleration of lung maturation, defined in terms of distensibility and the presence of surfactant by 26 to 27 days. No increase in lung weight took place. Similarly, the glucocorticoid had no effect on body size or the number of ossification centers, suggesting that regulators of maturation of these organs are independent.