Glucocorticoids and the preparation for life after birth: are there long-term consequences of the life insurance?

In mammals, survival both before and after birth depends on a number of key physiological processes such as the provision of 02, a supply of oxidative substrates and a means of removing the waste products of metabolism. In adults these processes are carried out by a number of different tissues, whereas in the fetus they are carried out primarily by the placenta. The successful transition from intrato extra-uterine life, therefore, depends on the ability of specific tissues and organ systems to take over the functions of the placenta at birth. Consequently, organs such as the lungs, liver, kidneys and gut undergo maturational changes during late gestation in preparation for extra-uterine life. Most of these changes are glucocorticoid dependent and can be induced prematurely by exogenous glucocorticoid administration (Silver, 1990; Liggins, 1994; Fowden, 1995). As a consequence, synthetic glucocorticoids are now routinely administered to women in threatened preterm labour to improve neonatal viability (NIH Consensus Development Conference, 1995). In all species studied so far, there is an increase in the circulating glucocorticoid concentration in the fetus towards term (Fig. 1). The magnitude and timing of this cortisol surge vary between species, as does the precise mechanism by which it occurs (Fig. 1; Fowden & Silver, 1995; Wood & Cudd, 1997). In most species, the prepartum cortisol surge is due to increased adrenal cortisol output, but in certain animals, such as the rat and horse, its effect is enhanced by a fall in the level of plasma corticosteroidbinding globulin (Challis et al. 1993; Wood & Cudd, 1997). In normal conditions, therefore, fetal tissues are exposed to increasing levels of bioactive glucocorticoid for periods of up to 10-15 d before delivery.