Effects of hormones on development of fetal enzymes

The adaptation of newborn mammals to extrauterine life is conditioned, to a large extent, by the maturity that their biochemical functions have attained during prenatal tissue differentiation. We concentrated on studying the development of enzyme systems in rat liver during late fetal and neonatal life; this was combined with the morphometry of cell types and of subcellular organelles. A cluster of enzymes that emerge during late fetal life and attain their adult concentrations at term has been distinguished from another cluster that makes its appearance immediately after birth. Different hormones (e.g., thyroxine, glucagon, and glucocorticoids), the secretion of which is the natural stimulus for the expression of specific groups of genes, have been identified. With the administration of these hormones to fetuses it is possible to evoke the synthesis of chosen enzymes before the scheduled time and to produce newborn mammals (delivered at term or prematurely) with precociously mature enzyme profiles. The potential medical application of the numerous experimental means by which we can now manipulate the course of enzymic differentiation remains to be evaluated.