An increase in fetal adrenal cortisol output signals the onset of parturition in many animal species but, in the fetal horse, plasma concentrations of cortisol remain low for much of late pregnancy, with a rise occurring only very close to the time of birth (term 320–360 days). Immunohistochemistry was used to determine the localisation and changes in distribution of key steroidogenic enzymes for cortisol production; P450SCC, P450C17 and 3β-hydroxysteroid dehydrogenase (3βHSD) in adrenal tissue from fetal and newborn horses and these findings were correlated with the appearance of immunoreactive (IR)-phenylethanolamine-N-methyl-transferase (PNMT), a cortisol-dependent enzyme. Five micron sections of adrenal tissue from fetuses at Day 100–156 (n=5), Day 244–295 (n=8), greater than Day 300 (n=4) and from newborn foals (n=6), were stained using specific antibodies and the avidin-biotin-peroxidase technique. All 3 steroidogenic enzymes were present by Day 150, but in less than 20% of the cortical cells. By late gestation the steroidogenic enzymes were present in approximately 30% of the cells, but the distribution varied. P450SCC and P450C17 predominated in cortical cells proximal to the medulla; 3βHSD was present throughout the cortex, but more in the zona fasciculata. In foals after birth, IR-3βHSD and IR-P450SCC had increased substantially throughout the adrenal cortex, and IR-P450C17 was present in most cells of the presumptive zonae fasciculata and reticularis. IR-PNMT was localised to nuclei of scattered medullary cells at the medullary-cortical interface by Day 150. It was present in the cytoplasm of a band of cells at the medullary-cortical interface by Day 310 and this pattern persisted into the post natal period. This study suggests that the low cortisol output of the adrenal of the fetal horse is associated with relatively poor expression of steroidogenic enzymes during most of fetal life, but that the presence of these enzymes increases rapidly just before or after birth. Differentiation of the IR-PNMT containing cells preceded complete adrenocortical expression of the enzymes on the cortisol biosynthetic pathway. This pattern of distribution is quite different from that seen in fetal adrenal tissue of other species such as the sheep.
This work provides further understanding of the part played by the equine fetal adrenal gland and placenta in the pathway of steroid metabolism leading up to parturition and associated processes of maturation in the fetal foal. These represent the basis of clinical conditions of prematurity and dysmaturity in the horse.