IMMUNOHISTOCHEMICAL LOCALIZATION OF STEROIDOGENIC ENZYMES AND PHENYLETHANOLAMINE-N-METHYL-TRANSFERASE (PNMT) IN THE ADRENAL-GLAND OF THE FETAL AND NEWBORN FOAL

An increase in fetal adrenal cortisol output signals the onset of part urition in many animal species but, in the fetal horse, plasma concent rations 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). I mmunohistochemistry was used to determine the localisation and changes in distribution of key steroidogenic enzymes for cortisol production; P450(scc), P450(C17) and 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) in adrenal tissue from fetal and newborn horses and these finding s were correlated with the appearance of immunoreactive (IR)-phenyleth anolamine-N-methyl-transferase (PNMT), a cortisol-dependent enzyme, Fi ve 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-p eroxidase technique. All 3 steroidogenic enzymes were present by Day 1 50, 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. P450(scc) and P450(C17) predominated in c ortical cells proximal to the medulla; 3 beta HSD was present througho ut the cortex, but more in the zona fasciculata. In foals after birth, IR-3 beta HSD and IR-P450(scc) had increased substantially throughout the adrenal cortex, and IR-P450(C17) 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 inter face 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 persiste d into the post natal period. This study suggests that the low cortiso l output of the adrenal of the fetal horse is associated with relative ly 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 preced ed complete adrenocortical expression of the enzymes on the cortisol b iosynthetic pathway. This pattern of distribution is quite different f rom that seen in fetal adrenal tissue of other species such as the she ep. This work provides further understanding of the part played by the equine fetal adrenal gland and placenta in the pathway of steroid met abolism leading up to parturition and associated processes of maturati on in the fetal foal. These represent the basis of clinical conditions of prematurity and dysmaturity in the horse.