THE DEVELOPMENT OF THE ADRENAL-CORTEX IN THE RAT - AN IMMUNOHISTOCHEMICAL STUDY

The Inner Zone Antibody (IZAb) is a monoclonal antibody which interacts with an antigen found predominantly in rat adrenal inner cortical zones. Since its expression increases after ACTH treatment the antigen may have a role in steroidogenesis although, so far, this has not yet been fully characterised. Due to its molecular weight, it cannot be any of the known cytochrome P450 proteins. In this study we examined the expression of IZAb in male and female rats throughout their postnatal development and in aged animals. In a different set of animals, blood was collected for hormonal assays and the adrenals stained with classical methods. The staining with IZAb was clear from the first post-natal day. The zona glomerulosa which was always present at birth, was easily distinguished and unstained. The staining in the inner zone cells was fainter at birth and increased progressively until postnatal day 20. Afterwards these cells were remarkably stained at all ages. Medullary cells were also present from birth although they were generally found in clusters instead of constituting a well defined zone. Cortical cells appeared in the medullary zone at all ages after its complete development. The zona glomerulosa increased in size until approximately postnatal day 40 while the inner zones increased until day 70. The area of the cortex was significantly different between the two sexes from day 50 onwards and this was predominantely due to differences in the zona fasciculata. Corticosterone levels increased until approximately day 25 in the male rat and until day 45 in the female. The male and female divergence was significant from this day on. We concluded that IZAb allows a clear distinction of adrenal cortex zones already at birth and may help in zonation studies. The increase in staining intensity from birth till day 20, occurs in parallel with the increased secretion of certain hormones by the adrenal gland at that time. IZAb also demonstrated the presence of cortical cells in the medulla at all ages which suggests a possible paracrine function.