Close Relationship between Cortical and Medullary Cells in the Fetal Mouse Adrenal: Morphological Evidence and Functional Implications

In adult adrenal glands, the close cellular contact between the cortical and medullary cells has been proposed as a morphological basis for the paracrine regulations between these two endocrine tissues. To address this issue in the fetal adrenal gland at late gestation, we characterized the cellular contact of cortical and medullary cells in the fetal mouse adrenal by electron microscopy. From embryonic day 16 (E16) to E18, isolated medullary cells or islands of medullary cells could often be detected in the cortical area and strands or clusters of cortical cells were found intermingled with medllllary cells in the centralized medullary region. In these regions, cortical and medullary cells were in direct contact without intervening fibrous tissue or vessels. We also found that the secretory vesicles in the medullary cells positioned close to the cell membrane opposing the cortical cells, and that filopodia from the cortical cells extended to the medullary cells, suggesting direct intercellular message exchange. These findings suggest a paracrine interaction between the cortical and medullary cells in the fetal adrenal gland similar to that in the adult.