The fine structure of the adrenal gland of the fetal armadillo was examined by electron microscopy to determine whether or not there were two cell types in the parenchyma of the fetal zone, as suggested by examination by light microscopy, and whether any of the cells of this zone had the cytological features associated with steroid production.

Two cell types were found. One cell type is composed of large cells situated in clusters and containing a relative paucity of organelles. The other cell type is composed of smaller, eosinophilic, PAS positive cells, situated primarily in a reticular pattern around the blood vessels. This second cell type has numerous spherical mitochondria with tubular cristae and an extensive tubular agranular endoplasmic reticulum. It is concluded that this latter cell type is responsible for the steroid production reported by other authors. Both of the cell types of the fetal zone appear to be derived from the original mesenchymal blastema of the adrenal gland.