Internal Structure of the Adrenal Cortex The outer covering (adrenal cortex) is derived from the fetal mesodermal ridge, a structure that also gives rise to the kidneys so that the juxtaposition of the two organs is not surprising. Within the adrenal cortex are three zones known as the outer (zona glomerulosa), the middle (zona fasciculata), and the inner (zona reticularis). Under the microscope the cells are rather typical endocrine cells; the distinction between zones is made by differing staining characteristics.