Embryology
Each adrenal consists of two functionally distinct endocrine glands within a single capsule. The cortex derives from mesenchymal cells of the coelomic cavity lining, adjacent to the urogenital ridge. The fetal adrenal gland is recognizable by the second month of gestation when it is invaded by neuroectodermal cells that will form the medulla.The adrenal becomes quite vascular, increases rapidly in size and is actually larger than the kidney at midgestation. By the second trimester, the thin, outer definitive zone that will form the adult cortex becomes distinct. The inner fetal zone comprises most of the adrenal mass and still represents three quarters of the cortex at birth.
The fetal zone degenerates rapidly after birth, accounting for only one quarter of the cortical mass at 2 months and vanishing by 1 year. The total adrenal weight declines until age 2 to 3 months, and the growth from this point on parallels growth of the body. The zona reticularis develops during the first year of life.
