Chapter 180. Development of The Adrenal (suprarenal) Gland - Review of Medical Embryology Book

Introduction: the adrenals are glandular masses found above the kidneys and consist of a cortical area of mesodermal origin and a medullary area of ectodermal origin

THE CORTICAL PRIMORDIUM comes from a wide plate of coelomic epithelium in the most internal region of the mesonephric blastema, between the mesenteric root and the gonadal primordium

This origin accounts for the presence of accessory paratesticular and paraovarian accessory cortical masses which are sometimes encountered
At the beginning of month 2 (8 mm fetus), under induction by the primary ureter (wolffian duct), mesothelial (coelomic epithelial) cells proliferate and penetrate the underlying mesenchym They multiply quickly and differentiate into large acidophilic cells which surround the medullary primordium and form the fetal or primitive suprarenal cortex
Shortly after, toward the end of month 3, a second wave of cells from the coelomic epithelium (mesothelium) penetrates the mesenchyme and surrounds the original acidophilic cell mass. These smaller cells form the definitive cortex of the gland

The small basophilic cells will form the future glomerular and fascicular zones of the definitive cortex
After birth, the fetal cortex regresses rapidly, except for its outer layer which differentiates into the reticular zone of the cortex

The adult structure of the cortex is not achieved until near puberty
Prior to month 5, the cortex appears to develop autonomously. After this time, its development depends on hypophyseal corticotropic hormone (ACTH)

THE MEDULLARY PRIMORDIUM occurs at about day 45 of gestation and results from the assembling of sympathogonia from the sympathetic chain (of ectodermal origin) in the region near the developing mesodermal cortical primordium

While the fetal cortex is forming, the invading sympathogonia cells become arranged in clusters and cords. The cells do not form nerve processes, but rather, stain yellow-brown with chrome salts and are called chromaffin cells. The staining is probably due to epinephrine and norepinephrine in the cells.
At birth, the medulla is only slightly developed and is not yet functional. The definitive cortex is only a peripheral ring and makes up only 15-20% of the glandular parenchyma

At birth: the structure of the external glomerular zone is still not precise, but the zona fasciculata is seen readily and is directly continuous with the fetal zone

The fetal zone begins to regress, but is not completely gone until the second year
While the fetal zone is regressing, the zona glomerulosa and fasciculata develop, and the zona reticularis makes its appearance
Development of the definitive cortex and its physiologic activity is regulated by ACTH, and it is not completely differentiated until 18 to 21 months after birth

On reaching the gland, the arteries ramify with some going directly to the medulla, but the majority nourish the extended capillary network of the glomerulosa and reticularis. From the latter, blood flows into vessels of the medulla and finally leaves the gland via a large central vein at the gland hilus
Thus, there is an important vascular relationship between the cortex and medulla

Innervation of the adrenal gland: preganglionic sympathetic fibers to the gland do not synapse in the sympathetic ganglia but go directly to the gland and synapse in ganglia in the cortex and medulla. From these, postganglionic fibers supply the blood vessels. However, the majority of preganglionic fibers go directly to the cells of the medulla