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Adrenal Gland Embryogenesis

For medical students2 min readUpdated 2026-10-10

The development of the adrenal glands represents a unique process of convergence of two embryonic primordia of completely distinct origins. During embryogenesis, genetically and functionally different tissues combine to form a single organ covered by a common capsule.

Dual originThe organ is formed from two independent sources: the mesoderm and the neural ectoderm.

Fetal cortexAt birth, it accounts for 80% of the adrenal cortex mass, but it subsequently degenerates.

MedullaIts precursors actively migrate from the neural crest region into the interior of the cortical primordium.

Mesodermal Phase: Formation of the Cortex

The formation of the adrenal cortex is strictly of mesodermal origin. The primary source for this structure is a thickening of the coelomic epithelium located in the region of the embryo's dorsal mesentery root.

During development, the cells of this epithelium begin to actively divide (proliferate). This proliferation results in the formation of paired thickenings that gradually transform into a dense, compact cellular mass. This specialized embryonic structure is termed the interrenal body.

Further differentiation of the interrenal body proceeds in two directions, forming two distinct zones of the cortex:

Fetal cortex (inner zone). This region consists of large cells whose cytoplasm exhibits prominent acidophilia. The fetal cortex plays a leading role during the prenatal period and, by the time of birth, constitutes the overwhelming majority of the organ — about 80% of the total cortical mass. However, its fate is predetermined: immediately after birth, this massive zone undergoes rapid degeneration and completely disappears.
Definitive cortex (outer zone). It is located peripherally, surrounding the fetal cortex, and is formed by smaller cells with basophilic properties. This zone will later transform into the permanent (definitive) adrenal cortex of the adult.

Neuroectodermal Phase: Development of the Medulla

Unlike the cortex, the adrenal medulla has a neuroectodermal origin. Its development is closely linked to the formation of the embryonic nervous system.

The source of the medulla is the neural crest (specifically, its ganglionic crest). Cells originating in this region do not remain in place, but undergo complex spatial migration. They direct themselves toward the primordium of the future organ and literally invade the interior of the already formed interrenal body.

Once inside the mesodermal primordium, the migrated neuroectodermal cells begin their differentiation:

First, they transform into poorly differentiated precursor cells known as chromaffinoblasts.
Following a series of cell divisions (proliferation), these precursors finally mature, transforming into fully functional chromaffin cells, which constitute the basis of the medulla.

Summary of Embryonic Development

The key characteristic of adrenal gland embryogenesis is convergence — the spatial coming together and physical fusion of two primordia of different origins.

After the fetal cortex undergoes physiological involution (degeneration) in the postnatal period, the organ acquires its definitive (final) appearance. As a result, a single endocrine gland is formed in which two genetically and functionally independent tissues — the cortex (former definitive cortex) and the medulla — are inextricably linked beneath a common connective tissue capsule.

Mnemonic

To remember the sources of development, use a simple matching rule: "Cortex — Coelom" (cortex from coelomic epithelium), and "Medulla — Crest" (medulla from neural crest).

Frequently asked questions

At which week of embryonic development does the initial formation of the interrenal body occur?

The initial formation of the adrenal cortical primordium and the development of the interrenal body occur during the 5th week of the prenatal period.

The process begins with the appearance of thickenings of the coelomic epithelium on both sides of the root of the mesentery. Subsequently, these epithelial thickenings, consisting of large cells with acidophilic cytoplasm, gather into a single compact structure that serves as the basis for the development of the organ's definitive cortex.

Into what three zones does the definitive adrenal cortex divide during final maturation?

The definitive adrenal cortex is divided into three consecutive zones:

Zona glomerulosa — a thin outer part beneath the capsule, formed by small cells that form rounded arches.
Zona fasciculata — the middle zone, forming the bulk of the cortex; consists of large oxyphilic vacuolated cells (spongiocytes) forming radially oriented cords.
Zona reticularis — the narrow inner region of the cortex.

From which primordia do the cortex and medulla develop?

They develop from different primordia: the cortex has a mesodermal origin (coelomic epithelium), while the medulla is neuroectodermal (neural crest).

What is the fetal cortex and what is its function?

It is the inner zone of the embryonic cortex, formed by large acidophilic cells. It makes up 80% of the cortex at birth, but subsequently fully degenerates and disappears.

How does the medulla form?

Cells of the ganglionic plate of the neural crest migrate and invade the interrenal body, where they differentiate first into chromaffinoblasts and then into mature chromaffin cells.

Go deeper

The process of coelomic epithelium proliferation and interrenal body formation.
Cytological features of the fetal cortex acidophilic cells.
Morphology of small basophilic cells of the definitive cortex.
Mechanisms of neural crest cell migration.
Stages of chromaffinoblast maturation into chromaffin cells.