Mesodermal phase: specification of the cortex
The formation of the cortex is strictly of mesodermal origin. The primary source for this structure is the thickening of the coelomic epithelium located in the region of the embryo's mesentery root.
During development, the cells of this epithelium begin to actively divide (proliferate). As a result of this proliferation, paired thickenings are formed, which gradually transform into a dense, compact cellular mass. This specialized embryonic structure is called 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 intrauterine 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 at the periphery, surrounding the fetal cortex, and is formed by smaller cells with basophilic properties. It is this zone that will later transform into the permanent (definitive) adrenal cortex of the adult.
Neuroectodermal phase: development of the medulla
Unlike the cortex, the adrenal medulla is of 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 plate). The cells generated in this region do not remain in place, but undergo complex spatial migration. They head toward the primordium of the future organ and literally invade the already formed interrenal body.
Once inside the mesodermal primordium, the migrated neuroectodermal cells begin their differentiation:
- First, they transform into poorly differentiated progenitor cells — chromaffinoblasts.
- Following a series of cell divisions (proliferation), these precursors fully mature, transforming into fully functional chromaffin cells, which constitute the basis of the medulla.
Summary of embryonic development
The key characteristic of adrenal embryogenesis is convergence — the spatial coming together and physical union 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 (formerly the definitive cortex) and the medulla — are inextricably linked under a single common connective tissue capsule.
Briefly: answers to topical questions
At what week of embryonic development does the initial specification of the interrenal body occur?
The initial specification of the adrenal cortex and the formation of the interrenal body occur at the 5th week of the intrauterine period.
The process begins with the appearance of thickenings of the coelomic epithelium on both sides of the mesentery root. 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 (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; it consists of large oxyphilic vacuolated cells (spongiocytes) forming radially oriented cords.
- Zona reticularis — the narrow inner region of the cortex.
To remember the sources of development, use a simple matching rule: "Cortex — Coelom" (cortex from the coelomic epithelium), and "Medulla — Crest" (medulla from the neural crest).
Class and exam questions
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 has a neuroectodermal origin (neural crest).
What is the fetal cortex and what is its function?
This is the inner zone of the embryonic cortex, formed by large acidophilic cells. It makes up 80% of the cortex by birth, but then completely degenerates and disappears.
How does the medulla form?
Cells of the neural crest ganglionic plate migrate and invade the interior of the interrenal body, where they differentiate first into chromaffinoblasts and then into mature chromaffin cells.
What else to study on this topic
- The process of coelomic epithelium proliferation and specification of the interrenal body.
- Cytological features of the acidophilic cells of the fetal cortex.
- Morphology of small basophilic cells of the definitive cortex.
- Mechanisms of neural crest cell migration.
- Stages of maturation of chromaffinoblasts into chromaffin cells.
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