General Characteristics and Embryogenesis
Externally, the adrenal gland is covered by a dense connective tissue capsule. Thin partitions of loose connective tissue carrying blood vessels and nerves extend inward from the capsule, and the secretory cells themselves are entwined by a network of reticular fibers. On cross-section, two distinct regions are clearly visible: the superficial yellowish cortex and the dark, soft medulla.
This anatomical duality is a result of embryonic development. In lower vertebrates, these structures exist as separate glands, but in humans, they are united by a common blood supply.
- The cortex is of mesodermal origin. It forms from the coelomic epithelium, initially developing into the interrenal body and the fetal cortex (which regresses after birth), followed by the definitive cortex.
- The medulla is of neuroectodermal origin. Neural crest cells migrate together with sympathetic ganglion precursors, differentiate into chromaffinoblasts, and invade the interior of the future cortex.
Structure of the Adrenal Cortex
Cortical endocrinocytes specialize in the synthesis of corticosteroids—steroid hormones derived from cholesterol. These cells are characterized by a well-developed smooth endoplasmic reticulum (where hydroxylation occurs) and mitochondria with tubular cristae. The cortex is divided into three zones:
- Zona glomerulosa (zona glomerulosa). Occupies 15% of the volume, lying immediately beneath the capsule. Cells are arranged in rounded arches ("glomeruli"). It secretes mineralocorticoids (e.g., aldosterone), which promote sodium and water retention in the kidneys. The activity of this zone is controlled by the renin-angiotensin system.
- Zona fasciculata (zona fasciculata). The widest region (78%). Cells are arranged in long cords. They synthesize glucocorticoids (cortisol, corticosterone), which are essential for survival during chronic stress (elevating blood glucose and mobilizing bodily resources). The cytoplasm of these cells is packed with lipids, turning this zone bright orange upon staining with Sudan III.
- Zona reticularis (zona reticularis). The deepest and thinnest layer (7%). It secretes androgens (androstenedione), which stimulate anabolic processes. The cells here are smaller and form a loose network.
Adrenal Medulla and Vascular Architecture
The medulla consists of cords of large basophilic cells. The bulk of the tissue comprises medullary endocrinocytes (chromaffin cells), which belong to the APUD system. Among them, two main types are distinguished:
- A-cells (the majority) — synthesize epinephrine (adrenaline).
- N-cells — synthesize norepinephrine (noradrenaline).
These catecholamines accumulate in secretory granules and are released into the bloodstream during acute stress, triggering a systemic response similar to sympathetic nervous system activation (lipolysis and glycogenolysis). Ganglion cells (about 1%) and supporting glial elements are also found in the medulla.
Vascular Architecture: The blood supply to the gland is centripetal. Arterial blood from the capsule flows through the entire cortex via sinusoidal capillaries with fenestrated endothelium, collecting steroid hormones. This blood then drains into the venous sinusoids of the medulla. Consequently, chromaffin cells are constantly bathed in blood with a high concentration of glucocorticoids.