Location and Structure
The adrenal glands (glandulae suprarenales) are paired glands weighing 11–18 g each, enclosed within the renal fascia. Externally, they are covered by a thin fibrous capsule from which connective tissue septa extend inward. The gland parenchyma is divided into an inner medulla and an outer yellowish-brown cortex.
The cortex is subdivided into three zones: the zona glomerulosa, the zona fasciculata, and the zona reticularis. The zona fasciculata lies deeper than the zona glomerulosa.
Architecture of the zone:
- Endocrinocytes are organized into long cords—fascicles.
- The thickness of each cell fascicle is 1–2 cells.
- The fascicles are oriented perpendicular to the surface of the adrenal cortex.
Cytological Features of the Cells
The cells of the zona fasciculata have a structure adapted to their function: steroid synthesis.
- Lipid inclusions. The cells of the zona fasciculata contain the highest amount of lipid droplets. In the zona glomerulosa and zona reticularis, lipids are significantly sparser, making up 5–10% of the cytoplasmic volume. These droplets contain neutral fats and stores of cholesterol, the precursor to steroid hormones. Due to the abundance of lipids, standard histological processing with alcohols leaves the cytoplasm looking honeycomb-like or "foamy." On histochemical staining with Sudan III, the zona fasciculata stains most intensely orange.
- Mitochondria. A hallmark marker of steroid-producing cells is the presence of mitochondria with tubular cristae, which appear as vesicles in cross-section.
- Intercellular junctions. Gap junctions (nexus junctions) predominate between cells for metabolic cooperation. Tight junctions and desmosomes are rare.
Synthesis and Target Effects
The zona fasciculata specializes in producing glucocorticoids—cortisol (hydrocortisone) and corticosterone.
Mechanism of action of cortisol: Cortisol binds to a cytosolic receptor; the complex then acts in the nucleus to regulate gene transcription.
Key effects:
- In the liver: $\uparrow$ gluconeogenesis.
- In muscles: $\uparrow$ proteolysis.
- In extremity adipose tissue: $\uparrow$ lipolysis.
- In peripheral tissues: $\downarrow$ glucose utilization.
Target organs for cortisol include the liver, skeletal muscles, adipose tissue, and lymphoid tissue.
Regulation of Secretion
Unlike the zona glomerulosa, which is controlled by the renin-angiotensin system, cortisol secretion is regulated by different factors.
Cortisol synthesis and release are stimulated by:
- ACTH (Adrenocorticotropic hormone).
- Stress.
- Circadian rhythms: morning peak.
Clinical Significance
Morphological features of zona fasciculata cells are important in oncopathology. The Weiss system is used in the differential diagnosis of adrenocortical carcinoma. One of the malignancy criteria is the presence of less than 25% clear cells with clear cytoplasm in the tumor. Clear cells, rich in lipids and with vacuolated cytoplasm, resemble the cells of the adrenal zona fasciculata.