Fasciculata

Fasciculata refers to the zona fasciculata, the middle layer of the adrenal cortex in mammals, including humans. The term derives from Latin and means “bundled,” reflecting the arrangement of its cells in cords or fascicles. It lies between the outer zona glomerulosa and the inner zona reticularis and is typically the thickest cortical layer in humans.

Histology and structure: The zona fasciculata is composed of large, polygonal cells with foamy cytoplasm due to lipid droplets. The cells are organized into columns or fascicles that are separated by capillaries, enabling rapid exchange of steroid hormones. The cytoplasm is rich in smooth endoplasmic reticulum and mitochondria, reflecting its role in steroid synthesis. Enzymes involved in glucocorticoid production, such as 11β-hydroxylase, are present in these cells.

Function and regulation: The zona fasciculata synthesizes glucocorticoids, primarily cortisol in humans (and corticosterone in some other species). These hormones influence glucose metabolism, protein and fat metabolism, and have anti-inflammatory and immunosuppressive effects. Secretion is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary, which is regulated by corticotropin-releasing hormone (CRH) from the hypothalamus. Cortisol exerts negative feedback on the hypothalamic-pituitary axis and follows a circadian rhythm, peaking in the early morning.

Clinical notes: Excess cortisol production can cause Cushing’s syndrome, often due to pituitary, adrenal, or exogenous glucocorticoid sources. Deficiency of zona fasciculata function contributes to adrenal insufficiency and related metabolic disturbances, underscoring the layer’s critical role in stress response and homeostasis.