The dromedary camel HPA axis: Baseline ultrastructure and functional correlates of cortisol following transport

The hypothalamic-pituitary-adrenal (HPA) axis is central to vertebrate adaptation, yet its structural organization remains poorly characterized in large desert-adapted mammals at the subcellular level. The dromedary camel (Camelus dromedarius) is a species of considerable comparative interest due to its tolerance of extreme environmental conditions, but foundational integrated neuroendocrine anatomy is lacking. Furthermore, whether subcellular structural variation within this axis correlates with functional HPA output has not been investigated enough.

This study provides the first combined histological, ultrastructural, and functional baseline description of the HPA axis in juvenile male dromedary camels. We compared animals sampled immediately following a 380 km transport (immediate post-transport group, IPT; n = 12) with animals sampled after a 72-hour lairage period (72-hour group; n = 8). Tissues from the hypothalamic paraventricular nucleus (PVN), anterior pituitary, and adrenal gland were examined using light microscopy, histomorphometry, transmission electron microscopy, and serum cortisol measurements. Pearson correlation analysis was performed to directly link structural parameters with circulating cortisol levels.

Routine histology demonstrated preservation of mammalian-typical cytoarchitecture across all HPA components. The camel PVN displayed a butterfly-shaped organization with distinct magnocellular and parvocellular neuronal populations. Transmission electron microscopy established, for the first time, baseline subcellular morphological features of the camel PVN, including myelinated and unmyelinated axons, synaptic vesicle populations, and mitochondria with well-preserved cristae. Ultrastructural analysis revealed that the IPT group exhibited significantly larger unmyelinated axonal area (p < 0.001) and mitochondrial dimensions (p < 0.001) within the PVN, together with a lower count of morphologically intact cells within the adrenal zona fasciculata (p < 0.05), compared with the 72-hour group. The anterior pituitary remained structurally similar across all parameters examined. Critically, Pearson correlation analysis directly linked systemic HPA output to subcellular structural variation: serum cortisol was significantly positively correlated with PVN mitochondrial area (r = 0.68, p = 0.002) and unmyelinated axon area (r = 0.73, p < 0.001). A strong negative correlation was observed between cortisol and the number of morphologically intact zona fasciculata cells (r = -0.79, p < 0.001). No significant correlation was found between cortisol and pituitary secretory granule area (r = 0.18, p = 0.45).

This study provides the first integrated histomorphological, ultrastructural, and functionally anchored baseline for the camel HPA axis. The significant correlations between circulating cortisol and subcellular structures in the PVN and adrenal cortex transform descriptive observations into functionally validated structural correlates. The pattern of correlations were strongest in the adrenal cortex, robust in the hypothalamus, and absent in the anterior pituitary. This suggests compartmentalized, organ-specific structural plasticity along the axis. These findings provides a foundational reference for future hypothesis-driven research on neuroendocrine adaptation in desert-adapted mammals and position Camelus dromedarius as a valuable comparative model for investigating the structural correlates of physiological resilience.