Morphological, Histological and Ultrastructural Characterization of the Common Dolphin's Adrenal Glands

The adrenal glands are central regulators of endocrine function and stress physiology, yet detailed species-specific anatomical baselines remain limited in cetaceans. This study provides a comprehensive gross, histological, morphometric, and ultrastructural characterization of the adrenal glands in 55 short-beaked common dolphins (Delphinus delphis) examined postmortem in the Canary Islands. Adrenal glands were evaluated macroscopically and microscopically, and histological corticomedullary ratios were calculated from mid-transverse sections. Associations with body length, sexual maturity, and cause-of-death category were assessed statistically. Transmission electron microscopy was used to characterize cortical and medullary cellular ultrastructure. Adrenal weight showed a positive correlation with body length. The histological corticomedullary ratio showed no lateral asymmetry but differed significantly between sexually immature and mature individuals, indicating ontogenetic remodeling of adrenal architecture. In contrast, the corticomedullary ratio did not differ significantly between adult dolphins that died from acute events and those that died following more progressive pathological conditions. Ultrastructural analysis identified characteristic steroidogenic cortical cells and two distinct chromaffin cell populations in the medulla. These findings establish the first integrated anatomical baseline for the adrenal gland in Delphinus delphis, providing essential reference data for comparative anatomy, veterinary pathology, and the interpretation of endocrine-related findings in cetaceans.