The Morphology of the Cetacean Adrenal Gland: A Cross-Species Comparison between Tursiops truncatus and Stenella attenuata
The general morphology of the cetacean adrenal gland is poorly understood. This study provides the first detailed comparison of adrenal glands between two cetacean species, the bottlenose dolphin (Tursiops truncatus) and the Pan-tropical spotted dolphin (Stenella attenuata). Thirty-one T. truncatus and twenty-seven S. attenuata were examined in this study. For T. truncatus, twelve immature and five mature males and thirteen immature and one mature female were included. For S. attenuata, six immature and three mature males, six immature and mature females, and six of unknown maturity (one male and five female) were included. Samples were collected from only fresh dead animals, preserved in 10% neutral buffered formalin, and processed for routine histology.
Morphometric measurements of the adrenal cortex to medulla ratios were performed using a 25-point hexagonal lattice reticle. Significant differences were found between the average cortex to medulla ratio of T. truncatus and S. attenuata (ANOVA, p=0.001). The average cortex to medulla ratio for T. truncatus was 1.22 ± 0.060 (range: 0.84-2.31). Approximately 48% of the cross-sectional area of the adrenal gland was composed of cortex, 41% was medulla, and the remaining 11% was categorized as "other", indicating blood vessels, connective tissue, and the gland capsule itself. In contrast, the average cortex to medulla ratio for S. attenuata was 2.36 ± 0.160 (range: 1.32-3.75). Adrenal glands of S. attenuata had a cross-sectional area where 64% was cortex, 29% was medulla, and 7% was "other". Since adrenal gland mass was not consistently collected on S. attenuata, no mass comparisons were performed.
The cortex of both species was pseudolobulated and contained the typical zonation (zona glomerulosa, zona fasciculata, and zona reticularis) found in most mammals. At least one to three medullary projections, where the medulla extended out to the gland capsule, were found in T. truncatus. Similar projections were not found in S. attenuata. A medullary band, consisting of highly basophilic staining cells, was found along the periphery of the medulla in both species. Immunolabeling with commercially available polyclonal antibodies against dopamine β hydroxylase and phenylethanolamine N-methyl transferase indicated that norepinephrine producing cells were distributed throughout the medulla, including the medullary band, while epinephrine producing cells were found only within the medullary band. In T. truncatus, transmission electron microscopy confirmed the presence of two distinct endocrine cell populations within the medullary band and a single endocrine cell population throughout the entire medulla.