In humans and mice, the glymphatic system, critical for central nervous system (CNS) health, relies on cardiorespiratory coupling, but has not yet been investigated in a diving mammal that routinely experiences apnea, bradycardia, and peripheral vasoconstriction. The glymphatic and meningeal lymphatic systems maintain CNS homeostasis by distributing nutrients and clearing metabolic waste via cerebrospinal fluid. We investigated meningeal lymphatic and glymphatic structures in stranded bottlenose dolphins (Tursiops truncatus; n = 9) using immunofluorescence microscopy, histochemical staining, and CT angiography. Results demonstrate that the bottlenose dolphin possesses prominent perivascular spaces and aquaporin-4 astroglial water channels required for glymphatic function, as well as meningeal lymphatic vessels in close anatomic proximity to dural venous sinuses, required for a functional meningeal lymphatic system. Notably, we also identified arachnoid granulations involved in cerebrospinal fluid resorption, marking the first such observation in this species. This study provides evidence of both glymphatic and meningeal lymphatic systems in the most well-studied cetacean, the bottlenose dolphin.

The glymphatic system, an analog of the peripheral lymphatic system in the brain, and the meningeal lymphatic system are critical to central nervous system health. The glymphatic system functions to distribute cerebrospinal fluid and important compounds throughout the brain and to remove metabolic waste. The flow of cerebrospinal fluid through this system is affected by changes in cerebral blood flow, intracranial pressure, and vascular tone. Cetaceans experience profound cardiorespiratory alterations while diving that can directly affect cerebrospinal fluid and blood flow and, thus, glymphatic function. Our goal was to investigate glymphatic and lymphatic system structures, including perivascular spaces, aquaporin-4 water channels, meningeal lymphatic, and dural venous sinus vessels in the common dolphin (Delphinus delphis), using immunofluorescent labeling, histochemical staining, and postmortem computed tomography (CT) angiography. We highlight perivascular spaces and aquaporin-4 water channels surrounding blood vessels in the parenchyma and demonstrate evidence of meningeal lymphatic vessels and associated dural venous sinuses. These results demonstrate that common dolphins possess the key anatomical structures required for functional glymphatic and meningeal lymphatic systems. Future studies can build upon these anatomical discoveries to study the function and role of these systems in brain health in this species.

The pygmy sperm whale (Kogia breviceps) possesses an exocrine gland associated with its false gill slit pigmentation pattern. The cervical gill slit gland is a compound tubuloalveolar gland that produces a holocrine secretion and displays maturational changes in size and secretory histology. While the morphology of the cervical gill slit gland has been described in detail, to date, the chemical composition of its secretion remains uncharacterized. This study used histochemical staining techniques and quantitative lipid analysis to identify and characterize the constituents expressed in the secretory cells and secretion of the cervical gill slit gland. Results demonstrate that the secretion, like those of terrestrial artiodactyls that function in chemical communication, includes a complex mixture of carbohydrates, proteins, and lipids. Differences in staining intensity across germinal and secretory epithelial layers demonstrate differential expression, or maturation, of mucins and proteins. Additionally, a highly unusual and primary constituent of the secretion is uric acid. Uric acid was identified within the secretion using histochemical stains and polarized light imaging, and chemically verified using scanning electron microscopy with energy dispersive spectrometry. While uric acid is not a common constituent of mammalian exocrine glands, urate‐based compounds are abundant in the secretions of marine organisms used in chemical communication. Thus, uric acid may contribute to the chemical message produced by K. breviceps in its marine environment. We hypothesize that the chemical signals produced by the gill slit gland may be shared at close‐range by conspecifics, and that the mode of sensory reception is likely gustation.