Central nervous system lymphatic structure in marine mammals: a morphological comparison of shallow and deep divers

the body where the lymphatic system has been described, there are specific lymphatic vessels, cells, and organs that carry excess fluids and wastes away from tissues and filter pathogens from blood. Therefore, several diseases and susceptibility to certain injuries can be linked to dysfunction of the lymphatic system. The recently discovered brain lymphatic system, comprised of the meningeallymphatic system and the glymphatic system, has been shown to play a key role in CNS health by driving clearance of metabolic wasteand neurotoxic proteins, modulating brain immune response, and distributing important compounds throughout the brain. Preliminary data in a rodent model of decompression sickness (DCS) has demonstrated disruption in brain lymphatic function in response to the presence of gas emboli. In fact, DCS in humans and marine mammals commonly results in the formation of gas and fat emboli in CNS and lymphatic tissues.Deep-diving cetaceans are known to be susceptible to injuries associated with apparent DCS. Mass-stranding events incetaceans throughout the world commonly occurs after naval sonar exercises have occurred in the area. Similar to humans with DCS, these stranded animals will often have gas and fat emboli in their CNS and lymphatic tissues. Despite increased susceptibility to injuries from DCS, and negative impacts from gas emboli on the function of the brain lymphatic system, it is not known if marine mammals have a brain lymphatic system. Determination of the presence and structure of a brain lymphatic system in different marine mammal species could therefore provide information on why certain species appear to be more susceptible to decompression injuries.Research Objectives: Our Specific Aims are to answer the following questions:(1) Do marine mammals have a brain lymphatic system? (2) If so, what is the gross and microscopic morphology of the marine mammal brain lymphatic system? (3) Do observable/quantifiable differencesin gross and microscopic morphology exist between deep and shallow divers? (4) Is there a difference in venous cerebral blood volume among deep and shallow divers?Technical Approaches: We will focus on sampling sites that are employed in terrestrial mammals. We will use histological, histochemical, and fluorescent immunohistological staining techniques to explore for glymphatic and lymphatic tissues of the cetacean and pinniped CNS. Computed tomography angiography will allow us to identify target areas of interest for histologic comparisons among deep and shallow divers and tearch: We hypothesize; 1) all marine mammals will have a brain lymphatic system, 2) cetaceans will have split-brain lymphatic systems to support clearance of waste associated with unihemispheric sleep (glymphatic system activated during sleep in humans and rodents), and 3) deeper-diving species will possess a higher density of venous associated lymphatic vessels based upon elaborate retia andvascular structures in extracranial tissues of beaked and kogiid whales.Naval Relevance: Our Specific Aims relate to the diving physiology Research Objectives of ONRs Marine Mammals and Biology program, and the basic research to understand the cognitive impacts and individual responses of the various exposures in the undersea environment of ONRs Undersea Medicine and Performance program. Our research also addresses several Research Concentration Areas of ONRs Marine Mammal Health program, which aims to improve the medical management and improve the prevention of infectious/non-infectious diseases in bottlenose dolphins and California sea lions (two species we will investigate in this study).