Dolphin brain

The brain from the common dolphin (Delphinus delphis) has been examined in respect to the pathway of the Vestibulo-cochlear nerve from the medulla to the cerebellum. The rationale for this study is to identify optimum locations for excising neurological tissue samples along the lower order regions of the auditory pathway, to look for evidence of trauma symptomatic of exposure to intense underwater noise or other environmental pollutant. By examining the auditory cortex for evidence of beta-amyloid deposits and neuritic plaques, a pathological diagnosis of a neurodegenerative disorder (if present) can be made. Extra-cellular deposits of beta-amyloid and neurofibrillary tangles display a sequential accumulation in the cerebral cortex and can be differentiated using the Transmission Electron Microscope or immunohistochemistry.

Materials and methods

The common dolphin (D. delphis) brain examined here was acquired on the 14/02/2006 from a dead stranding at Seaton in Cornwall (OS grid ref SX303544) (Link to Seaton pathology report). The animal was male with a length of 1.90 m from the beak tip to the notch in the tail and had most probably died as a result of entanglement in fishing gear. The brain was removed and weighed during autopsy; the total weight of 969.4 g was recorded using precision scales. After fixing in 20 % formaldehyde for three weeks, the external features of the brain were photographed and presented in this study. A second brain was acquired from a dead stranding on the 20/01/2006 from a male with a length of 2.23 m from the beak tip to the notch in the tail (Link to Bovisands pathology report). The brain was removed and weighed during autopsy, and a total weight of 850 g was recorded. Once fixed in 20 % formaldehyde, the brain was divided and sliced into 2 mm thick sections using a rotary blade and the pathway of the VIII nerve followed from the medulla to the cerebellum.

Results

Figure 1. Anterior ventral view of the brain from the male 1.90 m body length D. delphis

Figure 2. Close up of the brainstem and VIII auditory nerve from the 1.90 m body length D. delphis. II. Optic nerve, V. Trigeminal nerve VIII. Vestibulo-cochlear nerve

Figure 3. Lateral view of the brain from the 1.90 m D. delphis toward the mid-sagittal plane. II. Optic nerve, V. Trigeminal nerve VIII. Vestibulo-cochlear nerve

Figure 4. Mid-sagittal division of the 2.23 m D. delphis brain into left and right hemispheres

Figure 5. Annotated mid-sagittal division of the right hemisphere from the 2.23 m D. delphis brain, c. cerebellum, cc. corpus callosum, h. hypothalamus, ls. limbic system, mb. midbrain, p. pons, me. medulla sc. spinal cord. Bar = 50 mm

Figure 6. Sections through the left hemisphere showing the progress of the Vestibulo-cochlear nerve (hatched circle) from the medulla to the cerebellum

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