Download A whale of an eye - University of Wisconsin School of Veterinary

Sarah Miller
COPLOW Intern
University of Wisconsin
http://www.boatus.com/cruising/ithaka/200612-01.asp
Features of Mammalian Eyes
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No bone or cartilage in sclera
No skeletal muscle
No photomechanical movement in RPE
No fovea
Most have blood vessels within the retina
(Holangiotic)
Accommodation limited by passive action of lens
capsule on lens –No accommodation via lens
capsule
Dichromatic vision- cetaceans monochrome
– L-opsin (green) only
Evolution
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Artiodactyls (even-toed ungulates)
Closest relative thought to be hippo
Similar features
– Fibrous Tapetum lucidum
– indented blood vessels-whales not indented
http://www.sheppardsoftware.com/content/ani
mals/animals/mammals/animal_of_the_week/a
nimaloftheweek_whalehippo.htm
http://3rdarm.biz/images/2006/05/thehippobeluga2.JPG
Fibrous Tapetum Lucidum
Hippo
(H&E)
Dolphin
(trichrome)
The Difficulty of Underwater Vision
• Refractive Index
• Large changes in light
• Heat loss
• High Pressure
Difficulty of Underwater Vision-R.I.
• Refractive Index air
=1
– R.I. terrestrial cornea=1.35
• Refractive index water =1.35
– R.I. marine cornea
=1.37
Cornea doesn’t provide effective refraction!
Built for Underwater Vision-R.I.
Cornea of varying thickness ÆÆ
• Spherical lens
• Heterogeneous lens
Round lensÆ
– Lower R.I. in periphery
• Cornea
– Divergent lens
• R.I. 1.37 (central)
1.53 (periphery)
Built for Underwater Vision-Light
• covers 2/3 to entire fundus
Built for Underwater Vision-Light
• Modified iris (Operculum)
-Protects from
large changes in
illumination
Operculum
Iris muscle
Built for Underwater Vision-Temp
• Fatty eyelids
• Highly vascular plexus around optic nerve
– Heater
– Oxygen
Built for Underwater Vision-Temp
Built for Underwater Vision-Pressure
• Thick Sclera…why?
– Pressure and thermal protection?
• No adaptation needed
– Fluid continuum of body compensates
• Another suggestion
– Protect ON from high IOP
Terrestrial Vision
should be myopic due to R.I. of air but not…
http://media.canada.com/9ef63c09-385e-4b40bac2-da66c75d8b27/mdf41628.jpg
http://img2.travelblog.org/Photos/4797/34565/t/188488-Dolphins--TrainerHoop-Trick-0.jpg
http://upload.wikimedia.org/wikipedia/commons/c/c2/Peeking_Orca.jpg
Shaped for success
Flattened anterior segment
150° hemisphere
Not good enough!
Theory 1- Axial Displacement
• Backward movement of lens
• T. truncates seen to move eye 10-15 mm
forward
• Decreased IOP flattens cornea
Theory 2- “spoon-shaped” divots
• Depressed regions near nasal and
temporal margins
• Nasal and temporal regions of increased
ganglion cell density
*Line up via center of lens*
Theory 3- Heterogeneous lens
• Use different parts of lens with different
R.I.
– Underwater Æ center of lens
– Air Æ periphery
Additional Unique Features…
http://images.inmagine.com/img/designpics/dpic039/dp1766255.jpg
Movement
• Horizontal, vertical, and axial movement
• Independent eye movement
– horiz and vert
– Pupil reflexes
From Zhu et al
Anatomical Record
(2000)
Staying Focused
• Accomodation?
• Ciliary muscles underdeveloped or
absent
Dog
Whale
Staying Focused
• Large protractor/retractor muscles
• IOP increases when eye is pulled back, shifting
lens forward
• Lens moves backwards when eye moves forward
Complex nerve endings
• Found in sclerocorneal trabecular meshwork
• Pressure reception
ciliary body stroma
Bowman’s Layer
Very large ganglion cells
• Can get larger than 75 um
– Average 20-30 um
• Terrestrial large 15-30 um
• Y-cells
(movement)
Very large ganglion cells
Whale
Dog
References
• Ya Supin, Alexander, et al. The Sensory Physiology of Aquatic
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Mammals. Boston, Kluwer Academic Publishers, 2001. (pp 239264)
Mass A. & Ya Supin A. (2007) Adaptive Features of Aquatic
Mammals’ Eye. The Anatomical Record 290:701-715.
Ninomiya H. & Yoshida E. (2007) Functional Anatomy of the
Ocular Circulatory System: Vascular Corrosion Casts of the
Cetacean Eye. Veterinary Ophthalmology 10(4): 231-238.
Zhu Q. et al. (2000) Observations on the Muscles of the Eye of
the Bowhead Whale, Balaena mysticetus. The Anatomical Record
259:189-204.
Peichl L. et al. (2001) For Whales and Seals the Ocean is Not
Blue: A Visual Pigment Loss in Marine Mammals. European
Journal of Neuroscience 13: 1520-1528.
Kroger RHH & Kirschfiels K. (1992) Optics of the Harbor Porpoise
Eye in Water. J. Opt. Soc. Am. A 10:1481-1489.
Vrabec F. (1972) Encapsulated Sensory Corpuscles in the
Sclerocorneal Boundary tissues of the Killer Whale Orcinus
orca L. Acta Anat. 81: 23-29.