Figure 7. Morphological development and degeneration of the eye in A. mexicanus cavefish. A. The eye of a live juvenile surface fish showing a pigmented iris (dashed white arrow) encircling a dark central region containing a cornea and lens (white arrow). Dorsal to the top. B. Fixed eye showing a blueish lens (white arrow) within retracted iris tissue (dashed white arrow). The iris is pigmented with melanophores (black) and iridophores (iridescent dots). C. Isolated lens extracted from the fixed eye in ‘B’. D. Anterior left-lateral view of an adult cavefish showing the region of complete eye loss (dashed yellow ellipse). melanophores form a semicircle of pigmentation along the posterior edge of the eye field, which is completely closed by a cartilaginous sclera. E. Optic cup (dashed white arrow) and lens (white arrow) in a cavefish after 1d (day) of development. The ventral sector (white arrowhead) of the optic cup is morphologically reduced during eye formation in cavefish. F. left-lateral views of a larval cavefish eye undergoing apoptotic degeneration at 5d. The lens (white arrow) extends from an optic cup (dashed white arrow) pigmented with melanophores and iridophores (yellow arrow). Melanic pigment is visible in the olfactory pit (dashed black arrow) and margins (dashed yellow arrow) of the otic vesicle (black arrow). G. The lens has dropped into the ventral sector of degenerating optic cup (dashed white arrow) at 5d. Iridophores (yellow arrow) cover the optic cup. H. Lens (white arrow) and cornea attached to the optic cup (dashed white arrow) of a larval cavefish at 7d. I. Left-lateral view of larval cavefish head of at 15d with a remnant of the pigmented optic cup (dashed white arrow) nested in the center of the orbit (yellow arrowhead). J. Left-lateral view of the larval fish in ‘I’ at 15d, showing the fixed angle of the degenerate optic cup (dashed white arrow) and melanin pigmentation (dashed yellow arrow) on the larval body covering the viscera. K. Left-lateral view of larval cavefish head at 19d. Melanin is visible around the olfactory pit anterior to the eye, and within the left lobe of the optic tectum (midbrain) that is dorsal to the eye (dashed yellow arrows, respectively). The optic cup (dashed white arrow) is split open on its ventral side and fixed within the orbit (yellow arrowhead). L. Larval cavefish at 19d, with similar orientation and patterns of pigmentation and degeneration as shown in ‘K’. Note the ventral extrusion of pigmented and non-pigmented matter from the optic cup (dashed white arrow). M. Left-lateral view of a larval cavefish head at 26d of development. The optic cup and its contents (dashed white arrowhead) have been displaced into the orbit (yellow arrowhead). Melanic pigmentation on the optic tectum has increased in the number and spatial coverage of dendritic melanophores (dashed yellow arrow). Overall, the degenerative remnants of cavefish eyes remain visible during larval development through late stages of juvenile growth (not shown). Stereomicrographs in A–D by Michael J. Boyle; Stereomicrographs in E–M were prepared by M. J. Boyle, and obtained from video frames produced by Scott Arledge. BOYLE, ARLEDGE, THOMAS, TOMKINS, AND GULIUZZA Testing the cavefish model 2023 ICC 129
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