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Fig. 6 | Frontiers in Zoology

Fig. 6

From: Optic-nerve-transmitted eyeshine, a new type of light emission from fish eyes

Fig. 6

Optic nerve characteristics of T. delaisi (a1-a4), P. zvonimiri (b1), and A. ocellaris (c1-c4). a1-c1 Transverse sections showing elaborately pleated ONs that differ in pleat number, thickness, regularity, and spacing. In T. delaisi (a1), pleats are further apart than in A. ocellaris (c1) and more evenly spaced than in P. zvonimiri (b1). This regular pattern of alternating high-refractive-index neural tissue and low-refractive-index interstitial spaces may enable the ON to act as a biological light guide (T. melanurus' ON structure is identical to T. delaisi's). a2-c2 Coronal sections of the intra- and extraocular ON in T. delaisi (a2) and A. ocellaris (c2). The PTAH staining, which stains myelin dark blue, confirms the common pattern that axons are unmyelinated within the nerve fibre layer (NFL) of the retina (white arrowhead) and become myelinated outside the eyecup (black arrowhead) in T. delaisi. The axons of A. ocellaris stained dark blue throughout both the intraocular ON and the NFL of the retina (black and white arrowhead), indicating that at least some of them are myelinated within the NFL. a3-c3 Detail of the intraocular ON (coronal section). The RPE forms a pigment sheath (black arrowheads) that surrounds the ON in both T. delaisi (a1) and A. ocellaris (b1). In T. delaisi, the sheath comprises a distinct band of pigmented processes that criss-crosses the entire intraocular ON (white arrowheads). a4-c4 Semi-thin sections of the intraocular ON confirm that the pigment sheath (black arrowheads) is an extension of the RPE in both species. The sheath extends just far enough to shield the light-sensitive photoreceptor outer segments (ros) (between the black arrowheads). Within-nerve pigment processes (white arrowheads) seem to be only present in T. delaisi. Abbreviations: inl inner nuclear layer, ion intraocular optic nerve, onl outer nuclear layer, opl outer plexiform layer, ris receptor inner segments, ros receptor outer segments, rpe retinal pigment epithelium

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