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Occlusable corneas in toadfishes: light transmission, movement and ultrastruture of pigment during light- and dark-adaptation

Ulrike E. Siebeck^*, Shaun P. Collin, Majid Ghoddusi and N. Justin Marshall

School of Biomedical Sciences, University of Queensland, Brisbane QLD 4072 Australia

View larger version (86K): [in a new window]   Fig. 1. (A) The common toadfish Tetractenos hamiltoni. (B) The weeping toado Torquigener pleurogramma. The colour of the iridescent cornea appears to be different in dark- (DA) and light-adapted (LA) conditions. (C–F) Pigment cover of the corneas of both toadfish during dark- (C,D) and light-adaptation (E,F) during the day and night. The light-adapted T. pleurogramma cornea (F) has a less densely pigmented temporal area, the `pseudopupil' (asterisk). In each case (C–F), the isolated cornea (top) and the cornea with intact pupil (bottom) is shown. There are slight differences between the pigmentation during the night and the day, possibly indicating that there is an underlying diurnal rhythm in pigment migration. D, dorsal, N, nasal orientation.

View larger version (15K): [in a new window]   Fig. 2. Spectral composition of the two light regimes used in the experiment. The spectrum of the sun is much broader than that of the combination of the halogen lamp and the background room lighting. The illumination of the lamp combination is relatively poor in short wavelengths. The sharp spikes in the spectrum of the lamp combination are due to the overhead fluorescent room illumination.

View larger version (29K): [in a new window]   Fig. 3. Time course of pigment movement (expansion and retraction) during the day in both species of toadfish. (A) Pigment movement across the whole cornea and (B) within the pupil zone, and (C,D) a comparison of pigment movement in the ventral and dorsal hemifields of the whole cornea. (A) Corneal pigment cover varies from 53% during dark-adaptation (broken lines) to 100% during light-adaptation (solid lines) in T. hamiltoni and from 40% to 80%, respectively, in T. pleurogramma. (B) During light-adaptation (solid lines) the entire pupil zone is 100% pigmented, but is free of pigment after dark-adaptation (broken lines). (C,D) Pigment movement is symmetrical in the ventral (V) and dorsal (D) corneal hemifields of both species. The time course of the pigment movement is similar in both species during pigment expansion (C) while there are differences during pigment retraction (D). In T. pleurogramma the pigment retracts at a slow constant rate, whereas in T. hamiltoni it retracts in two phases, quickly between 10–20 min and slowly between 20–60 min.

View larger version (29K): [in a new window]   Fig. 4. Transmission change in different areas of the corneas of T. hamiltoni (Th; Aii–Eii) and T. pleurogramma (Tp; Ai–Ei) during three stages of light- and dark-adaptation (LA, light-adapted; LA+30 min dark, the halfway point of dark-adaptation; DA, dark-adapted). Mean transmission measurements are shown for five areas of each cornea (see inset). Due to the symmetrical distribution of the pigment in the ventral and the dorsal hemifields, the measurements for Areas 1 and 2 were averaged. The central cornea (Area 3; Ci,ii) shows the largest change in transmission properties during the adaptation change while the properties of the dorsal and ventral rims (Area 1; Ai,ii) remain constant. During dark-adaptation the central areas of the cornea (Areas 3–5; Ci,ii–E,i,ii) transmit increasing amounts of light in the 400–500 nm wavelength band while the reverse is true in Area 2 (Bi,ii), indicating that the pigment is shifted from the center towards the rim of the corneas.

View larger version (141K): [in a new window]   Fig. 5. Structure of the toadfish cornea. (A) Schematic diagram showing the thickness of the different layers in the central cornea. ASS, anterior scleral stroma; BM, base membrane; D, Desçemet's membrane; DS, dermal stroma; EN, endothelium; EP, epithelium; IR, iridescent layer; ML, mucoid layer; PL, pigment layer; PSS, posterior scleral stroma. Insets indicate which areas of the cornea are enlarged in B. Scale bar, 5 µm. (B) Electron micrographs showing a transverse section of (i) the epidermis (EP), base membrane (BM) and dermal stroma (DS); arrow, microprojections, N, nucleus; (ii,iii) the different structures of the iridescent layer (IR) comprising a series of aligned endoplasmic reticula in T. hamiltoni (ii) and T. pleurogramma (iii), and (iv) the posterior scleral stroma (PSS), a monolayer of cells (M), Desçemet's membrane (D) and endothelium (EN). Scale bar, 2 µm.

View larger version (127K): [in a new window]   Fig. 6. Section through the scleral corneas of T. hamiltoni (A) and T. pleurogramma (B). The white arrowheads indicate the beginning of the iridescent layer (IR). In T. pleurogramma, the pigment processes can be found on either side of the iridescent layer, whereas in T. hamiltoni the processes are only found posterior to the iridescent layer. The anterior scleral stroma (ASS) has divided during histological processing. A, annular ligament; I, iris; P, pigment layer; PR, pigment reservoir with cell bodies; PSS, posterior scleral stroma. Scale bar, 500 µm.

View larger version (141K): [in a new window]   Fig. 7. Transverse sections through the central cornea showing the pigment layer during light- (A) and dark-adaptation (B). Scale bar, 1 µm. (C) Differences in width of the central iridescent layer (IR), the cells in the reservoir (RC), the central pigment layer (PL) and individual pigment processes (PP) in the central cornea in both species in the light- (LA) and dark-adapted (DA) condition. The difference between the widths of the iridescent layer is significant for both species (T. pleurogramma, F=2400, P<0.001; T. hamiltoni, F=33.3, P<0.001). Note that the IR width in T. pleurogramma is larger during dark-adaptation compared to light-adaptation, whereas the opposite is true for T. hamiltoni. During dark-adaptation, the cells in the reservoir of both species swell, the pigment processes in the central cornea empty leading to a decrease in width of the pigment layer. During light-adaptation, the central pigment processes swell, leading to an increased width of the pigment layer and a decreased size of the peripheral pigment reservoir cells.

View larger version (188K): [in a new window]   Fig. 8. (A) Tangential section through the pigment layer (PL) and posterior scleral stroma (PSS) in the light-adapted condition. The pigment cells have a sheet-like appearance. Scale bar, 5 µm. (B) Transverse section through a pigment reservoir cell in the dark-adapted condition. The cell contains granular material (GM) and vacuoles (V). Scale bar, 0.5 µm. (C) Transverse section through two pigment processes in the central cornea during light-adaptation. The cytoplasm contains granular material (GM) as well as vacuoles (V) of different sizes containing material with varying osmophilic affinity. Microtubules (MT) are arranged parallel to each other and the cell membrane. These may be involved in the transport of the pigment from the pigment reservoir into the pigment cell processes. Scale bar, 0.5 µm.