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R. F. Nelson, V. P. Connaughton; Morphology of L- and C-Type Horizontal Cells in Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5793. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the morphologies of L1-type, L2-type, C-type biphasic, C-type blue tri/tetraphasic, and C-type UV triphasic horizontal cells (HCs) identified in zebrafish retina (Connaughton & Nelson 2007).
Sharp, 500-1300 mΩ microelectrodes penetrated retinal neurons in zebrafish retina-eyecups vitreally perfused with MEM (0.3 ml/min, 95% O2, 5%CO2). Alexa 594 hydrazide iontophoresed from electrode tips stained penetrated cells (-270 pA, 15 sec). Photographed in wide-field z-axis stacks with a 40x water-immersion objective, HCs were reconstructed in Adobe photoshop and imagej. Full field light responses for wavelengths from 650 nm (red) to 330 nm (ultraviolet) were used to generate spectral sensitivity curves defining spectral types.
HC stains consisted of either (1) just cell bodies (CBs), (2) CBs with dendritic fields (DFs), (3) CBs with DFs and axons, (4) CBs with DFs, axons and tube-like axon terminals (ATs), or (5) just ATs. L-type responses were recorded from HC CBs (n=6) or ATs (n=2). Axons (200-400 µm in length, n=5) ended in ATs located among bipolar CBs. L1-type CBs were larger (150 µm2, n=2) than L2-type CBs (40 µm2, n=5), but DFs were similar (L1, 480 µm2, n=2; L2, 420 µm2, n=5). Dendrites were short and wide, often extending as protoplasmic sheets or wedges with occasional short spines sprouting laterally. C-type biphasic HC CBs were large (90µm2, n=6). Axons (~300 µm in length, n=2) were less common, and lacked ATs. DFs were 360 µm2, similar to L-types; but more short spines protruded laterally, sometimes giving a pin cushion appearance. One C-type blue triphasic response was found in an HC with long slender dendrites and terminal puncta, but no axon (CB 45 µm2; DF 860 µm2). C-type UV triphasic HCs also had long slender dendrites with terminal puncta. Axons (300-400 µm in length, n=2) lacked ATs. CBs (75 µm2, n=3) supported the widest of HC DFs (1050 µm2, n=2).
Stains of spectrally characterized zebrafish HCs suggest stages of morphological maturity, ranging from just CBs, to CBs with dendrites, to CBs with axons, ATs and dendrites. C-type biphasic HCs more typically lacked axons, while L-types more typically had them. C-type HCs of all kinds lacked prominent ATs. The AT 'nematode' appears characteristic of L-types. Long slender dendrites bearing terminal and en passage puncta characterize both C-type blue and UV HCs, similar to the blue- or UV-cone selective H3 types of Song et al (2008).
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