Sugiyama Y et al. (2024), Concave-to-convex curve conversion of fiber cel...

XB-ART-60892

Exp Eye Res 2024 Nov 02;248:110066. doi: 10.1016/j.exer.2024.110066.

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Concave-to-convex curve conversion of fiber cells correlates with Y-shaped suture formation at the poles of the rodent lens.

Sugiyama Y , Murthy VV , Mbogo I , Morohashi Y , Masai I , Lovicu FJ .

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The eye lens contains convexly curved fiber cells that align in concentric layers around the lens anterior-posterior pole axis. For lens fiber differentiation at the equator, cells elongate with their apical and basal tips migrating towards the anterior and posterior poles, respectively. At each pole, the fiber tips meet opposing tips of other fiber cells, to form a suture. Although umbilical or point sutures are observed in fish and birds, line, Y- or star-shaped sutures are detected in other vertebrate lenses. Sutures that do not converge at the point are thought to result from intricate movements of the fiber tips, rather than a straightforward migration along a meridional path. The triggers that give rise to these variations are currently not understood. Our findings revealed that in the mouse embryo, the early-stage lens contains only concave curved fibers, and later, a zone of concave-to-convex curve conversion develops. At this point, a nascent suture in a linear shape appears at the posterior pole and subsequently progresses into a V-shape. This V-shape appears to further develop into a Y-shape as a branch extends from the apex of the V-shape. In lens of zebrafish and Xenopus larvae that form point sutures, this curve-conversion zone is not observed. In lens of adult birds (e.g. zebra finch) that form a point suture, these too also lack a curve-conversion zone. In our previous studies, we demonstrated that murine lens fibers undergoing curve conversion extend membrane protrusions, or lamellipodia, at their basal membranes. In line with this, we did not observe protrusions at the basal tips of fibers in the non-mammalian lenses of zebrafish, Xenopus, and zebra finch in which curve conversion does not occur. We propose that the concave-to-convex conversion in rodent lenses introduces defined paths for fiber cell tips, leading to a more elaborate and complex suture formation, compared to the simple point suture of lower vertebrates.

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Species referenced: Xenopus laevis

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	Graphical abstract
	Fig. 1. Changes in lens fiber cell morphology in the mouse during early development. Embryonic lens sections were cut along the anterior-posterior axis (A–D) or in the planes parallel to the equator (E–L). Panels E–H are images of the area near the anterior pole. Panels I–L present images of the regions near the posterior pole. Enlarged images of panels A–D are indicated as A′–D′, respectively. Cellular morphology was visualized by immunolabeling with anti-β-actin (A–D, A′–D′, and H) and anti-β-catenin (E–G and I–L). Scale bars for panels A–D, A′, B′, C′, D′, E and I, F and J, G and K, H and L: 100 μm.
	Fig. 2. Lens fiber cells of Xenopus larvae do not form a concave curve. Lenses of Xenopus larva (day 8) were whole-stained with phalloidin (green) and Hoechst (red). (A–C) Anterior surface, including the anterior pole. (D–F) Equatorial view. (G–J) Posterior hemisphere, including the posterior pole. 3D reconstructed images of surface (A,B,D,E,G,H) are shown with matching XY- (C,I inset) and YZ- (F,J) sections. To center the posterior pole, a stack of oblique sections is shown (I). Fulcrum positions are indicated with yellow lines. An enlarged image of F around the fulcrum is shown in F'. Scale bar: 20 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
	Fig. 3. Basal membranes of fibers in the adult zebrafish do not extend lamellipodia. Staining of a whole lens from a six-month-old zebrafish using phalloidin (green) and Hoechst (red). Positions of the lens equator and the posterior boundary of the axis-turn zone are indicated by yellow and blue lines, respectively. (A–C) A surface reconstructed view (A, B) and Z-section (C) around the equator. (D) A Z-section around the fulcrum at higher magnification. Pairs of arrowheads indicate the width of lateral cross-sections of axis-turning fibers. (E–J) Basal membranes of epithelial cells (E), fiber cells at the lens equator (F), the posterior end of the axis-turn zone (G), the region posterior to the axis-turn zone (H), the peri-posterior pole (I), and the posterior pole (J). (K) Surface reconstructed view of the posterior pole. (L, M) Stacked sections of the anterior (L) and posterior (M) poles. Scale bars: A–C, K 100 μm; D, 10 μm; E–J, 25 μm; L, M, 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
	Fig. 4. Lens fiber morphology of adult zebra finch. (A) Bright-field microscopic image of a lens from an adult zebra finch showing the annular pad (asterisk), a unique structure found only in the lenses of birds and reptiles. The yellow arrowhead indicates the boundary between the annular pad and lens fiber cells. (B–K) Whole lens staining with phalloidin (green) and Hoechst (red), including low-magnification views of the equatorial region (B), fulcrum (C), and the posterior pole (D), and high-magnification views of the anterior pole (E), the equator (F), fulcrum (G, panel H shows a higher-magnification image), posterior to the fulcrum (I, panel J shows a higher magnification image of the same region), and the posterior pole (K). Panels B and F-I contain reconstructed cross-sections. Scale bars: A, 1 mm; B-D 200 μm; E–G, I, K 100 μm; H, J 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
	Graphical abstract
	Fig. 1. Changes in lens fiber cell morphology in the mouse during early development. Embryonic lens sections were cut along the anterior-posterior axis (A–D) or in the planes parallel to the equator (E–L). Panels E–H are images of the area near the anterior pole. Panels I–L present images of the regions near the posterior pole. Enlarged images of panels A–D are indicated as A′–D′, respectively. Cellular morphology was visualized by immunolabeling with anti-β-actin (A–D, A′–D′, and H) and anti-β-catenin (E–G and I–L). Scale bars for panels A–D, A′, B′, C′, D′, E and I, F and J, G and K, H and L: 100 μm.
	Fig. 2. Lens fiber cells of Xenopus larvae do not form a concave curve. Lenses of Xenopus larva (day 8) were whole-stained with phalloidin (green) and Hoechst (red). (A–C) Anterior surface, including the anterior pole. (D–F) Equatorial view. (G–J) Posterior hemisphere, including the posterior pole. 3D reconstructed images of surface (A,B,D,E,G,H) are shown with matching XY- (C,I inset) and YZ- (F,J) sections. To center the posterior pole, a stack of oblique sections is shown (I). Fulcrum positions are indicated with yellow lines. An enlarged image of F around the fulcrum is shown in F'. Scale bar: 20 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
	Fig. 3. Basal membranes of fibers in the adult zebrafish do not extend lamellipodia. Staining of a whole lens from a six-month-old zebrafish using phalloidin (green) and Hoechst (red). Positions of the lens equator and the posterior boundary of the axis-turn zone are indicated by yellow and blue lines, respectively. (A–C) A surface reconstructed view (A, B) and Z-section (C) around the equator. (D) A Z-section around the fulcrum at higher magnification. Pairs of arrowheads indicate the width of lateral cross-sections of axis-turning fibers. (E–J) Basal membranes of epithelial cells (E), fiber cells at the lens equator (F), the posterior end of the axis-turn zone (G), the region posterior to the axis-turn zone (H), the peri-posterior pole (I), and the posterior pole (J). (K) Surface reconstructed view of the posterior pole. (L, M) Stacked sections of the anterior (L) and posterior (M) poles. Scale bars: A–C, K 100 μm; D, 10 μm; E–J, 25 μm; L, M, 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
	Fig. 4. Lens fiber morphology of adult zebra finch. (A) Bright-field microscopic image of a lens from an adult zebra finch showing the annular pad (asterisk), a unique structure found only in the lenses of birds and reptiles. The yellow arrowhead indicates the boundary between the annular pad and lens fiber cells. (B–K) Whole lens staining with phalloidin (green) and Hoechst (red), including low-magnification views of the equatorial region (B), fulcrum (C), and the posterior pole (D), and high-magnification views of the anterior pole (E), the equator (F), fulcrum (G, panel H shows a higher-magnification image), posterior to the fulcrum (I, panel J shows a higher magnification image of the same region), and the posterior pole (K). Panels B and F-I contain reconstructed cross-sections. Scale bars: A, 1 mm; B-D 200 μm; E–G, I, K 100 μm; H, J 50 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)