A complex choreography of cell movements shapes the vertebrate eye

Abstract

Optic cup morphogenesis (OCM) generates the basic structure of the vertebrate eye. Although it is commonly depicted as a series of epithelial sheet folding events, this does not represent an empirically supported model. Here, we combine four-dimensional imaging with custom cell tracking software and photoactivatable fluorophore labeling to determine the cellular dynamics underlying OCM in zebrafish. Although cell division contributes to growth, we find it dispensable for eye formation. OCM depends instead on a complex set of cell movements coordinated between the prospective neural retina, retinal pigmented epithelium (RPE) and lens. Optic vesicle evagination persists for longer than expected; cells move in a pinwheel pattern during optic vesicle elongation and retinal precursors involute around the rim of the invaginating optic cup. We identify unanticipated movements, particularly of central and peripheral retina, RPE and lens. From cell tracking data, we generate retina, RPE and lens subdomain fate maps, which reveal novel adjacencies that might determine corresponding developmental signaling events. Finally, we find that similar movements also occur during chick eye morphogenesis, suggesting that the underlying choreography is conserved among vertebrates.

Analysis of opo cis-regulatory landscape uncovers Vsx2 requirement in early eye morphogenesis

“…In the light of current evidence, this seems an unlikely possibility. Although cell proliferation was initially hypothesized as a cellular mechanism operating during optic cup morphogenesis 59 , more recent reports using DNA synthesis inhibitors have shown that proliferation is largely dispensable for the morphogenesis of the nervous system in Xenopus 60 and of the optic cup in zebrafish 21 . In fact, optic cup invaginates between 16 and 20 h.p.f.…”

“…In fact, optic cup invaginates between 16 and 20 h.p.f. in zebrafish, a 4-h period in which only a modest increase in cell number takes place 21 . Moreover, here we show no significant correlation between retinal size and optic cup folding defects on knockdown of vsx2 in zebrafish (Fig.…”

“…As tissue patterning takes place, eye precursors within their different domains undergo a coordinated series of morphogenetic movements that shape the primordium into a hemispheric optic chamber [21][22][23] . Two major characteristic movements have been described by anatomical observations and live-imaging recordings in all vertebrate models.…”

“…The first, known as evagination, results in the formation of bilateral optic vesicles either by the lateral bulging of coherent epithelial sheets, as reported in tetrapods 24,25 , or by the rearrangement of individual cells, as described in teleost models 26,27 . In a second series of coordinated movements (that is, invagination), the neural retina and RPE domains fold to form a bilayered cup that covers the lens vesicle, which simultaneously invaginates from the surface ectoderm 21,25,28 . Although contractile interepithelial filopodia have been shown to participate in the finetuning mechanism that coordinates optic cup and lens morphogenesis 29 , a number of studies indicate that both tissues fold following cell intrinsic-independent mechanisms.…”

Mechanisms of cranial placode assembly

“…The formation of segregated placode primordia correlates in time with upregulation of transcription factors specifically expressed in individual or groups of placodes Pieper et al, 2011;Schlosser, 2010;Streit, 2008). Pax2 and Pax8, expressed in the otic and epibranchial placode progenitors (and in lateral line placodes in fish and amphibians), show saltand-pepper expression in chick (Groves and Bronner-Fraser, 2000;Streit, 2002), Xenopus (Pieper et al, 2011;Schlosser and Ahrens, 2004), zebrafish (Bhat et al, 2013;Bhat and Riley, 2011;Hans and Westerfield, 2007;McCarroll et al, 2012; Kozlowski et al, 1997;Bhattacharyya and Bronner, 2013;Breau et al, 2012;Dutta et al, 2005;Harden et al, 2012;Knaut et al, 2005;Kwan et al, 2011;Pieper et al, 2011;Streit, 2002;Whitlock and Westerfield, 2000;Xu et al, 2008) al., 2012) and mice (Ohyama and Groves, 2004). Pax6, a transcription factor expressed in the lens placode, exhibit mosaic expression in zebrafish (Dutta et al, 2005;Hans and Westerfield, 2007).…”