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At first glance, the eye of a fruit fly (Drosophila melanogaster) and a human are very different. However, increasing evidence indicates that many conserved mechanisms are used for specifying the eye field and for generating different neuronal cell populations in the retina. However, whether molecular pathways used for generating the focusing structures of the eye – the lens and cornea – are conserved throughout evolution remains unclear. Indeed, while some progress has been made in understanding how these structures form in mammalian systems, little to nothing is known regarding how they form in invertebrates such as flies.
Thus, one question in our lab is aimed at understanding how the corneal lens forms in Drosophila and testing the hypothesis that these processes are similar to those used in humans. If true, this would allow us to take advantage of the powerful genetics available in Drosophila to better understand human lens development and to create helpful therapies for patients suffering from cataracts and other cornea / lens-associated diseases.
Genesis of the corneal lens in Drosophila occurs during the latest larval stages, just prior to pupation. Initially, a group of cells known as the “R7 equivalence group” have the potential to become either a photoreceptor neuron (the R7) or a lens-secreting cell (cone cell, CC). This retina vs. lens fate decision requires input from two primary signaling pathways: Ras and Notch. Our recent studies have found that two direct downstream targets of these signaling pathways, the transcription factors Prospero (Pros) and dPax2, function antagonistically during R7 vs. CC differentiation, but act synergistically to promote lens formation.
Importantly, vertebrate factors related to Pros and dPax2, known as Prox1 and Pax6, are also important for lens development in mice and humans, supporting the hypothesis that lens formation in vertebrates and invertebrates use evolutionarily conserved pathways. Likewise, Prox1 and vertebrate Pax2 and Pax6 are important regulators for many aspects of nervous system development. Therefore, our studies on understanding the fundamental mechanisms underlying Pros and dPax2 function during fly eye development should provide a useful platform for developing better treatments for human eye and brain diseases.
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Diagram showing the retina, corneal lens and pigment epithelia in individual fly eye units
The fly compound eye is made up of ~750 individual eye units called ommatidia. This diagram represents all the components required to build a single unit. Even though the general eye structure externally appears quite different from a human eye, each ommatidia, like the human eye, requires a lens for focusing light (light pink structure at top of the eye), photoreceptors for dim light detection (gray cells) and color discrimination (UV-sensitive cells = dark pink R7s, and blue/green-sensitive cells = dark blue R8s) and highly pigmented cells important to absorb excess light (orange, light blue and gray cells). See our work on Photoreceptor Differentiation to learn more about R7 vs. R8 cell fate decisions.
Membrane staining of individual ommatidial cell clusters in the developing larval eye
Eye tissue isolated from larvae just prior to pupation is immunostained with two membrane markers to visualize developing individual eye units (ommatidia). Left of the tightly packed area of cells, called the morphogenetic furrow, cells continue to proliferate. Right of the furrow, cells begin to differentiate into photoreceptors and cone cells. E-cadherin (green) is present on apical surfaces of all cells in this epithelial sheet while N-cadherin (blue) is restricted to a smaller subset of cells. As more cells become recruited to the ommatidial clusters, the staining becomes more obvious. Each row of ommatidia is approximately two hours older than the row before it, allowing us to watch development of different cells in “real time.”
Staining of lens-secreting and pigment-producing cells during their formation in pupil eyes
During pupil development, the four cone cells that will give rise to the lens are stained with a nuclear marker (Cut, magenta). These cells are separated by pigmented interommatidial cells that can be visualized with membrane markers (E-cadherin, green).
The exterior surface of the lens during (left) and after (right) secretion
Scanning electron microscopy of the exterior surface of pupil and adult eye units. During secretion of the lens material, beaded wispy material is noticeable on the surface of the pupil eye (left). This material becomes refined into a rigid beaded structure, required for light refraction and focused vision in the adult (right).
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