Retinal determination the beginning of eye development

Abstract

The road to producing an eye begins with the decision to commit a population of cells to adopting an eye tissue fate, the process of retinal determination. Over the past decade and a half, a network of transcription factors has been found to mediate this process in all seeing animals. This retinal determination network is known to regulate not only tissue fate but also cell proliferation, pattern formation, compartment boundary establishment, and even retinal cell specification. The compound eye of the fruit fly, Drosophila melanogaster, has proven to be an excellent experimental system to study the mechanisms by which this network regulates organogenesis and tissue patterning. In fact the founding members of most of the gene families that make up this network were first isolated in Drosophila based on loss-of-function phenotypes that affect the eye. This chapter will highlight the history of discovery of the retinal determination network and will draw attention to the molecular and biochemical mechanisms that underlie our understanding of how the fate of the retina is determined.

Figure 1.1

Structure of the adult compound eye. (A) Scanning electron micrograph of the adult eye. (B) Section of the adult retina showing the photoreceptor neurons. (C) Section of the pupal retina showing all cone, pigment, and bristle cells. (D) A high magnification of a single ommatidium from the adult retina. Note that each photoreceptor neuron is given a unique identifier number. (E) A schematic describing the orientation of ommatidia in the dorsal and ventral quadrants in the left and right compound eyes. Anterior is to the right in all images.

Figure 1.2

The morphogenetic furrow and ommatidial assembly. (A) A confocal section of a third instar larval eye disk that has been stained with phalloidin, which marks F-actin. Note that cells ahead of the furrow are unpatterned while those behind the furrow are organized into ommatidial rudiments. The blue circle marks one individual unit eye. (B) A confocal section of a third instar larval eye disk that has been stained with an antibody that is directed against the ELAV protein. The blue circle marks a unit eye that is in roughly the same position as the one in panel A. (C) A schematic drawing of the order of ommatidial assembly within the eye disk. Anterior is to the right in all images. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this chapter.)

Figure 1.3

The retinal determination network: genes and phenotypes. (A) A list of the known retinal determination genes, the vertebrate homologs, and the known functional domains. (B) A scanning electron micrograph of a sine oculis loss-of-function mutant. Note that the compound eyes are missing and have been replaced by head cuticle. (C) A light microscope image of an animal in which the ey gene has been expressed in the wing and haltere disks using an ap-GAL4 driver. Anterior is to the right in all images.

Figure 1.4

The workings of the retinal determination network. A schematic of all known interactions within the retinal determination network. Note that the cascade does not function as a linear pathway but rather contains autoregulatory circuits and feedback loops.

Figure 1.5

Expression patterns of retinal determination genes within the developing eye field. The eye disk is divided into six zones based on the expression patterns and functions of the known retinal determination genes. The zones are listed at the bottom of the figure. The expression patterns of each gene are represented by the colored horizontal lines. The ongoing developmental processes within each zone are listed at the top of the figure. The morphogenetic furrow is shown in gray. Anterior is to the right in this schematic diagram. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this chapter.)