Stop or Go in the Target Zone

The eye and brain develop coordinately during the period of photoreceptor axon ingrowth in the larval stage. The spatiotemporal pattern of ommatidial differentiation in the eye (posterior earlier than anterior) results in a temporal pattern of photoreceptor axon arrival in the brain. In the eye, new ommatidia initiate differentiation at the anterior (left), while older ommatidia add additional cells (R1–R6 in blue). The initial R8 cell (red color) sends its axon into the brain first; its growth cone (colored red) transits the lamina en route to its medulla target (e.g., the most anterior axon shown). The outer (R1–R6) photoreceptors send their growth cones (blue color) along the expanding ommatidial fascicle. The R1–R6 growth cones travel along the R8 fiber into the lamina (the middle fascicle shown). As shown for the most posterior fascicle, the R1–R6 growth cones stop between the layers of epithelial (E-) and marginal (M-) glia (green color). The R8 growth cones meanwhile have traveled on, beyond the lamina to their deeper targets in the medulla layer.