In order to characterize further the morphogenesis of Crx-/- photoreceptors, the developing outer segment layer was viewed by scanning electron microscopy at P7, P14 and P21 (Figure 4). In Crx+/+ retinas, photoreceptor inner segments, connecting cilia, and the first rudimentary outer segment structures were noted at P7. In the Crx-/- retina, only an occassional connecting cilium was noted emerging from inner segments at this stage (Figure 4A and 4B). This observation was confirmed by comparison with transmission electron micrographs (Figure 5). These differences are the earliest noted differences between Crx+/+ and Crx-/- photoreceptors. At P14, elongating outer segments were noted on Crx+/+ photoreceptors, occasionally demonstrating a paddle-like structure at their apex (Figure 4C, os). In Crx-/- retinae, the vast majority of photoreceptors at this stage demonstrated connecting cilia without outer segments (Figure 4D, cc). Sporadically, Crx-/- photoreceptors would exhibit an irregular structure extending from a connecting cilium (Figure 4D, cc*) perhaps representing a malformed outer segment. Such structures were also observed at P21 (Figure 4F, cc*). These putative, abnormal outer segments were only rarely noted in Crx+/+ at P14, and never at P21 (Figure 4C and 4E, cc*). Further, in Crx-/- photoreceptors, unusually long connecting cilia were noted (Figure 4F, cc). Serial examination of Crx-/- photoreceptors at P7, P10, P14, and P21 by TEM, demonstrated a distinctive lack of any structure interpretable as orderly stacks of discs or forming discs. These data demonstrate a complete absence of normal outer segment formation in the Crx mutant mouse, and the arrest of development of the photoreceptor appendage at the elongation stage of outer segment formation.