Fig. 3. Developmental changes in horizontal cell morphology. (A) A single optical section at the level of the horizontal cell layer. Horizontal cells are identified by their expression of Prox1. (B-G) Projection images (upper panels) and optical vertical sections (lower panels) of mEGFP-expressing horizontal cells at E9 (B), E12 (C,D) and E14 (E-G). Vertical processes are shown by arrowheads in B-D. (C) At E12, some horizontal cells have a long process reminiscent of an axon (arrow). (D) Other horizontal cells have less branched dendrites extending in one direction. (E-G) At E14, the three subtypes of horizontal cells have become apparent. Arrows in G indicate the dendrite terminals with growth cone-like morphology. Scale bar: 10 µm.