Figure 10. Formation of lamina-specific connections.

A. Inner plexiform layer. Processes of many amacrine cells ramify from the outset in appropriate sublaminae. Development of RGC dendrites exhibits subtype-specific variations: arbors of some types are initially diffuse then remodel, other are lamina-restricted from an early stage, and still others develop in discrete steps. The first two are shown here. Bipolar cells are born and extend axons late; their processes initially span multiple sublaminae, then become lamina-restricted.

B. Lateral geniculate nucleus. Axons from the ipsi- and contralateral eyes initially arborize broadly. Activity-dependent processes promote refinement to appropriate target laminae.

C. Fly lamina. The growth cones of R1-6 axons from the same ommatidium initially terminate in a tight cluster in a temporary layer bounded by glia. Interactions between growth cones promote extension away from one another in defined orientations where each projects to a different set of lamina neurons in surrounding cartridges and forms synapses with them. Some of the molecules implicated in discrete steps are indicated.

D. Fly medulla. The axons of R7, R8, L3 and L5 exhibit cell type-specific behaviors as they form lamina-specific terminals in the medulla.