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. 2015 Jul 9;9:33. doi: 10.3389/fncir.2015.00033

Figure 5.

Figure 5

Possible origins of ME cell inputs to T4 and T5. (A,B) Profiles of Mi1 and Tm3 (L1 pathway, green) and Tm1, 2 and 4 (L2 pathway, orange), from Golgi impregnations (after Fischbach and Dittrich, 1989). Tm3 and Tm4 are proposed to have arisen by duplication, and extend axons to terminals in the LO. (C) The original T4/T5 cell population of an ancestral fused ME/LO is proposed to have received input from an ancestral ME intrinsic (Mi) neuron from which later Tm cells derived. Gray arrows indicate direction of accretion of new columns to the neuropil, corresponding to the posterior-anterior axis of the visual field (cf. Figure 4A). (D) When the LO separated from the inner layers of the ME, Tm1 and Tm2 cells are proposed to have derived from the Mi cells, losing their connection with the T cells in stratum M10 but retaining synaptic connection with newly differentiated T5 cells in LO stratum Lo1. The Mi cell type survived as Mi1. (E) The LO split from the ancestral ME. The axon of the Tm1/Tm2 cells entered the LO from its distal end and terminated in Lo1, located on one side of the ME. (F) Tm cell axons enter the LO from the side of the LOP. In a final step, Tm1/Tm2 segregate into two separate Tm cell types, both retaining input from L2.