8/31/2023 0 Comments Dendrite axon synapse micrograph(2004) first described 10 cell types (T1–T9 and RB), Mataruga et al. These molecular genetic data are consistent with the reported morphological data. Recently, transcriptomics experiments defined 15 types of bipolar cells in the mouse retina ( Shekhar et al., 2016). These findings imply that the most sensitive scotopic signal may be conveyed to the center by ganglion cells that have the most numerous synapses with type 2 and 6 cells. Both type 2 and 6 cells gain the greatest access to AII amacrine cell signals also share those signals with other types of bipolar cells via networked gap junctions. Type 2 cells have 69% of the total number of OFF bipolar chemical synaptic contacts with AII amacrine cells and type 6 cells have 46% of the total area of ON bipolar gap junctions with AII amacrine cells. However, the number of synapses is dependent on bipolar cell types. AII amacrine cells are connected to five of six OFF bipolar cell types via conventional chemical synapses and seven of eight ON (cone) bipolar cell types via electrical synapses (gap junctions). We also found that almost all types of ON cone bipolar cells frequently have a minor group of midway ribbons along the axon passing through the OFF sublamina as well as a major group of terminal ribbons in the ON sublamina. Formation of bipolar cell output at each ribbon synapse may be effectively regulated by a few nearby inhibitory inputs of amacrine cells which are chosen from among many amacrine cell types. We found a strong correlation between the number of amacrine cell synaptic contacts and the number of bipolar cell axonal ribbons. After validation of bipolar cell types, we examined their relationship with amacrine cells then particularly with AII amacrine cells. Next, while two groups of rod bipolar (RB) cells, RB1, and RB2, were previously proposed, we clarify that a half of RB1 cells have the intermediate characteristics, suggesting that these two groups comprise a single RB type. Although whether the previous type 5 cells consist of two or three types was uncertain, they are here clustered into three types based on the vertical distribution of axonal ribbons. We confirmed the classification of 15 morphological types of mouse bipolar cells by serial section transmission electron microscopy and characterized each type by identifying chemical synapses and gap junctions at axon terminals. 2Department of Biology, Hyogo College of Medicine, Nishinomiya, Japan.
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