Cell numbers, cell ratios, and developmental plasticity in the rod pathway of the mouse retina

The precise specification of cellular fate is thought to ensure the production of the correct number of neurons within a population. Programmed cell death may be an additional mechanism controlling cell number, believed to refine the proper ratio of pre‐ to post‐synaptic neurons for a given species. Here, we consider the size of three different neuronal populations in the rod pathway of the mouse retina: rod photoreceptors, rod bipolar cells, and AII amacrine cells. Across a collection of 28 different strains of mice, large variation in the numbers of all three cell types is present. The variation in their numbers is not correlated, so that the ratio of rods to rod bipolar cells, as well as rod bipolar cells to AII amacrine cells, varies as well. Establishing connectivity between such variable pre‐ and post‐synaptic populations relies upon plasticity that modulates process outgrowth and morphological differentiation, which we explore experimentally for both rod bipolar and AII amacrine cells in a mouse retina with elevated numbers of each cell type. While both rod bipolar dendritic and axonal arbors, along with AII lobular arbors, modulate their areal size in relation to local homotypic cell densities, the dendritic appendages of the AII amacrine cells do not. Rather, these processes exhibit a different form of plasticity, regulating the branching density of their overlapping arbors. Each form of plasticity should ensure uniformity in retinal coverage in the presence of the independent specification of afferent and target cell number. Across a panel of different mouse strains, there exists a large variation in the numbers of the first three cell types of the rod pathway of the retina. That variation is not correlated, however, yielding substantial variation in the ratios of these synaptic partners. Plasticity during morphological differentiation ensures uniformity of retinal coverage and connectivity, yet the mechanisms can vary, even between different sets of processes arising from the same cell.