The horizontal cells are known to form a mono-layered mosaic in the
adult retina, but are scattered at different retinal depths in early
development. To help clarifying when and which spatial constraints appear
in the relative positioning of these cells, we have performed a
quantitative analysis of the three-dimensional (3D) organization of the
horizontal cell mosaic at different developmental stages in the postnatal
rat retina. We first analyzed the two-dimensional (2D) distribution of the
horizontal cell projections onto a plane parallel to the upper retinal
surface in retinal flat-mounts, and thus to the future mature horizontal
cell mosaic. We found that this 2D distribution was non random since
postnatal day 1 (P1), and had a subsequent stepwise improvement in
regularity. This preceded the alignment of cells in a single monolayer,
which was observed on P6. We then computed true horizontal cell spacing in
3D, finding non-random 3D positioning already on P1. Simulation studies
showed that this order might simply derive from the 2D order observed in
the projections of the cells in flat-mount, combined with their limited
spread in retinal depth. Throughout the period analyzed, the relative
positions of horizontal cells are in good agreement with a minimal spacing
rule in which the exclusion zone corresponds to the average size of the
inner core of the cell dendritic tree estimated from P1 samples. These
data indicate the existence of different phases in the process of
horizontal cell 3D spatial ordering, supporting the view that multiple
mechanisms are involved in the development of the horizontal cell
mosaic.