Development of A-type (axonless) horizontal cells in the rabbit retina

The development of A-type horizontal cells (HC) was studied in the rabbit retina between embryonic day (E)24 and adulthood [the day of birth was called postnatal day (P)1 and corresponds to E31-32]. The cells were visualized by several methods 1) by immunolabeling with antibodies to neurofilament 70,000 (NF-70kD), 2) by immunolabeling with antibodies to a calcium binding protein (CaBP-28kD), 3) by two different methods of silver impregnation, and 4) by histochemical demonstration of NADH-diaphorase activity. Most methods labeled A-type HC only in the dorsal retina; thus, our study is restricted to HC of this region. HC densities were determined at each developmental stage. The cells were drawn at scale, and size, quotient of symmetry, and topographical orientation of dendritic trees were studied by image analysis. The growth of HC dendritic fields was correlated with data on the postnatal local retinal expansion, which is known to be driven by the intraocular pressure (after cessation of retinal cell proliferation at P9). This expansion was evaluated in an earlier paper (Reichenbach et al. [1993] Vis. Neurosci. 10:479-498) by using local subpopulations of Müller cells as "markers" of distinct topographic regions of the retinae. After E24, when the final number of HC is established, we can discriminate three distinct developmental stages of A-type HC. During the first stage, between E24 and E27, the young cells are often vertically oriented and may extend their first short dendrites within (the primordia of) both plexiform layers. The irregular HC mosaic at E24 shows a significant difference to all other stages. The second stage begins after birth when the dendritic trees of the cells are already restricted to the outer plexiform layer. Between P3 and P9, their dendritic trees enlarge more than the surrounding retinal tissue expands, and the coverage factor almost doubles from 2.5 to 4.4. The third stage occurs after P9 when the growth rate of dendritic tree areas corresponds to that of the local retinal tissue expansion caused by "passive stretching" of the postmitotic tissue, and the coverage factor remains constant. This is compatible with the view that mature synaptic connections of A-type HC are mostly established after the first week of life and are then maintained.