Rod Bipolar Cells Require Horizontal Cells for Invagination Into the Terminals of Rod Photoreceptors

Transgenic mice provide a new approach for studying the structure and function of the mammalian retina. In the past, the cellular organization of the mammalian retina was investigated preferentially in primates, cats, and rats but rarely in mice. In the current study, the authors applied 42 different immunocytochemical markers to sections of the mouse retina and studied their cellular and synaptic localization by using confocal microscopy. The markers applied were from three major groups: 1) antibodies against calcium-binding proteins, such as calbindin, parvalbumin, recoverin, or caldendrin; 2) antibodies that recognize specific transmitter systems, such as glycine, gamma-aminobutyric acid, or acetylcholine; and 3) antibodies that recognize transmitter receptors and show their aggregation at specific synapses. Only a few markers labeled only one cell type: Most antibodies recognized specific groups of neurons. These were analyzed in more detail in double-labeling experiments with different combinations of the antibodies. In light of their results, the authors offer a list of immunocytochemical markers that can be used to detect possible changes in the retinal organization of mutant mice. Copyright 2000 Wiley-Liss, Inc.

The photoreceptor ribbon synapse is a highly specialized glutamatergic synapse designed for the continuous flow of synaptic vesicles to the neurotransmitter release site. The molecular mechanisms underlying ribbon synapse formation are poorly understood. We have investigated the role of the presynaptic cytomatrix protein Bassoon, a major component of the photoreceptor ribbon, in a mouse retina deficient of functional Bassoon protein. Photoreceptor ribbons lacking Bassoon are not anchored to the presynaptic active zones. This results in an impaired photoreceptor synaptic transmission, an abnormal dendritic branching of neurons postsynaptic to photoreceptors, and the formation of ectopic synapses. These findings suggest a critical role of Bassoon in the formation and the function of photoreceptor ribbon synapses of the mammalian retina.

To understand the mechanisms of cell fate determination in the vertebrate retina, the time course of the generation of the major cell types needs to be established. This will help define and interpret patterns of gene expression, waves of differentiation, timing and extent of competence, and many of the other developmental processes involved in fate acquisition. A thorough retinal cell "birthdating" study has not been performed for the laboratory rat, even though it is the species of choice for many contemporary developmental studies of the vertebrate retina. We investigated the timing and spatial pattern of cell genesis using 3H-thymidine (3H-TdR). A single injection of 3H-TdR was administered to pregnant rats or rat pups between embryonic day (E) 8 and postnatal day (P) 13. The offspring of prenatally injected rats were delivered and all animals survived to maturity. Labeled cells were visualized by autoradiography of retinal sections. Rat retinal cell genesis commenced around E10, 50% of cells were born by approximately P1, and retinogenesis was complete near P12. The first postmitotic cells were found in the retinal ganglion cell layer and were 9-15 microm in diameter. This range includes small to medium diameter retinal ganglion cells and large displaced amacrine cells. The sequence of cell genesis was established by determining the age at which 5, 50, and 95% of the total population of cells of each phenotype became postmitotic. With few exceptions, the cell types reached these developmental landmarks in the following order: retinal ganglion cells, horizontal cells, cones, amacrine cells, rods, bipolar cells, and Müller glia. For each type, the first cells generated were located in the central retina and the last cells in the peripheral retina. Within the sequence of cell genesis, two or three phases could be detected based on differences in timing, kinetics, and topographic gradients of cell production. Our results show that retinal cells in the rat are generated in a sequence similar to that of the primate retina, in which retinogenesis spans more than 100 days. To the extent that sequences reflect underlying mechanisms of cell fate determination, they appear to be conserved. Copyright 2004 Wiley-Liss, Inc.