3D-EM Analysis Of Dendritic Tree Ultrastructure and Rod Convergence in Two Types of Horizontal Cells in a Functionally Plastic Retina
By: Yaqoub Yusuf
Department: Biology
Faculty Advisor: Dr. Ivan A. Anastassov
The overall goal of this project is to determine what molecular and structural adaptations have allowed the pure-rod skate (Leucoraja erinacea) retina to achieve functionality over a full range of brightness conditions. The visual systems of most vertebrate species have two different kinds of photoreceptors: rods and cones. Vertebrates can distinguish color and bright light by combining and comparing signals from cones sensitive to different wavelengths of light. Rods are sensitive to low light levels and drive vision in dim light. L. erinacea only possesses rods, which can function across large ranges of light intensity. We aim to understand how the skate retina achieves functional duality with a monotypic photoreceptor population. Specifically, we aim to describe how the diversity of horizontal cells (i.e., a downstream partner of rods and cones) and their structural characteristics contribute to the ability of the skate retina to transmit visual information across vastly different levels of illumination. We analyze differences between internal and external horizontal cells (IHC and EHC, respectively) by using serial section EM data to obtain 3D representations of cell structure and cell connectivity. SB-3DEM was performed on skate retina; the dataset analyzed here was from adult animals with width=57.344μm, height=315.392μm, and depth=34.79μm. Voxel size was 7x7x70nm. Additional reconstructions of connections of each cell type to rods is ongoing, but preliminary data suggests EHCs contact significantly more rods than IHCs. Large differences in structural characteristics between IHCs and EHCs suggest a possible division along functional lines. Given that in mixed rod-cone retinas, horizontal cells contact rods or cones very selectively, we hypothesize that IHCs and EHCs also contact rods selectively. However, we propose that this selectivity is based on the numbers and functional range of the contacted rods within the dendritic field of each type of horizontal cell.