# Horizontal cell connectivity in the anchovy retina—a 3D electron microscopic study

## TL;DR

This study uses 3D electron microscopy to reveal how horizontal cells connect in the anchovy retina, shedding light on its unique polarization and color vision systems.

## Contribution

The paper provides the first detailed 3D connectomics analysis of horizontal cell types in the anchovy retina.

## Key findings

- Horizontal cell types H1, H2, and H3 have distinct connectivity patterns with cones and rods.
- The polarization contrast system connects with a bichromatic color contrast mechanism in the retina.
- Polycones likely evolved from red and green cones, with blue single cones disappearing in the studied region.

## Abstract

Block-face scanning electron microscopy has opened a new era of connectomics research, in which it is possible to make dense reconstructions of all cells in a clipping of a neuronal network, such as the retina, resolving synaptic contacts. Anchovies, exceptionally abundant marine teleosts, have retinae with regions for triple cone-based color vision and a region with specialized cone photoreceptors, so-called polycones, made of long and short cones with axially oriented outer segment lamellae for polarization contrast vision. This modality, discovered in the 1970s, is unique in vertebrates, but the neural wiring for contrast generation in deeper retinal layers is unknown so far.

To elucidate the retinal connectomics of the European anchovy Engraulis encrasicolus (Linnaeus, 1758), in a first project, we investigated the shapes and cone-specific wiring rules of 3 horizontal cell types using volume electron microscopy and subsequent computer-aided reconstruction: H1 cells contact both cone types of the polycone, H2 cells contact only the short cones, and H3 cells are exclusively connected to rods. In addition, a distinctive double band of Müller fibers and a layer of H1 axon terminals were structurally clarified.

The findings suggest that (1) the monochromatic polarization contrast system based on fine structure specializations in the outer retina is connected to an inherited (bichromatic) color contrast mechanism in the inner retina, (2) the anchovy polycones arose from red (now long) and green (now short) cones, and (3) the blue single cones disappeared in the relevant retinal region.

The online version contains supplementary material available at 10.1186/s12915-025-02242-7.