# Ocular Motoneuron Pools Develop Along a Dorsoventral Axis in Zebrafish, Danio rerio

## TL;DR

This study shows how eye movement neurons in zebrafish develop along a top-to-bottom axis, influencing their roles in eye movement control.

## Contribution

The study reveals a dorsoventral neurogenesis order that correlates with motoneuron identity and circuit organization in zebrafish.

## Key findings

- Oculomotor motoneuron pools in zebrafish are organized dorsoventrally.
- Dorsal motoneurons develop before ventral ones, linking birth order to identity.
- Neurogenesis timing may shape functional oculomotor circuit assembly.

## Abstract

Both spatial and temporal cues determine the fate of immature neurons. A major challenge at the interface of developmental and systems neuroscience is to relate this spatiotempo-ral trajectory of maturation to circuit-level functional organization. This study examined the development of two ocular cranial motor nuclei (nIII and nIV), structures in which a motoneuron’s identity, or choice of muscle partner, defines its behavioral role. We used retro-orbital dye fills, in combination with fluorescent markers for motoneuron location and birth-date, to probe spatial and temporal organization of the oculomotor (nIII) and trochlear (nIV) nuclei in the larval zebrafish. We described a dorsoventral organization of the four nIII motoneuron pools, in which inferior and medial rectus motoneurons occupy dorsal nIII, while inferior oblique and superior rectus motoneurons occupy distinct divisions of ventral nIII. Dorsal nIII motoneurons are, moreover, born before motoneurons of ventral nIII and nIV. Order of neurogenesis can therefore account for the dorsoventral organization of nIII and may play a primary role in determining motoneuron identity. We propose that the temporal development of ocular motoneurons plays a key role in assembling a functional oculomotor circuit.