Local retinoic acid signaling directs emergence of the extraocular muscle functional unit.

Glenda Comai ,Muriel Rhinn,Jozef Kaiser,Tomáš Zikmund,Sigmar Stricker,Shahragim Tajbakhsh,Andreas Schedl,James Briscoe,Valérie Dupé,Ronen Schweitzer,Pascal Dollé,François Spitz,Markéta Tesařová,Fabio Da Silva,Leif Carlsson,Norbert B Ghyselinck,Katherine Exelby,Pedro Vallecillo-García,Betty Féret ,Brian A Pryce

doi:10.1371/journal.pbio.3000902

Abstract

Coordinated development of muscles, tendons, and their attachment sites ensures emergence of functional musculoskeletal units that are adapted to diverse anatomical demands among different species. How these different tissues are patterned and functionally assembled during embryogenesis is poorly understood. Here, we investigated the morphogenesis of extraocular muscles (EOMs), an evolutionary conserved cranial muscle group that is crucial for the coordinated movement of the eyeballs and for visual acuity. By means of lineage analysis, we redefined the cellular origins of periocular connective tissues interacting with the EOMs, which do not arise exclusively from neural crest mesenchyme as previously thought. Using 3D imaging approaches, we established an integrative blueprint for the EOM functional unit. By doing so, we identified a developmental time window in which individual EOMs emerge from a unique muscle anlage and establish insertions in the sclera, which sets these muscles apart from classical muscle-to-bone type of insertions. Further, we demonstrate that the eyeballs are a source of diffusible all-trans retinoic acid (ATRA) that allow their targeting by the EOMs in a temporal and dose-dependent manner. Using genetically modified mice and inhibitor treatments, we find that endogenous local variations in the concentration of retinoids contribute to the establishment of tendon condensations and attachment sites that precede the initiation of muscle patterning. Collectively, our results highlight how global and site-specific programs are deployed for the assembly of muscle functional units with precise definition of muscle shapes and topographical wiring of their tendon attachments.

Highlights

Acquisition of shape and pattern during development depends on the orchestrated crosstalk between a variety of tissues and cell types
The periocular mesenchyme (POM) is a heterogeneous cell population surrounding the optic cup that gives rise to specialized structures of the anterior segment of the eye and connective tissues associated with the extraocular muscle (EOM) [37]
We simultaneously traced the contribution of neural crest cell (NCC) (Fig 1A–1B”, S1A, and S1B Fig) and mesodermal (Fig 1C– 1D”, S1C and S1D Fig) derivatives using Tg:Wnt1Cre driver (Tg):Wnt1Cre and Mesp1Cre mice, respectively [45,46], in combination with the R26Tom reporter [74]

Summary

Introduction

Acquisition of shape and pattern during development depends on the orchestrated crosstalk between a variety of tissues and cell types. The vertebrate musculoskeletal system serves as an ideal model to study these processes as different tissues including muscle, tendon and their attachments need to be articulated in 3D for proper function [1,2]. The basic EOM pattern is shared among all vertebrate classes [3,4,5] and includes 4 recti muscles (the superior rectus, the medial rectus, the inferior rectus, and the lateral rectus) and 2 oblique muscles (superior oblique and inferior oblique) for movement of the eyeball. The EOMs constitute an archetypal and autonomous functional unit for the study of how muscles, tendons, and tendon attachments are integrated with the development of the eyeball, their target organ

Methods

Results

Discussion

Conclusion