Local retinoic acid directs emergence of the extraocular muscle functional unit

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 where 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 retinoic acid 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.

Scx-positive tendon cells are required for correct muscle patterning

“…The fact that muscle did not attach randomly to nearby bone but changed its morphology toward the distal tendon tissue implied a hypothesis that diffusible molecule(s) secreted from tendon cells could attract myofibers. Indeed, recent studies reported the involvement of retinoic acid in the formation of the extraocular functional unit (Comai et al, 2020), and FGF or BMP signaling were active at the interface of the embryonic tendon and muscle (Eloy-Trinquet, Wang, Edom-Vovard, & Duprez, 2009; Wang et al, 2010). However, cell adhesion molecule (Hasson et al, 2010) or ECM (Kutchuk et al, 2015) can also play parallel roles.…”

The impact of Drew Noden's work on our understanding of craniofacial musculoskeletal integration

“…In addition to the ear, tendons that connect six extraocular muscles to the eye are essential for normal eye movement and defects in these tendons have been linked to ocular movement disorders including Brown syndrome and congenital fibrosis of the extraocular muscles (CFEOM), 100,101 where a shortened superior oblique tendon contributes to limited movement and passive elevation of the eye. While more is known about mechanisms governing the development of extraocular muscles, 102,103 there is a paucity of research on ocular tendon development. Ocular tendons have been identified in developing mice and zebrafish and the initiation of their formation during development is muscle independent, similar to their limb counterparts 47,104 .…”