Local retinoic acid signaling 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 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.

Identification of bipotent progenitors that give rise to myogenic and connective tissues in mouse

“…We thus examined the EOM, larynx and upper back muscles in the early fetus at E14.5 using a Myf5-lineage reporter mouse (Myf5 Cre/+ ; R26 TdTomato/+ ) combined with a contemporary reporter for Pdgfra+ non-myogenic cells (Pdgfra H2BGFP/+ ) (Figure 3). Notably, we observed tdTom/H2BGFP double-positive cells in regions of EOM, laryngeal and upper back muscles that were partially or fully deprived of neural crest (Adachi et al, 2020;Comai et al, 2020;Heude et al, 2018) (Figure 3A-C'). Conversely, no double-positive cells were detected in muscles that are fully embedded in neural crest derived connective tissues such as mandibular and tongue muscles (Heude et al, 2018) (Figure 3D-E').…”

“…Mesenchymal tissue that is associated with the EOM arises from mesoderm in its most dorso-medial portion and from neural crest in its ventro-lateral portion (Comai et al, 2020). This dual origin makes it a prime candidate to explore the relative contribution of Myf5-derived cells to the associated connective tissues within a single functional unit.…”

“…The majority of muscle-associated connective tissues in the head is derived from cranial neural crest cells (NCCs), an embryonic cell population that contributes to most of the structural components of the "new head", a vertebrate innovation (Douarin and Kalcheim, 1999;Gans and Northcutt, 1983;Grenier et al, 2009;Heude et al, 2018;Noden and Trainor, 2005). Recently, the extent of this contribution was redefined in muscles derived from cranial mesoderm, including extraocular (EOM), laryngeal and pharyngeal muscles (Comai et al, 2020;Grimaldi et al, 2015;Heude et al, 2018;Noden and Epstein, 2010). Interestingly, these muscles contain mesenchyme that is mesoderm-derived in their dorso-medial component, whereas the remaining muscle mass is embedded in mesenchyme that is neural crestderived.…”

Distinct Embryonic Origin and Injury Response of Resident Stem Cells in Craniofacial Muscles

“…One distinct characteristic of craniofacial muscle connective tissue is its heterogeneity in embryonic origin ( Adachi et al, 2020 ). Among the seven EOM, the connective tissue associated with four recti muscles is derived from mesoderm, while that associated with the rest three muscles is of neural crest origin ( Comai et al, 2020 ). Furthermore, the composition of craniofacial FAPs undergoes temporal changes after birth ( Lemos et al, 2012 ).…”

“…The impetus to study the functional role of FAPs in muscle regeneration only began several years ago, and investigations focusing on craniofacial FAPs are rare. Nevertheless, the putatively disparate performance of neural crest cell-derived craniofacial FAPs has been suggested ( Lemos et al, 2012 ; Formicola et al, 2014 ; Paylor et al, 2014 ; Stuelsatz et al, 2014 ; Comai et al, 2020 ). Take the EOM muscles, for example, the proportion of FAPs in the MuSC niche in the EOM is much higher than that in the limb, and this trend is maintained in aged individuals and in dystrophic muscles ( Formicola et al, 2014 ).…”