Fig. 1

Models of development of the esophageal ME. a Lineage progression in development of ESM. Mesp1⁺ cranial mesoderm progenitors express Tbx1 to give rise to migratory Isl⁺ ESM progenitors. It is hypothesized that after arrival at the proximal end of the esophageal ME, these cells express Pax7 and subsequently Myf5 and MyoD. The dashed arrows indicate uncertainty as to the precise linear order of this process; see text for additional details. b Model of striated myogenesis in the TZ. The TZ contains proliferating skeletal muscle-like progenitor (Pax7⁺) cells, muscle progenitor cells in the process of commitment to the skeletal muscle-like lineage (Pax7⁺/Myf5⁺/MyoD⁺ cells), myoblasts (Myf5⁺/MyoD⁺ cells), and differentiating myoblasts (myogenin⁺ [MyoG⁺] cells). The TZ moves in a proximal-distal manner, leaving ESM in its wake. SMCs are mainly found distal to the TZ where they undergo fascicular reorientation (see c). Some SMCs are also found dispersed within the TZ. c Model for reorientation of SMC fascicles and proximal-distal movement of the TZ between P0 and P14. SMCs of the circumferential layer of the ME are initially grouped into fascicles that have an end-to-end configuration and an orientation parallel to the lumen (note that, for simplicity, the external, longitudinal layers of both smooth and striated muscles are not shown in the figure). Fascicles reorganize in a distal-to-proximal manner via a globular intermediate (indicated by the tear drop-shaped cluster of SMCs) and culminate in a side-by-side configuration with an orientation that is nearly parallel to the lumen; as a consequence, the fascicles ultimately occupy only the most distal portion of the ME. The blue triangle represents a hypothetical distal signal that promotes SMC fascicular reorientation. The identity of this signal is not known. The orange triangle represents a hypothetical TZ-based signal that promotes movement of proximal SMC fascicles in the distal direction. The identity of this signal is not known but cell proliferation of ESM progenitors in the TZ may contribute to this activity. b and c are adapted from reference [26]