Ultrastructural analysis of the smooth‐to‐striated transition zone in the developing mouse esophagus: Emphasis on apoptosis of smooth and origin and differentiation of striated muscle cells

Abstract

The exact mechanism of smooth-to-striated muscle conversion in the mouse esophagus is controversial. Smooth-to-striated muscle cell transdifferentiation vs. distinct differentiation pathways for both muscle types were proposed. Main arguments for transdifferentiation were the failure to detect apoptotic smooth and the unknown origin of striated muscle cells during esophageal myogenesis. To reinvestigate this issue, we analyzed esophagi of 4-day-old mice by electron microscopy and a fine-grained sampling strategy considering that, in perinatal esophagus, the replacement of smooth by striated muscle progresses craniocaudally, while striated myogenesis advances caudocranially. We found numerous (1) apoptotic smooth muscle cells located mainly in a transition zone, where smooth intermingled with developing striated muscle cells, and (2) mesenchymal cells in the smooth muscle portion below the transition zone, which appeared to give rise to striated muscle fibers. Taken together, these results provide further evidence for distinct differentiation pathways of both muscle types during esophagus development. Developmental Dynamics 233:964 -982, 2005.

Deletion of Pax7 changes the tunica muscularis of the mouse esophagus from an entirely striated into a mixed phenotype

“…؉/؊ and Pax7 ؊/؊ Esophagi Electron microscopy revealed that the tunica muscularis of the esophagus was composed of the same cell types recently described in P4 mice in both wild-type and mutants (Wörl and Neuhuber, 2005b). We focused further on differences of esophageal myogenesis in wild-type and mutants and demonstrated the results by representative images from P4 and P25 Pax7 Ϫ/Ϫ esophagi.…”

“…It was originally suggested that striated muscle fibers of the mouse esophagus develop in the late embryonic and early postnatal period by transdifferentiation from smooth muscle fibers (Patapoutian et al, 1995;Kablar et al, 2000;Stratton et al, 2000;Reddy and Kablar, 2004). However, the smooth-to-striated transdifferentiation hypothesis was finally falsified, based on the following evidence: (1) no cells with both striated and smooth muscle type morphological features were detected in the developing esophagus by immunocytochemis-try or electron microscopy (Zhao and Dhoot, 2000;Wörl and Neuhuber, 2005b); (2) fate mapping studies in which smooth muscle cells were lineage-tagged through expression of the lacZ gene demonstrated that esophageal striated myocytes show no evidence of a prior smooth muscle phenotype (Rishniw et al, 2003); (3) the demonstration of numerous apoptotic smooth muscle cells located in a transition zone, where smooth intermingled with developing striated muscle cells (Wörl and Neuhuber, 2005b); and (4) the detection of mesenchymal cells in the smooth muscle portion caudal to the smooth-to-striated transition zone, which appeared to give rise to striated muscle fibers (Wörl and Neuhuber, 2005b). Thus, it has been proposed that distinct differentiation pathways exist for smooth and striated muscle precursors rather than transdifferentiation (Zhao and Dhoot, 2000;Rishniw et al, 2003;Wörl and Neuhuber, 2005b).…”

“…However, the smooth-to-striated transdifferentiation hypothesis was finally falsified, based on the following evidence: (1) no cells with both striated and smooth muscle type morphological features were detected in the developing esophagus by immunocytochemis-try or electron microscopy (Zhao and Dhoot, 2000;Wörl and Neuhuber, 2005b); (2) fate mapping studies in which smooth muscle cells were lineage-tagged through expression of the lacZ gene demonstrated that esophageal striated myocytes show no evidence of a prior smooth muscle phenotype (Rishniw et al, 2003); (3) the demonstration of numerous apoptotic smooth muscle cells located in a transition zone, where smooth intermingled with developing striated muscle cells (Wörl and Neuhuber, 2005b); and (4) the detection of mesenchymal cells in the smooth muscle portion caudal to the smooth-to-striated transition zone, which appeared to give rise to striated muscle fibers (Wörl and Neuhuber, 2005b). Thus, it has been proposed that distinct differentiation pathways exist for smooth and striated muscle precursors rather than transdifferentiation (Zhao and Dhoot, 2000;Rishniw et al, 2003;Wörl and Neuhuber, 2005b). Moreover, numerous satellite cells were seen enclosed in the basal lamina of mature striated esophageal muscle fibers (Samarasinghe, 1972;Faussone-Pellegrini and Cortesini, 1986;Zhao and Dhoot, 2000;Wörl and Neuhuber, 2005b), which raised the question of origin of these quiescent undifferentiated striated muscle precursor cells and the mechanism underlying postnatal growth of esophageal striated muscle fibers.…”

“…Thus, it has been proposed that distinct differentiation pathways exist for smooth and striated muscle precursors rather than transdifferentiation (Zhao and Dhoot, 2000;Rishniw et al, 2003;Wörl and Neuhuber, 2005b). Moreover, numerous satellite cells were seen enclosed in the basal lamina of mature striated esophageal muscle fibers (Samarasinghe, 1972;Faussone-Pellegrini and Cortesini, 1986;Zhao and Dhoot, 2000;Wörl and Neuhuber, 2005b), which raised the question of origin of these quiescent undifferentiated striated muscle precursor cells and the mechanism underlying postnatal growth of esophageal striated muscle fibers. Adult skeletal muscle fibers are terminally differentiated such that muscle growth and regeneration are accomplished by a small population of cells, which were termed satellite cells (Mauro, 1961).…”

“…The aims of the present study were (1) to extend a recent qualitative analysis of esophageal striated myogenesis (Wörl and Neuhuber, 2005b) by quantitative data about development of striated muscle fibers and satellite cells in pre-, postnatal, and adult stages; (2) to examine, whether satellite cells are responsible also for postnatal growth of esophageal striated muscle fibers as recently described for skeletal muscle by using Pax7 mutant mice; and (3) to test, whether deletion of Pax7 can change the muscle phenotypic composition of mouse esophagus. Because replacement of smooth by striated muscle advances craniocaudally, while the stages of striated myogenesis are aligned caudocranially, we intended to investigate entire esophagi of pre-, postnatal, and adult Pax7 ϩ/ϩ , Pax7 ϩ/Ϫ and Pax7 Ϫ/Ϫ mice divided into 1-mm portions by means of electron microscopy.…”