Morphology of the developing muscularis externa in the mouse esophagus

Muscularis externa of mouse esophagus is composed of two skeletal muscle layers in the adult. But less attention is paid to the histogenesis of the muscularis externa of the esophagus, and controversies still exist about the developmental process and the spatio-temporal expression characteristics of muscle-specific proteins during the development of esophageal muscularis externa. To further probe into the developmental pattern of muscularis externa of the mouse esophagus and the expression characteristics of different muscle-specific proteins, immunohistochemical and terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP)-digoxigenin nick-end labeling apoptotic staining methods are used to investigate the expression patterns of different muscle-specific proteins and to elucidate the relationship of these protein expressions with the development of muscularis externa of the mouse esophagus. Thus, an understanding of the developing esophageal muscularis externa may be important for developing therapeutic strategies for the treatment of human esophagus diseases. Serial sections of mouse embryos from embryonic day (ED) 12 to ED18, and full-length esophagi from postnatal first to 5th day were stained with monoclonal antibodies against α-smooth muscle actin (α-SMA), α-sarcomerical actin (α-SCA), desmin, and monoclonal anti-skeletal myosin (MHC), while apoptosis was determined using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick-end labeling assay. The expression of α-SMA was started at ED12. During the development of ED14-ED15, α-SMA positive cells were seen extending from the walls of left three, four, and six arch arteries toward the dorsal wall of esophagus. Stronger expression of α-SCA and desmin could be detected at ED14 and ED15, expression intensity in caudal segment and inner layer was stained stronger than that of cranial segment and outer layer, but after ED16, strong expression of α-SCA and desmin was found in the outer layer of muscularis externa. Expression of MHC was first detected in the outer layer of cranial segment of muscularis externa at ED17. At ED18, MHC had extended to the level of thyroid gland, staining intensity in the outer layer and cranial segment was stronger than that of inner layer and caudal segment. One to five days after birth, the thickness of the esophageal muscle layer was obviously increased. Most of the muscle cells in the cranial segment of esophagus showed strong expression of α-SCA and clear cross striations at higher magnification. With progression toward the caudal segment, expression intensity of α-SCA became weaker, but the expression intensity of desmin was the same at different levels of esophagus. The muscle fibers were arranged densely with high expression of MHC in the cranial segment. During the development of esophageal muscularis externa, few apoptotic cells were observed. α-SMA, α-SCA, desmin, and MHC show different expression patterns. The differentiation of outer layer of esophageal muscularis externa is quicker than that of inner layer, and the caudal segment is quicker than that of the cranial segment. Besides, apoptosis may not participate in the development of esophageal muscularis externa. The smooth muscle cells from arch arteries may participate in the development of esophageal muscularis externa.