Both smooth and skeletal muscle precursors are present in foetal mouse oesophagus and they follow different differentiation pathways.

Muscularis externa of mouse oesophagus is composed of two skeletal muscle layers in the adult. Unlike rest of skeletal muscle in the body, the oesophageal skeletal muscle in the mouse has been proposed to be derived from fully differentiated smooth muscle cells by transdifferentiation during later foetal and early postnatal development (Patapoutian et al. 1995] Science 270:1818-1821). Here we characterised the nature of cells in muscularis externa of the mouse oesophagus by ultrastructural and immunoctyochemical analyses. The presence of differentiated skeletal muscle cells identified by positive staining for skeletal muscle specific myosin heavy chain became first apparent in the outer layer of cranial oesophagus at 14 days gestation. The transient expression of smooth muscle type alpha-actin in mouse oesophageal muscle was also apparent during foetal development. This isoform, however, was not smooth muscle specific during early development as it was also detected in foetal skeletal muscles. Compared with oesophagus, the suppression of this smooth muscle type alpha-actin during foetal development was faster in non-oesophageal skeletal muscle cells. The development of skeletal muscle in oesophagus showed a cranial to caudal and an outer layer to inner layer progression. During early foetal development, mouse oesophagus is composed of undifferentiated mesenchymal cells that formed cell clusters. Two types of cells with different staining densities could be distinguished within these cell clusters by electron microscopy. The centrally located pale staining cells gave rise to skeletal muscle cells while the peripherally positioned dense staining cells gave rise to smooth muscle cells, indicating the existence of both skeletal and smooth muscle cell precursors in mouse oesophagus during early foetal development. Further development showed an increase in the proportion of skeletal muscle cells and a decrease in size and number of the smooth muscle type cells. Apart from decrease in cell size, some other morphological features of smooth muscle cell degeneration were also observed during later foetal and early neonatal development. No smooth muscle cells undergoing transdifferentiation were observed. Both immunochemical and ultrastructural observations, thus, demonstrated the presence of skeletal muscle cells in early foetal oesophagus. It is concluded that the transient appearance of smooth muscle cells may provide a scaffold for the laying down of skeletal muscle layers in mouse oesophagus, the final disappearance of which may be triggered by lack of smooth muscle innervation.