STUDIES IN THE DYNAMICS OF HISTOGENESIS
Authors: Carey Eben J. 
Published: May 2004

Esophageal Development.
1. The region of most active mitosis per mm. of cross-section in the esophagus is the entodermal epithelial tube. The mitotic figures follow a spiral path in the manner of a left-handed helix from the cephalic to the caudal direction.
2. The region of least active growth per mm. of cross-section in the esophagus is the mesenchyme surrounding the epithelial tube.
3. The helicoidal activity of the epithelial tube causes a vortical reaction in the surrounding mesenchyme. The mesenchymal whirlpool represents a reaction to the spirally grooving epithelial tube.
4. In embryos 9.5 to 14 mm. in length the esophageal epithelial tube grows relatively more rapidly in width than in length. During this period the myoblasts which form the inner, close spiral, muscle coat of the esophagus are becoming rapidly differentiated in the outer condensed margin of the mesenchymal maelstrom.
5. The nuclei, first spherical then oval, and finally rod shaped with rounded ends, are drawn out in the direction of the circumference of the mesenchymal rim which is directed tangentially.
6. The cytoplasm is also drawn out in the direction of the mesenchymal rim of the vortex. The elongated rows of isolated granules appear which subsequently, by confluence, form the myofibrillæ. These cytoplasmic derivatives are elongated in the direction of the circumference of the vortex.
7. Between the epithelial tube and the myoblastic rim at the periphery of the mesenchymal whorl is found the embryonic connective tissue. From this direct observation the conclusion is made that an optimum tensional stress stimulus is necessary to elicit the formation of muscular tissue at the circumference of the mesenchymal vortex. Consequently, the formation of a specific derivative from a pluripotent mesenchymal cell is due to the fortuitous circumstance of position.
8. In embryos from 14 to 24 mm. in length, the esophagus grows relatively more rapidly in length than in width. This elongation is due to two factors; first, the descent of the stomach, and, second, the resistance to diametrical growth presented by the inner close spiral musculature. The epithelial tube, still the dominant zone of mitotic activity, pursues the lines of least resistance, and consequently growth in length takes place. This is due to the shifting of the planes of cell division on account of the compression of the inner, close spiral, muscle coat.
9. The undifferentiated mesenchyme peripherad to the inner, close spiral musculature is elongated and the histogenetic changes in muscular formation are gradually taking place between 14 and 24 mm. A very attenuated, outer, elongated, spiral, or longitudinal muscle coat is detected in the esophagus of a 24 mm. pig embryo.
10. The characteristic intestino-colic flexure is a torsional reaction of the mesenchyme. The mesenchymal cells are thrown into a left-handed helicoidal series, corresponding to the activity in the epithelial tube. The right-handed helicoidal reaction of the mesenchyme, therefore, is due to the left-handed helicoidal growth of the epithelial tube.
11. The normal asymmetry of the abdominal viscera as well as the position of the gut is dependent upon the clockwise reaction of the stretched mesenchymal cell. These cells are stretched by the left-handed helicoidal growth of the epithelial tube. One factor producing situs inversus viscerum could be the reversal of the spiral growth of the epithelial tube resulting in a reaction of the mesenchyme in a direction opposite, namely counterclockwise, to that which occurs normally.