Abstract
The distal colon of the developing rat embryo was studied by electron microscopy on specimens taken at each gestational day from 11 to 21. The circular muscle and Auerbach's plexus were identified in the mesenchyme on day 15. Prior to this, nerve cells were not easily distinguishable in the colonic wall. However, many clear cells in the subserosa were found to have abundant microtubules and filaments in the cytoplasm and also in their processes. These cells were presumed to be a primitive form representing undifferentiated migratory neuroblasts.
To examine whether neuroblasts were present in the distal colon before day 15, colons were dissected from day 14 embryos and cultured in vitro for 1 to 3 days. Circular muscle and Auerbach's plexus differentiated after 3 days in culture. These findings support the view that undifferentiated neuroblasts have already migrated into the colonic mesenchyme by day 14, although their morphology is equivocal.
