Acetylcholinesterase and polyingression in the epiblast of the primitive streak chick embryo.

Cholinesterase histochemistry and SEM were performed on whole chick blastoderms, stage 4 Hamburger-Hamilton, to study the relationship between acetylcholinesterase (AChE) and cell movement in the epiblast. Correlation of LM photomicrographs of enzyme sites with SEM micrographs of surface topography permitted the determination of the three dimensional morphology of enzyme-positive cells. On the epiblast under surface two cell configurations were observed indicating movement of cells out of the epiblast at sites distant to the streak. In one configuration, cells are flask-shaped with tapered tails. In the other configuration, roughly spherical cells seem to be loosening contact with mounds that bulge from the epiblast. Flask cells and detaching mound cells are AChE-positive. On the upper surface of the hypoblast some cells are spherical and attached to the surface by thin cytoplasmic processes while others are flattened and elongated. The former are intensely stained for AChE and the latter are faintly stained or unstained. Ultrastructural studies indicate the presence of AChE at discontinuous sites in the perikaryon, endoplasmic reticulum and golgi. Reaction product was observed at the aforementioned sites when acetylthiocholine iodide was the substrate but not when butyrylthiocholine iodide was used or substrate was omitted. The results support the use of AChE as an endogenous marker of cell movement and provide morphological information on the process of polyingression.