Landmarks for orienting transverse EM sections

Originally posted on WormAtlas by “Andrew Chisholm”, June 21, 2004

Could you post the simplest landmarks to find the dorsoventral axis in an EM section of a worm?

e.g. in the midbody it is simple–look for the ventral cord. How does one rapdly orient in the head (anterior to pharynx and within pharynx) and tail?


Originally posted on WormAtlas by “hall”, June 22, 2004

In the nose, orientation can be done in two steps: 1) identify the left/right axis by finding the amphids or the amphid nerves. The amphids open at the left and right lips, and the amphid nerves then extend along the left and right sides, containing more processes and several distinctly larger processes than the other mechanosensory nerves. So this gives you left vs right sides. 2) identify dorsal vs. ventral by inspecting the pharynx, which has a triangular shaped lumen in cross-section. One point of the triangle will point to the ventral side, the other two points point to left and right sides. To see these effects, you can look on “Slidable Worm” at sections #2,4,6 and 13. The pharynx lumen will continue to help in locating ventral pole along its full length, throughout the head

In the midbody, the dorsal/ ventral axis is easily recognized by the larger ventral nerve cord, whereas the dorsal cord is quite small. Left and right sides are marked by much larger hypodermal ridges than the dorsal and ventral hyp. See Slidable Worm sections # 273 and 315. This information works in the midbody back to the rectum.

In the tail tip, the last dorsal muscle adopts an extreme dorsal position, while two ventral muscles remain at ventral positions, giving a triangular shape surrounding a large hypodermal ridge in the center, the anal ridge. Left and right sides have very thin hyp ridges (both part of hyp 11) and hold the phasmid nerves. So the nerves will give you the left/right axis, and the dorsal muscle points to the dorsal pole. Slidable Worm shows many examples in the #700s.

David Hall