early 17 ISN, focal plane 1

This image begins a series of high-power (63X and 40X) views of the motor axons. Anterior is to the left and dorsal is up in all cases. The ISN develops quite early, and reaches the terminus region near muscle 1 in stage 16. Late 16 embryos can thus be used for scoring most ISN guidance events. The ISN is pioneered by aCC, which innervates muscle 1, and also contains the axons of the RP2 and U neurons. In this focal plane we see muscles 1, 2, 3, and 4. The ISN terminus is slightly out of focus. The ISN has crossed the main tracheal trunk, which has been dissected away (see 1D4/twist timeline and distal ISN detail for the position of this landmark). The ISN has several characteristic side branches. One of these, the first branch (FB) is at the junction of muscles 3 and 11. The branch is always positioned on top of a FasII- and twist-expressing cell, PT3 (PT=persistent twist). PT3 does not define the position where this branch is actually made, but rather is contacted by the branch after it leaves the ISN. Later, PT3 migrates to a position at the branchpoint (see 1D4/twist series). FasII expression in PT3 is weak by stage early 17. A second PT cell, PT2, has a variable position at this stage, and is often located along the ISN between FB and the second branch position (SB), as in this image. The third PT cell along the distal ISN, PT1, is located at the ISN terminus. PT1 expression of FasII is not clearly visible in this image (see 1D4/twist series for PT1 position), but in the right-hand segment PT1 is probably the upside-down teardrop shaped cell immediately under the terminal growth cone. SB is normally at the junction of muscles 2 and 10. Another branch innervating muscle 19 is visible in these images. The ISN also has a side branch onto muscle 4, not visible here.

To early 17 ISN focal plane 2

To late 16 ISN

To early 17 brightfield

To twist/ID4 series stages 14-15.

To late 16 distal ISN detail.

To diagram of muscles.

To 2nd diagram (muscles and motor axons).

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