Fig. 1. Distribution of 5-HT-immunoreactive (5-HT-ir) neurons and associated processes in the subesophageal (SEG) and supraesophageal (SPEG) ganglia comprising the leech head brain. The brain of an embryonic leech at stage embryonic day 20 (E20) is shown, which was immunostained and imaged in situ (N=6). (A) The neuropil of the SPEG is filled with a dense array of 5-HT-ir fibers, probably originating from within the CNS because no peripheral 5-HT-ir cell bodies were observed. The four neuromeres of the SEG (marked 1–4) contain sets of large 5-HT-ir somata, some of which project toward the SPEG, e.g. the large lateral (LL) cells in neuromere 1. Several 5-HT-ir fibers exit the brain, such as those projecting throughout the stomatogastric nerve ring (large arrow points to one half of nerve ring). The rectangular broken box outlines the image shown at higher magnification in B. (B) Punctate 5-HT-ir fibers were visible in a putative neurohemal region of the SPEG. The terminations of varicose 5-HT-ir fibers appear to be concentrated in the upper region of the image shown, which lies in the approximate location of a neurohemal release site (Webb, 1980). Optical sections/image, 39 (A); 9 (B). Section intervals, 2 µm (A); 1.5 µm (B). Scale bars, 100 µm.