Double immunofluorescence staining of serotonin receptors with pan-neuronal marker (pgp) and glial fibrillary acidic protein (GFAP) in developing human spinal cord. Human spinal cord (sc) in the 5th developmental week (a–c), 6th week (d–f), 8th week (g–i), 10th week (j–o): ventricular zone (vz), intermediate zone (iz), marginal zone (mz), floor plate (f), roof plate (r), lumen (l), ventral horns (v), intermediate horns (i) and dorsal horns (d), notochord (n). In the 5th week co-expression of sr1 (a) and pgp (b) (inset in (c)) with DAPI nuclear stain (c) reveals co-expression of sr1/pgp in the intermediate zone neuroblasts (arrow) and in the notochord. Similar co-expression of sr1/pgp (inset in (f)) is observed in the outer part of the intermediate zone in the 6th week (d–f). In the 8th developmental week, sr3 (g) co-localizes with pgp (h) in the ventral horns (motor neurons) (inset in (i)) of the intermediate zone (g–i). Co-localization of sr1/GFAP is observed in the roof plate area (arrow) of the 10th week spinal cord (j–l), while it is absent in the ventral horn motor neurons (m–o). Magnification ×10, scale bar 400 μm (a–i); magnification ×40, scale bar 100 μm (j–o).