Fetal development of membrane water channel proteins aquaporin‐1 and aquaporin‐4 in the human brain

Aquaporin-1 and aquaporin-4, water channel membrane proteins reported in both experimental animals and in adult humans, have been detected in different, non-overlapping areas of the central nervous system. This immunohistochemical study describes the developmental expression pattern of the water channel membrane proteins, aquaporin-1 and aquaporin-4, in various structures of human fetal brain over the gestational period of 14-40 weeks. Aquaporin-1 immunostaining was exclusively found in the epithelial cells of the choroid plexus from the 14th gestational week, and the staining pattern altered slightly over time. At week 14, immunostaining appeared only in the apical cell membranes. By the 18th gestational week, the entire plasma membrane of these apical cells was immunopositive, as well as was the cytosol. These changes in immunoreactivity indicate an increasing production of aquaporin-1 in the epithelial cells during the period between the 14th and 24th weeks of gestation. Aquaporin-4 immunostaining was first detected in the archicortex, from gestational week 14 and was detected in the neocortex, 6-7 weeks later. Immunostained structures were always astrocytes, particularly the astrocytic endfeet in the ventricular wall, at the developing ependymal lining, at the pial surface, and around the capillaries. Neuronal labeling was not observed. These results in human fetal brain lend morphological support to the previous findings that aquaporin-1 and aquaporin-4 play different roles in the regulation of the water homeostasis of the brain.