specialized Malpighian tubule cell Malpighian tubule Type I cell Malpighian tubule principal cell Morphologically distinct cell type found throughout the Malpighian tubules. It is a large epithelial cell joined by septate junctions to other Type I cells, with apical microvilli and infolding of the basal membrane. There are small differences between Type I cells in different tubule segments: in the initial segment, the cells have few microvilli and are relatively narrow compared with the larger, flatter cells of the main segment. Type I cells originate from division of the ectodermal Malpighian tubules primordial cells that are found in the junction between the hindgut and midgut. Type I cell proliferation is complete by late stage 13 of embryogenesis. In the mature tubules there are on average 484 Type I cells. These cells are involved in primary urine production via the secretion of K+ into the lumen by a vacuolar-type H+ ATPase transporter and K+/H+ exchanger (Davies et al., 1996; Linton and O'Donnell, 1999). PC FBbt:00005796 fly_anatomy.ontology