Ticks of the genus Ixodes are the most importanttransmitters of pathogens of tick-borne encephalitis andborreliosis (Lyme disease). However, specific featuresof circulation of these or other pathogens in tick organ-ism are obscure. The midgut is the main habitat ofmany pathogens in the body of ticks of the genus Ixodes. To understand the mechanisms of tick infesta-tion with the pathogen and further pathogen transmis-sion, specific changes in the gut lumen and midgut cellcomposition of feeding tick specimens should be iden-tified. In this work, we studied the morphological andfunctional changes in the gut epithelium and gut con-tent throughout the period of tick feeding and after thesated tick had dropped off.The middle segment of the gut (midgut) in ticks ofthe genus Ixodes consists of a short tubular central part(stomach) and seven pairs of tubular diverticula locatedabove the other internal organs. In spite of the complexmorphological subdivision, all parts of midgut havesimilar histological structure. The walls of the gut arecomposed of simple epithelium. The epithelial cellsrest on a thin basement membrane, which is overlainwith a network of muscle fibers from the outside. Gutepithelium is represented by cells of three types:reserve, gastric, and connective [1, 2]. The structure ofthe gut cells and specific features of intracellular andcavity digestion were primarily studied in species of thesubfamily Amblyomminae ( Hyalomma asiaticum [1], Boophilus microplus [3–5], Dermacentor variabilis [6]), and, to a lesser extent, in Ixodidae [7].Female ticks Ixodes pacificus, I. pavlovsky, I. per-sulcatus, I. ricinus , and I. scapularis used in this workwere obtained as laboratory cultures from the Labora-tory of Parasitology, Zoological Institute, RussianAcademy of Sciences. Both hungry and feeding femaleticks were studied histologically. Feeding ticks werecollected 9–12 h after suction and then every daythroughout the period of feeding. Female ticks werealso studied 3, 5, 10, 15, 20, and 33 days after they haddropped off. Ticks were dissected in a phosphate buffersolution (pH 7.4). The tick’s stomach and pairs of diver-ticula with a common base (1 + 2 +3, 4 + 5, 6 + 7 pairs)were fixed in 9% formalin and ethanol–formalin, andthen studied separately. The fixed material was embed-ded in paraffin through methylbenzoate–celloidin andcut into 5- µ m-thick sections. The resulting sectionswere stained with azure–eosine, Heidenhein’s azan,and hematoxylin–eosine. The presence of divalent ironin the preparations was detected by turnbul blue forma-tion.Deep cyclic changes in the composition of the gutepithelium were observed in female ticks of the sub-family Ixodidae during 7- to 10-day feeding simulta-neously with food absorption. The gut epithelium ofhungry specimens was represented by the gastric cellsof the preceding (nymphal) stage of development andreserve cells (Fig. 1a). Gastric cells of 2.5- to 3-month-old female ticks before feeding were cubic, and thecytoplasm of the cells was loaded with hematin. Gastriccells began to grow 12–18 h after suction (Fig. 1b).Meanwhile, homogenic weakly eosinophilic substancebegan to fill the gut lumen of the female tick. Individualgranulocytes from the inflammation focus of the hostorganism were also found at that time in the tick’s gutcontents. The gastric cells assumed a columnar shape24–36 h after suction. Vacuoles emerged near the apicalsurface of the cells, and both the size and the count ofthe vacuoles gradually increased. The amount of theinflammatory cell infiltrate in the gut lumen also rose,and individual erythrocytes appeared in the gut lumenat that time.The majority of gastric cells began to tear away intothe gut lumen about 1.5–2 days after suction. By thattime, the cells assumed a claviform shape, the cellnucleus migrated to the dilated apical part of the cell,and hematin crystals and gastric vacuoles appeared inthe cytoplasm (Fig. 1c). In the stomach, the gastric vac-uoles were functionally active for 2 to 2.5 days aftersuction, whereas in the diverticula, they were retainedfor as long as three days.New cells, which were differentiated into secretorycells (Fig. 1c), appeared at the sites of location ofreserve cells 24–36 h after the tick attachment. These cellsbecame dominant in the layer of gut cells on day 3. Thesecells grew rapidly, assumed a claviform shape, numer-ous vacuoles appeared in the cytoplasm of the cells, andthe cytoplasm of the cells did not contain hematin.