FIG 7.

Roadblocks along the journey of Plasmodium sporozoites (SPZ) through Anopheles mosquito salivary glands. Schematic diagram depicting a cross-sectional view of a distal lateral (DL) lobe, infected by SPZs that encountered every barrier to salivary gland (SG) entry and exit observed in this study. Apical (SG lumen) and basal (mosquito hemocoel) compartments are noted (blue text). SPZs must interact with (step 1) and traverse (step 2) the basement membrane (gray), enter (step 2) and exit (step 3) the secretory cell cytoplasm (outlined in black) into the secretory cavity, enter the lumen (step 4), and finally enter the salivary duct (step 5) (the asterisk indicates the duct lumen) to proceed out of the mosquito during the next blood meal. Salivary ducts were sometimes (Fig. 1E, panel iii; see also Fig. 4B, panel ii, and D, panel ii) fused shut (step 6), as seen previously in male salivary glands (27, 28). Some lobes developed basement membrane and secretory cell disruptions (sections 7 and 8) that allowed SPZs a direct route (not requiring invasion) either to the lumen (step 7) or to the secretory cell cytoplasm (step 8). Some SPZs were seen in the secretory cell cytoplasm (sections 9 and 10), either as individuals (step 9) or groups (step 10). Rounded and/or fragmented SPZs (step 11) (36–38) were observed at the basement membrane and inside secretory cells. SPZs sometimes traversed secretory cell lateral cytoplasmic extensions en route to a neighboring secretory cavity (step 12). Other SPZs localized to pockets of lumenal saliva (step 13) were sometimes observed between the basement membrane and secretory cells (Fig. 1A, panel iv; see also Fig. 6B, panel ii). SPZs often bundled together when they reached a physical barrier, such as an aberrant WGA-positive chitinous wall (step 14) that sometimes lined the lumen (Fig. 5A). Some bundles cleared the barrier to enter the lumen (step 15), while in other cases, bundles represented a barrier that individual sporozoites navigated beyond (step 16). Some bundled SPZs traversed the basement membrane (step 17) (Fig. 6E, panel iv), exiting the SG and presumably entering the hemocoel. Certain SPZ groups, found primarily within secretory cavities, were captured by confocal imaging in a circling/swirling pattern (step 18) (Fig. 6B, panel vi; see also Movies S2 and S3).