Fig. 5. Vesicles contain MSP and form by budding. (A) Detection of MSP by immunoEM. Low-power view of two spermatozoa in the spermathecal-uterine junction region of an adult hermaphrodite. Intense labeling in pseudopods (ps). (B) Detection of MSP at the plasma membrane of the spermatozoa cell body. (Left) MSP labeling excluded from cellular organelles, including mitochondria (m), membranous organelles (mo), and the nucleus (n). Inset is a magnified view showing MSP associated with the plasma membrane. (Middle) MSP associated with the plasma membrane and a protrusion (arrow), magnified in the inset. Note free labeling in the extracellular space (bracket). (Right) MSP is not detected in distal germ cells. (C) MSP is contained within the vesicles; a gallery of seven vesicles (the lower right two panels are views of the same vesicle in non-adjacent sections) is shown. MSP is located in the annulus between the inner and outer membranes. (D) Vesicle budding from spermatids. Shown are non-adjacent sections of two different spermatids in the hermaphrodite gonad. Views 1 and 1', and 2 and 2', are corresponding pairs of non-adjacent sections. The budding vesicles contain MSP in both views. Vesicles connect to the cell body by a stalk (thin arrows), and the plasma membrane at the budding site appears to be intact (thick arrow). MSP is enriched in a cross-sectional view at the base of the budding projection (arrowheads in lower panels). (E) Section of epon-embedded material showing a lipid whorl deposit in an extracellular space of the spermatheca between a portion of two spermatozoa. (F) MSP associated with a lipid whorl structure in the extracellular space of the spermatheca. Scale bar: in A, 500 nm; in B, 500 nm (inset, 125 nm); in C,D, 100 nm; in E, 500 nm; in F, 100 nm.