Endocytic origin for periaxonemal vesicles along the flagellum during mouse spermiogenesis.

In mouse spermatogenesis, formation of the flagellum is associated with the presence of numerous periaxonemal vesicles. These are present in the cytoplasmic portion, limited by the deep invagination of the plasma membrane surrounding the axoneme; the number and size of these vesicles varies during spermiogenesis. The vesicles appear at step 10 in young spermatids and increase in number and size until step 14; they then rapidly decrease and disappear at step 16. Cationic ferritin (CF), an endocytic marker, directly injected in the lumen of the seminiferous tubules, labels periaxonemal vesicles, 1 hour after the injection, showing their endocytic origin. Some vesicles are membrane invaginations, still in continuity with the extracellular space, whereas others probably come from a phagocytic mechanism. The CF also shows that some vesicles flow along the axoneme and they accumulate in small cytoplasmic extensions before disappearing. All these complex endocytic phenomena go on to form certain components of the flagellum.