Does fertilization in the filarial nematode Dirofilaria immitis occur through endocytosis of spermatozoa?
Information on the ultrastructural details of fertilization in filarial nematodes are still unavailable. Here we report new data on this process in Dirofilaria immits, the heartworm of dogs and cats. Electron microscopy allowed us to observe oocytes engulfing spermatozoa through an endocytosis-like process. We also observed spermatozoa inside the oocytes which still possessed their plasma membrane and which were clearly enveloped by a further membrane, likely derived from the endocytosis process. At this stage, at the interface between the sperm membrane and the endocytotic membrane (vacuolar space), we observed flocculent material in the proximity of the membranous organelles (MOs) of the sperm. In the proximity of the MOs, we also observed the enlargement of the vacuolar space. Other images showed the dissolution of the sperm membrane, and the release of nuclear masses and organelles in the egg cytoplasm. We did not observe the fusion of lysosomes to the endocytotic vacuoles. In addi
