Spermiogenesis

This series of scanning electron micrographs show the process of spermiogenesis. In the left panel, a round spermatid begins to extend long thin projections all around its periphery. Soon the projections become fewer, longer, and congregated at one pole of the cell (second panel). Next, the projections begin to fuse and balloon out into a pseudopod (third panel). At right is a fully mature spermatozoon, with knobby projections called filopodia extending from its pseudopod. The cell body (right half of cell) is pockmarked with pores from the fusion of membranous organelles to the plasma membrane. (Scanning electron micrographs by Greg Nelson and Sam Ward.) See video to see spermiogenesis in action.

Note: In C. elegans, spermiogenesis is the final maturation of sperm, when the cell builds its pseudopod, acquires the ability to crawl, and becomes capable of fertilizing an oocyte. For a hermaphrodite's own sperm, spermiogenesis begins when the spermatids (the immature undifferentiated cells) arrive in the spermatheca, the chamber in the reproductive tract where fertilization will take place. Spermiogenesis of male spermatids is triggered upon mating. Spermiogenesis is a very rapid process; it only takes about 5 minutes to complete. During spermiogenesis, a set of dramatic cellular rearrangements take place, including plasma membrane flow at the site of the newly forming pseudopod, fusion of mysterious organelles, called membranous organelles, to the plasma membrane of the cell body, and the formation of a dynamic cytoskeleton in the pseudopod from the polymerization of a unique protein called the major sperm protein. Remarkably, all of these changes are accomplished without any new gene expression or protein synthesis, since spermatids lack transcription and translation machinery.

Original images published in: Nelson, G.A. and Ward, S. 1980. Vesicle fusion, pseudopod extension and amoeboid motility are induced in nematode spermatids by the ionophore monensin. Cell 19: 457-464. Abstract