Vesicle involvement in the egg cortical reaction of the horseshoe crab, Limulus polyphemus L

Ultrastructural observations (TEM) of the cortical reaction in Limulus polyphemus have been difficult to obtain due to the relative impermeability of the transparent egg envelope to standard fixatives. With the application of trialdehyde fixation techniques [Kalt, M. R., and Tandler, B. (1971). J. Ultrastruct. Res. 36, 633–645], the cortical reaction has now been examined and the role of cortical vesicles has been determined. The size of these vesicles in uninseminated eggs is heterogeneous, with small vesicles (0.5 μm) being apposed to the plasmalemma and with large vesicles (4 μm) located in a lower layer of the egg cortex. The contents of the small vesicles are translucent under the electron beam. With the onset of egg activation these vesicles fuse with the overlying plasmalemma. The contents of the large vesicles appear electron dense and exhibit distinctly different morphologies. Shortly after insemination these large vesicles begin to enlarge by fusing together. By 9 min after insemination some enlarged vesicles fuse with the plasmalemma to form pits on the egg surface. The remaining enlarged vesicles continue to fuse with the plasmalemma until approximately 60 min after insemination when few vesicles are remaining.