Egg exudate-induced reduction of sperm lysin sensitivity in the vitelline coat after fertilization of Bufo japonicus and its participation in polyspermy block

Behaviour of the vitelline envelope in Bufo arenarum oocytes matured in vitro in blockade to polyspermy

During activation of amphibian eggs, cortical granule exocytosis causes elaborate ultrastructural changes in the vitelline envelope. These changes involve modifications in the structure of the vitelline envelope and formation of a fertilization envelope (FE) that can no longer be penetrated by sperm. In Bufo arenarum, as the egg traverses the oviduct, the vitelline envelope is altered by a trypsin-like protease secreted by the oviduct, which induces an increased susceptibility of the vitelline envelope to sperm lysins. Full-grown oocytes of B. arenarum, matured in vitro by progesterone, are polyspermic, although cortical granule exocytosis seems to occur within a normal chronological sequence. These oocytes can be fertilized with or without trypsin treatment, suggesting that the vitelline envelope is totally sperm-permeable. Vitelline envelopes without trypsin treatment cannot retain either gp90 or gp96. This suggests that these glycoproteins are involved in the block to polyspermy and that trypsin treatment of matured in vitro oocytes before insemination is necessary to enable vitelline envelopes to block polyspermy. The loss of the binding capacity in vitelline envelopes isolated from B. arenarum oocytes matured in vitro with trypsin treatment and activated by electric shock suggests that previous trypsin treatment is a necessary step for sperm block to occur. When in vitro matured oocytes were incubated with the product of cortical granules obtained from in vitro matured oocytes (vCGP), vitelline envelopes with trypsin treatment were able to block sperm entry. These oocytes exhibited the characteristic signs of activation. These results support the idea that B. arenarum oocytes can be activated by external stimuli and suggest the presence of unknown oocyte surface receptors linked to the activation machinery in response to fertilization. Electrophoretic profiles obtained by SDS-PAGE of solubilized vitelline envelopes from oocytes matured in vitro revealed the conversion of gp40 (in vitro matured oocytes, without trypsin treatment) to gp38 (ascribable to trypsin activity or cortical granule product activity, CGP) and the conversion of gp70 to gp68 (ascribable to trypsin activity plus CGP activity). Taking into account that only the vitelline envelopes of in vitro matured oocytes with trypsin treatment and activated can block sperm entry, we may suggest that the conversion of gp70 to gp68 is related to the changes associated with sperm binding.

Structural and functional aspects of Bufo americanus spermatozoa: effects of inactivation and reactivation

Very little is known about the effects of manipulating toad sperm activity in vitro, and such information is important in the development of a genetic resource bank for bufonid species. The specific objectives of this study were to: 1). identify the optimal inactivation and reactivation solutions for toad spermatozoa collected in urine; 2). establish the length of time toad spermatozoa can be exposed to an inactivation buffer and still resume motility upon reactivation; 3). evaluate the consequence of inactivation on specific sperm characteristics; and 4). characterize the sperm mitochondria vesicle (MV) and its relationship to motility. Reactivated sperm motility was similar after inactivation in either Simplified Amphibian Ringers (SAR) solution or DeBoer's (DB) solution. Diluting the buffer by 80% with water provided the best method for reactivating sperm. Dilutions with NaCl solutions (10-50 mM) produced inferior results. SAR-inactivated spermatozoa could remain suspended up to 4 hr and still regain 25% of initial motility upon reactivation in water. Compared to the controls, sperm motility was greater (P<0.01) over time for samples treated with SAR, although forward progression was significantly lower. Furthermore, SAR treatment resulted in sperm samples with a greater number of viable, morphologically normal, and intact MVs over time. Electron microscopy and fluorescent staining confirmed that the toad sperm's MV contains a large number of active mitochondria with very few other cytoplasmic structures. Nearly all spermatozoa exhibiting motility had an intact MV, and dissociation of this structure was clearly related to motility loss. In conclusion, toad spermatozoa can be effectively inactivated and reactivated by varying the osmolality of the external solutions and, although sperm forward progression is reduced, all other characteristics are well maintained. Moreover, the increased number of spermatozoa with intact MV after inactivation suggests the process may help preserve this important structure.