UREA PARTHENOGENETICALLY ACTIVATES THE CORTICAL REACTION AND ELONGATION OF MICROVILLI IN EGGS OF THE SEA URCHIN, STRONGYLOCENTROTUS PURPURATUS

¹ Department of Anatomical Sciences, SUNY at Buffalo, Buffalo, NY 14214, and Marine Biological Laboratory, Woods Hole, MA 02543

Isotonic urea is believed to activate sea urchin eggs by triggering event(s) that normally follow cortical granule secretion at fertilization, particularly surface perturbations that result in elongation of microvilli (Mazia et al., 1975). However, Moser (1940) reported that urea triggered the cortical reaction. Transmission electron microscopy showed that unfertilized Strongylocentrotus purpuratus eggs discharge their cortical granules in isotonic urea (containing 1.0 to 0.1 mM CaCl₂ or 25 mM EGTA) to form incipient fertilization envelopes and hyaline layers. These investments quickly disperse in urea. Elongation of microvilli follows cortical granule discharge. Urea-activated eggs can be fertilized after return to sea water and fail to elevate fertilization envelopes but do form hyaline layers. Hyalin must be secreted from a secondary reservoir in these eggs, since the cortical granule store is discharged during the prior urea activation. Cortical granule secretion and elongation of microvilli do not occur in urea plus 10 mMCaCl₂. These eggs form normal fertilization envelopes and hyaline layers when fertilized after return to sea water. Our results show that: (1) urea triggers an early event in sea urchin egg activation that stimulates cortical granule secretion; (2) cortical granule discharge precedes elongation of microvilli in urea-activated eggs as it does during normal fertilization; and (3) reduction or removal of external calcium is required for activation by urea.