REGULATED EXOCYTOSIS AND SEQUENTIAL CONSTRUCTION OF THE EXTRACELLULAR-MATRIX SURROUNDING THE SEA-URCHIN ZYGOTE

After fertilization most eggs become surrounded by a complex extracell ular matrix. This study examines those matrix assembly processes that are triggered by fertilization of the sea urchin egg. The study uses a ntibodies that identify five different storage compartments in the egg . These compartments release their protein contents in a highly regula ted fashion to assemble and modify the extraembryonic layers. The exoc ytosis sequence begins with a fertilization wave that progresses from the site of sperm entry and elevates the fertilization envelope above a water-filled perivitelline space. The immediate surface of the zygot e then becomes covered by a newly secreted hyaline layer. Prior to fer tilization some of the antigens are localized to cortical granules. Ot hers are found in ''basal laminar vesicles'' that are released in a wa ve beginning at about 30 sec, or roughly at the same time as cortical granule exocytosis. The remaining antigens are exocytosed with a rathe r precise timing, but with a delay of several to tens of minutes relat ive to the first wave of exocytosis. ''Apical vesicles,'' so named bec ause antigens from this class are preferentially exocytosed toward the apical cell surface of polarized cells, include antigens that are exo cytosed beginning at about 5 min postfertilization. The fourth compart ment, named ''echinonectin vesicles'' release echinonectin, a protein that is deposited to the inner side of the hyaline layer. Surface stai ning of echinonectin is first detected about 10-15 min following sperm contact. Finally, maternal cadherin, which is stored in yet a fifth d istinct compartment, is not detected on the surface until at least 30 min following fertilization. The data are also consistent with the not ion that the tightly regulated timing of exocytosis contributes to the ordered assembly of the hyaline layer and elevation of the fertilizat ion envelope. Finally, two of the vesicle classes continue to exocytos e after the cells become polarized. In polarized cells apical and basa l laminar antigens are trafficked toward opposite sides of the same ce ll after passing through the same trans-Golgi network-like compartment . (C) 1997 Academic Press.