A detailed developmental study of eggshell architecture of the fish Salmo gairdneri (rainbow trout) was performed using transmission and scanning electron microscopy. Thioglycollic acid treatment and freeze-fracturing reveal that fibrils ca. 5-10 nm in diameter constitute each lamella of the helicoidal eggshell. Freeze-fracturing also permits a direct visualization of the helicoidal architecture. Laser-Raman studies of the eggshell indicate abundant antiparallel beta-pleated-sheet conformation in the eggshell proteins of S. gairdneri during all developmental stages. Apparently, this conformation dictates formation of the helicoidal structure. Disulfide bonds, together with isopeptide bonds, crosslink S. gairdneri eggshell proteins throughout development. (C) 1996 Academic Press, Inc.