Fine Structure of the Envelope and Micropyles in the Eggs of the White Sturgeon, Acipenser transmontanus Richardson

The structure of the egg envelope and micropyles of the white sturgeon was examined with light and electron microscopy. The mature ovum is 3.5–4.0 mm in diameter and is covered by a thick envelope (50 μm) that consists of four distinct layers. The outermost layer, L4, is electron dense and amorphous. Interior to this is layer three (L3), containing numerous pores, or ductules. Layer two (L2) contains screwlike projections that anchor L3 and L4 to the egg. The innermost layer, L1, is closely apposed to the oolemma. Numerous micropyles (average 7) are restricted to a 100–200 μm region at the animal pole and penetrate the egg envelope. The outer opening of these rnicropyles measures 15 μm in diameter. The micropylar canal tapers twice, eventually terminating at the oolemma with an inner opening diameter of 1.2 μm. The micropyles of the white sturgeon egg appear more complex than micropyles in other fish eggs.