Teleost yolk cell function on blastoderm differentiation and morphogenesis

Next to a prominent nutritional role during larval growth the teleost yolk cell may function during early developmental processes as a source of induc ing signals. In Xenopus mesoderm-inducing signals reside in the vegetal cel ls of the blastula and exert their function on the more animal located cell s in the margin. Because of its position reminiscent of the vegetal cells i n Xenopus, the yolk cells of teleost fishes may have a similar inductive ca pacity. Removal of the blastoderm from the yolk cell (YC) offers an experim ental approach to study to what extent blastoderm differentiation and morph ogenesis depend on inductive interaction with the YC. Expression of marker genes for dorsal-ventral and anterior/posterior determination such as no ta il, goosecoid and caudal was examined in blastoderm explants, which were in cubated until sibling controls reached the tail bud stage. Blastoderms removed at 3 h post fertilisation expressed rudimentary dorsal- ventral polarity. Apparently signals leading to this expression had reached the margin of 3-h blastoderms. Blastoderms removed at 4 h showed in additi on to dorsal-ventral polarity notochord-like structures and also expression patterns of goosecoid and caudal suggesting that gastrulation movements ha d occurred.