Early differentiation in zebra fish blastula: Role of yolk syncytial layer

The blastomeres of the zebra fish embryo can be classified into two types-cells stained densely (D) or lightly (L) with a mixture of toluidine and methylene (T-M) blue. The dense staining of D cells is largely due to the high density of mitochondria, rough endoplasmic reticulum and polyribosomes. The presence of partially dense stained cells during early blastula stage shows that L cells are transformed into D cells. That the yolk syncytial layer (YSL) plays some role in this transformation is suggested by the proximity of these cells to the YSL and by their distinct spatial orientation with densely stained cytoplasmic regions always facing towards the interior of the embryo.