Membrane differentiation in the pregastrula of the teleost, Brachydanio rerio Hamilton-Buchanan (Teleostei: Cyprinidae). A scanning electron microscope study

The pregastrula stages of the teleost, Brachydanio rerio, have been studied in intact or fractured eggs by scanning electron microscopy. The internal surface of the enveloping blastomeres was smooth, while the external surface was covered with cristae; these cristae disappeared during epiboly but reappeared when the cells were in mitosis. At the end of the morula stage, the smooth yolk cytoplasmic layer formed a ring of periblastic cristae; up to the late blastula stage that ring was attached to "star-like" complexes. Light and transmission electron microscopy images of these complexes are given and we propose hypotheses regarding their function. At first closely apposed, the deep cells seemed to be maximally dispersed during the mid-blastula stage. At the blastula stage, the yolk syncitium formed both enveloping and deep cells. This suggests a primordial role of the yolk syncitium layer in pregastrula differentiation. A new hypothesis concerning the functions of the membrane protrusions is proposed: they might play a role in the postmiotic reassociation of the blastomeres. We suggest that, during early development, the three modes of cell association, i.e. monolayer, mass aggregation, syncitium, might be correlated with the composition of the three media (perivitelline liquid, blastocoelic liquid, yolk) present at that time.