About the Cover
Cover
Early in the development of multicellular animals, information that determines the specific fate of cells and their progeny is distributed unequally at each cell division. As cleavage and the specification of embryonic territories proceeds, the location of each blastomere with respect to its neighbors must be fixed, ensuring normal development of the embryo. In mammals, the positions of the blastomeres are fixed by desmosomes. But specification begins early in many invertebrates, and long before desmosomes are formed; thus, other mechanisms are needed to fix the relative position of the blastomeres in these species.
In this issue of The Biological Bulletin, Shin-ichi Nemoto and his colleagues report on a novel mechanism that fixes the relative positions of blastomeres in developing embryos of the starfish Astropecten scoparius (see Matsunaga et al., p. 213). After fertilization and elevation of the fertilization envelope the centrally positioned egg cleaves; and the daughter cells move peripherally, away from each other and toward the envelope, to which they adhere. Moreover, during subsequent cleavages, through the blastula stage, the blastomeres-mothers and daughters-remain firmly anchored in place on the fertilization envelope, as on a scaffold. This critical binding to the envelope is due to specialized microvilli that already exist on the surface of the unfertilized egg. After the fertilization envelope is raised, each microvillus extends and becomes a thin filamentous process that adheres to the inner layer of the envelope. The authors call this structure a "fixing process."
The stabilizing mechanism observed in Astropecten scoparius seems, at first, to be different from that in sea urchins, where the blastomeres are held in position by a hyaline layer lacking in starfish. However, Nemoto and colleagues provide evidence from immuno-electron microscopy that the material constituting the echinoid hyaline layer may be a homolog of the inner layer of the asteroid fertilization envelope-a satisfying, and unifying, conclusion.
Three important components of this story are illustrated on the cover: the starfish (about 15 cm from tip to tip, on average); the 2-cell stage (about 250 &mgr;m in diameter); and of course the fixing processes, which extend (for about 32 &mgr;m) from the surface of the fertilized egg to the inside of the fertilization envelope.
The cover images were provided by Shin-ichi Nemoto and Miwa Tamura, Ochanomizu University, Tokyo, Japan. Cover design by Beth Liles, MBL.
[Table of Contents]
Copyright © 2003 by the Marine Biological Laboratory.
