HOME :: CHAPTER 7  :: 7.1 SEA URCHIN CELL SPECIFICATION :: TRANSCRIPTION FACTOR ACTIVATION IN THE SEA URCHIN EMBRYO

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Transcription Factor Activation in the Sea Urchin Embryo

It should be noted that the biochemical nature of the vegetalizing signal(s) has not yet been elucidated. Hörstadius interpreted these experiments in terms of gradients of animalizing and vegetalizing substances. More recently, Davidson (1989) sought to explain these phenomena in terms of transcriptional regulatory proteins (such as those that bind to promoters or enhancers) that are localized throughout the egg and that become activated by inductive interactions between adjacent cells. One of these factors, the one in the micromere precursors of the primary mesenchyme cells, is thought to become active autonomously, early in cleavage. This factor would cause the determination of the micromeres, alter their cell membranes so they could react with the cells above them, and initiate the cascade of determination. By the end of blastulation, there would be five territories that would constitute both the precursor cells for certain regions of the pluteus larva and the regions of specific gene activation. For example, the cytoskeletal actin gene, which is expressed only in the aboral ectoderm cells, cannot be activated anywhere else in the embryo. Similarly, if one injects the gene for the skeletal matrix protein anywhere else in the embryo except the region of skeletogenic mesenchyme, it will not be activated (Hough-Evans et al., 1987; Sucov et al., 1988). Therefore, each territory would constitute a region of differential gene expression that would be reflected in different parts of the organism being formed.

However, if the embryo is perturbed, the neighbors would differ. New fates would ensue as the new vegetal cells activated their skeletogenic mesenchyme determinants and started activation of their neighbors. To see if this was indeed the case, Ransick and Davidson (1993) transplanted micromeres into the animal cap of an 8-cell sea urchin embryo. These skeletogenic mesenchyme precursor cells induced the formation of the archenteron and the expression of the archenteron-specific genes. The fluorescently labeled micromeres did not contribute structurally to the secondary archenteron, but provided a short-range inductive signal that respecified the animal cap cells.

Literature Cited

Davidson, E. H. 1989. Lineage-specific gene expression and the regulative capacities of the sea urchin embryo: a proposed mechanism. Development 105: 421-446.

Hough-Evans, B. R., Franks, R. R., Cameron, R. A., Britten, R. J. and Davidson, E. H. 1987. Correct cell type-specific expression of a fusion gene injected into sea urchin eggs. Dev. Biol. 121: 576-579.

Ransick, A. and Davidson, E. H. 1993. A complete second gut induced by transplanted micromeres in the sea urchin embryo. Science 259: 1134-1138.

Sucov, H. M., Hough-Evans, B. R., Franks, R. R., Britten, R. J. and Davidson, E. H. 1988. A regulatory domain that directs lineage-specific expression of a skeletal matrix protein in the sea urchin embryo. Genes Dev. 2: 1238-1250.

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