Specification of endoderm and mesoderm in the sea urchin

The biology of the germ line in echinoderms

The formation of the germ line in an embryo marks a fresh round of reproductive potential. The developmental stage and location within the embryo where the primordial germ cells (PGCs) form, however, differs markedly among species. In many animals, the germ line is formed by an inherited mechanism, in which molecules made and selectively partitioned within the oocyte drive the early development of cells that acquire this material to a germ-line fate. In contrast, the germ line of other animals is fated by an inductive mechanism that involves signaling between cells that directs this specialized fate. In this review, we explore the mechanisms of germ-line determination in echinoderms, an early-branching sister group to the chordates. One member of the phylum, sea urchins, appears to use an inherited mechanism of germ-line formation, whereas their relatives, the sea stars, appear to use an inductive mechanism. We first integrate the experimental results currently available for germ-line determination in the sea urchin, for which considerable new information is available, and then broaden the investigation to the lesser-known mechanisms in sea stars and other echinoderms. Even with this limited insight, it appears that sea stars, and perhaps the majority of the echinoderm taxon, rely on inductive mechanisms for germ-line fate determination. This enables a strongly contrasted picture for germ-line determination in this phylum, but one for which transitions between different modes of germ-line determination might now be experimentally addressed.

Patterning the sea urchin embryo: gene regulatory networks, signaling pathways, and cellular interactions

We discuss steps in the specification of major tissue territories of the sea urchin embryo that occur between fertilization and hatching blastula stage and the cellular interactions required to coordinate morphogenetic processes that begin after hatching. We review evidence that has led to new ideas about how this embryo is initially patterned: (1) Specification of most of the tissue territories is not direct, but proceeds gradually by progressive subdivision of broad, maternally specified domains that depend on opposing gradients in the ratios of animalizing transcription factors (ATFs) and vegetalizing (beta-catenin) transcription factors; (2) the range of maternal nuclear beta-catenin extends further than previously proposed, that is, into the animal hemisphere, where it programs many cells to adopt early aboral ectoderm characteristics; (3) cells at the extreme animal pole constitute a unique ectoderm region, lacking nuclear beta-catenin; (4) the pluripotential mesendoderm is created by the combined outputs of ATFs and nuclear beta-catenin, which initially overlap in the macromeres, and by an undefined early micromere signal; (5) later micromere signals, which activate Notch and Wnt pathways, subdivide mesendoderm into secondary mesenchyme and endoderm; and (6) oral ectoderm specification requires reprogramming early aboral ectoderm at about the hatching blastula stage. Morphogenetic processes that follow initial fate specification depend critically on continued interactions among cells in different territories. As illustrations, we discuss the regulation of (1) the ectoderm/endoderm boundary, (2) mesenchyme positioning and skeletal growth, (3) ciliated band formation, and (4) several suppressive interactions operating late in embryogenesis to limit the fates of multipotent cells.