Regulative development of the sea urchin embryo: Signalling cascades and morphogen gradients

Differentiation of sea urchin embryo ectoderm, endoderm and mesenchyme cell s, whose anlagen are arrayed along the animal-vegetal axis, relies on both maternally regulated localized transcription factor activities and cell-cel l signalling. Classic models proposed that fates are determined by opposing animal and vegetal morphogenetic gradients, whereas current models emphasi ze unidirectional and sequential vegetal-to-animal signalling cascades betw een adjacent blastomeres. Recent data support aspects of both models: the v egetal micromeres send one or more signals, which depend on a nuclear beta- catenin-dependent pathway, that both activate Notch signalling required for secondary mesenchyme fate and promote endoderm differentation and gastrula tion. This is opposed by an animalizing domain of BMP4 signals that regulat es ectodermal cell fates and establishes the ectoderm-endoderm border.