A gene regulatory network for apical organ neurogenesis and its spatial control in sea star embryos.

How neural stem cells generate the correct number and type of differentiated neurons in appropriate places is an important question in developmental biology. Although nervous systems are diverse across phyla, many taxa have a larva that forms an anterior concentration of neurons, or apical organ. The number of neurons in these organs is highly variable. We show that neurogenesis in the sea star larvae begins with soxc-expressing multipotent progenitors. These give rise to restricted progenitors that express lhx2/9. Soxc- and lhx2/9-expressing cells are capable of undergoing both asymmetric divisions, which allow for progression towards a particular neural fate, and symmetric proliferative divisions. Nested concentric domains of gene expression along the anterior-posterior (AP) axis, which have been observed in a great diversity of metazoans, control neurogenesis in the sea star by promoting particular division modes and progression towards becoming a neuron. This work, therefore, explains how spatial patterning in the ectoderm controls progression of neurogenesis. Modification to the sizes of these AP territories provides a simple mechanism to explain the diversity of neuron number found among apical organs.