The multiple functions of hindbrain boundary cells: Tinkering boundaries?

Abstract

Embryonic boundaries were first described in Drosophila, and then in vertebrate embryos, as cellular interfaces between compartments. They display signaling properties and in vertebrates might allocate cells fated to different anatomical structures, or cells that will play different functions over time. One of the vertebrate embryonic structures with boundaries is the hindbrain, the posterior brain vesicle, which is transitory segmented upon morphogenesis. The hindbrain is formed by iterative units called rhombomeres that constitute units of gene expression and cell-lineage compartments. Rhombomeric cells are segregated by interhombomeric boundaries, which are prefigured by sharp gene expression borders. Hindbrain boundaries were first described as static groups of cells. However, later discoveries demonstrated the dynamic behavior of this specific cell population. They play distinct functional properties during brain morphogenesis that partially overlap on time, starting as a mechanical barrier to prevent cell intermingling, becoming a signaling hub, to finally constitute a group of proliferating progenitors providing differentiated neurons to the system. In this review, I try to give a more functional overview of this segmentation process and in particular of hindbrain boundaries. I will discuss the new challenges in the field on how to integrate cell fate specification and morphogenesis during brain embryonic development.

The author thanks the current members of her lab for insights and critical discussions, and Adrià Voltes -a former member of the lab- for his illustrations. This work was funded by the Spanish Ministry of Science, Innovation and Universities AEI-PGC2018-095663-B-I00 and AEI-RED2018-102553-Tgrants (MICIU-FEDER) to CP. DCEXS-UPF is a Unidad de Excelencia María de Maeztu funded by the AEI (CEX2018-000792-M). CP is a recipient of ICREA Academia award (Generalitat de Catalunya).