The molecular mechanisms controlling morphogenesis and wiring of the habenula.

The habenula is an evolutionarily conserved brain region comprising bilaterally paired nuclei that plays a key role in processing reward information and mediating aversive responses to negative stimuli. An important aspect underlying habenula function is relaying information between forebrain and mid- and hindbrain areas. This is mediated by its complex organization into multiple subdomains and corresponding complexity in circuit organization. Additionally, in many species habenular nuclei display left-right differences at the anatomical and functional level. In order to ensure proper functional organization of habenular circuitry, sophisticated molecular programs control the morphogenesis and wiring of the habenula during development. Knowledge of how these mechanisms shape the habenula is crucial for obtaining a complete understanding of this brain region and can provide invaluable tools to study habenula evolution and function. In this review we will discuss how these molecular mechanisms pattern the early embryonic nervous system and control the formation of the habenula, how they shape its asymmetric organization, and how these mechanisms ensure proper wiring of the habenular circuit. Finally, we will address unexplored aspects of habenula development and how these may direct future research.