We investigated how retinoic acid, a vital molecule produced during early development, affects the timing and spatial organization of neural structures in embryos. The nervous system initially forms as a tube of cells known as the neural tube. A group of cells from the dorsal side of this tube, called neural crest cells, migrates throughout the embryo to create a rich assortment of cell types, including most components of the peripheral nervous system. Eventually, the neural tube ceases production of neural crest cells and instead generates the roof plate, an essential structure for organizing the central nervous system. We previously uncovered that retinoic acid is crucial in halting the formation of neural crest cells and initiating the development of the roof plate.
Using quail embryos and advanced methods like single-cell RNA sequencing to analyze individual cells, we now report that in the absence of retinoic acid, cells destined to become distinct types (neural crest, roof plate, or dorsal interneurons of type 1) fail to separate properly. These cells express markers for multiple cell types simultaneously, leading to confusion in their identities. Notably, retinoic acid was found to influence Notch signaling, a pathway essential for maintaining boundaries between different cell types, particularly between the roof plate and adjacent dorsal interneurons. Hence, Notch acting downstream of retinoic acid may underlie the lack of segregation between roof plate and dorsal interneurons.
Additionally, the lack of retinoic acid impacted the peripheral nervous system, where some neural crest cells did not segregate appropriately into their designated roles, resulting in abnormal cells that exhibited characteristics of both glial cells and melanocytes.
This study offers new insights into how retinoic acid ensures the correct formation of cell types at the right times and places during neural development, underscoring its vital role in maintaining the organization and separation of the central and peripheral nervous systems.
Figure 1: In the early stages of embryonic development, the dorsal neural tube shifts from generating neural crest cells to creating the definitive roof plate. Neural crest cells (in green) migrate to differentiate into glial cells, sensory and sympathetic neurons, melanocytes, and additional cell types. By implementing single-cell RNA sequencing on neural tubes with suppressed retinoic acid activity, we revealed how retinoic acid regulates the timing and spatial separation of cell types and establishes boundaries between these regions.
