Resumo

During development of the cerebral cortex, a fine-tuning of cell proliferation and differentiation accounts for the generation of distinct anatomo-functional areas that harbor different numbers and types of neural cells. Such control is achieved through the action of both extrinsic and intrinsic factors in neural progenitors, which in turn change their cell cycle parameters, mode of cell division and survival. For example, it has been shown that overexpression of the morphogen Sonic Hedgehog (Shh) increases proliferation of neural progenitors leading to an increase in the generation of neurons and, consequently, on the size of cortical regions affected (Komada et al., 2008). Yet, the precise mechanisms responsible for this increased proliferation are still unclear. In this study, we used a video-microscopy system to observe neural progenitor cell behavior in real time and analyze the potential effects of Shh signaling in the cell cycle and mode of cell division. Cortical cells were isolated from 13-day old embryos (E13) and cultured in presence of Shh or its blocker cyclopamine and imaged by time lapse vídeo-microscopy every four minutes. Images were assembled into a movie, allowing the analysis of individual clones, in which parameters such as cell cycle length and cell division mode can be quantified. We observed that Shh increases the number of cell division, both shortening the cell cycle length and increasing the proportion of symmetric proliferative divisions. In contrast cyclopamine leads to a decrease in the number of proliferating progenitors and increase cell cycle exit, resulting in differentiated cells. Our data suggest that Shh could act as a mitogen in the developing cerebral cortex, influencing both the cell cycle and mode of cell division of neural progenitor cells.