## 4 Discussion  
Here we have demonstrated that human NSCs proliferate while in space and that they remain as NSCs after 39.3 days unattended, in the same culture medium, and in space microgravity. Moreover, these cells also keep their ability to become young neurons in the appropriate conditions. [PERSON]'s team reported data supporting that simulated microgravity arrests cell proliferation, albeit in a transient manner, and they also found that these changes were not permanent as NSCs recovered their proliferative capacity when they were returned to normal gravity. The apparent contradiction between [PERSON]'s group reporting the arrest of proliferation data and this study may reside in the fact that the cells they used were from a different species (i.e., mouse vs. human). Additionally, their cells were kept in simulated microgravity for 3 to 5 days, as opposed to our NSCs, which were exposed to SPC microgravity for 5 days at 37 \({}^{\circ}\)C and in BrdU for 3 days. Therefore, the reasons for the apparent contradiction may be due to the following: (1) the extent of the exposure to microgravity, which might be a determinant for the arrest of cell proliferation in [PERSON]'s study or (2) perhaps simulated microgravity differs from space microgravity with respect to the regulation of neural cells and their biological processes. Why a higher bioenergetic state is elicited by microgravity is  
Figure 4: **SPC-NSCs gave rise to neurons after space flight. ** (**A**)—(left panels) Whole-cell voltage clamp recording using K-gluconate in the internal solution: (top) step voltage commands from \(-80\) to \(+10\) mV elicited only minimal inward currents, whereas outward currents were predominant, and (bottom) graph represents an IV relationship, illustrating the presence of large outward currents starting at \(-30\) mV. (**B**) When the recording was switched to current clamp mode, small step current pulses evoked large negative voltage deflections and only an incipient action potential with depolarizing current injections, demonstrating a neuronal phenotype, yet still very immature. (**C**) Infrared video-microscopy images of human neurons derived from NSCs flown into space. The top left panel shows a cluster of cells with multiple processes emanating from different cells. The top right panel shows an NSC with bipolar morphology. The bottom left panel shows the same cell after patch and electrophysiological recordings (illustrated in A and B) were obtained. The bottom right panel shows another NSC after patching. not clear at the present time. The fact that we demonstrated that NSCs proliferate in space needs to be studied in more detail, as it might be that this phenomenon contributes to the visual impairment and intracranial pressure syndrome (VIIP) exhibited by astronauts in space and upon their return to Earth [9]. Elucidating the origin of the increased energy needs by brain cells is crucial in order to design effective countermeasures for astronauts working onboard the International Space Station and for long-term space travel. Moreover, here we found that NSCs displayed an elevated energy metabolism; both oxygen consumption and glycolysis were higher than in the ground control counterparts, confirming our previous findings from work using \"simulated microgravity\" [10]. We have previously reported that simulated microgravity enhances mitochondrial function in oligodendrocytes (OLs), the myelin forming cells in the CNS [11]. Two of the main sources of energy for respiration and energy production, glucose and pyruvate, were decreased after just 3 days in microgravity, while lactate levels increased considerably indicating that OLs switched to anaerobic glycolysis in order to meet the energy demands elicited by microgravity [11].