# Behavior of Astrocytes Derived from Human Neural Stem Cells Flown onto Space and Their Progenies
## 4 Discussion  
Glia, also called glial cells or neuroglia, are non-neuronal cells (astrocytes and oligodendrocytes) in the brain and spinal cord that maintain CNS homeostasis, form and maintain myelin, and provide support and protection for neurons. In the present study, we obtained astrocytes derived from NSCs flown into space. These astrocytes were generated in 1G upon return of the SPC-flown NSCs. This study is important because in the CNS, there are five times more astroglial cells (astrocytes) than neurons. In the past, the concept of neuroglia was introduced by [PERSON] _neuro-glia_ (nerve-cement) [18,19], when astrocytes were considered solely a physical support material for neurons. Currently, it is well known that astrocytes have a multitude of essential functions in the healthy CNS, including primary roles in synaptic transmission and information processing by neural circuit function [20]. Moreover, their secreted molecules such as gliotransmitters, neuromodulators, and trophic factors help maintain adult neurogenesis in the normal brain, while an imbalance of such molecules may be deleterious and contribute to CNS pathologies [21]. Astrocytes respond to all forms of CNS insults through reactive astrogliosis, which is a pathological hallmark of CNS structural lesions [11]. This means that instead of dying after injury or disease, astrocytes survive and undergo fundamental changes, as they display different degrees of astrogliosis proportional to the extent of injury [22] by proliferating and forming a \"gliotic scar\" to fill spaces left when neurons and glia die. Unfortunately, often the gliotic scar is the main obstacle for regeneration of the CNS tissue lost to injury or disease. In order to ascertain if NSCs flown to space would acquire the astroglial phenotype post-flight in 1G, NSCs were cultured in DF-10% FCS. As expected, the NSCs transitioned into the astrocyte phenotype as ascertained by the expression of the astrocyte markers S-100 and GFAP, indicating that after space flight NSCs had preserved their pluripotency, and with the right environmental cues they are able to generate astroglial cells. Literature on the proliferation of non-injured astrocytes is not as extensive as it is for models of CNS injury [11]. [PERSON] and collaborators have shown that astrocytes proliferate in response to a variety of insults, nonetheless, they described this proliferative potential as modest in both pediatric and adult populations, suggesting that there may be a limit to the potential of these cells to generate numerous new astrocytes in the injured CNS [23]. According to Dr. [PERSON], cell division rate for normal human astrocytes has been determined to be approximately 2.8 days for astrocyte progenitors and 4 days for astrocytes  
Figure 5: After 39.3 days in space, astrocytes derived from SPC-flown NSCs, increased their numbers slowly, steadily and in a significant manner with time in culture. Between one and 2 cells divided in an interval of 17 h. Values are expressed as mean + SD (shown inside the corresponding bar per time point) of three separate experiments. * \(p\) < 0.05, ** \(p\) < 0.01. (personal communication). It has been shown that when normal human astrocytes were re-programmed to become tumor initiating cells, thus expected to be more proliferative, their doubling time was longer than 43.6 h [24]. Another paper using human embryonic stem cell-derived astrocytes in the presence of growth factors (bFGF and EGF) and human serum exhibited a doubling time of 21 \(\pm\) 2.6 h [25]. We further found that SPC-flown NSCs-derived astrocytes proliferated modestly and steadily. Their numbers increased in a statistically significant manner with time in culture. Between one and two cells divided in an interval of 17 h, thus, in a shorter time than previously reported for astrocytes grown in 1G.