# Nanotechnology and Early Human Development
## 3 Nervous System  
Nanoparticles are currently being studied as drug carriers to cross the blood-brain barrier (BBB) for the therapy of several central nervous system diseases, as nanoscaffolds for axonal regeneration, tools for neurological interventions, and brain diagnostics. The main obstacle drugs have to face to reach their molecular targets in the central nervous system is the blood-brain barrier, as shown in Figure 2.  
The BBB is a highly selective barrier that separates circulating blood from the brain and prevents free diffusion of circulating substances from the blood into the brain. Intrauterine inflammation has been associated with preterm birth and also with a variety of neurobehavioral disorders, ranging from cognitive and learning disabilities to motor dysfunction disorders such as cerebral palsy. In a mouse model of intrauterine inflammation, [PERSON] et al. [8] demonstrated that intra-amniotic delivery of hydroxyl polyamidoamine dendrimer (G4-OH)-based nanodrugs may be effective in preventing complications such as preterm birth and its consequences (i.e., neurodevelopmental delay, chronic lung disease, and/or poor growth)  
Nanoparticles can also exert adverse effects on the central nervous system via several possible mechanisms, including oxidative stress, autophagy, lysosome dysfunction, and the activation of specific pathways which result in cell damage, dysfunction, or death. Other studies suggested that nanoparticles could induce considerable neurotoxicity in animals (i.e., blood-brain barrier destruction and/or neuronal degeneration). Since nervous tissue regeneration is limited, most of the nerve damage is irreversible. Furthermore, animal models have shown that nanoparticles administered to the mother during gestation may harm fetal development through direct or indirect mechanisms. Even traces of particles in maternal blood can migrate to the fetal compartment [9]. However, it is difficult to predict the toxicological behavior of nanoparticles in humans, given the lack of standards in previous studies and the complexity of the biological responses to these compounds.  
Regeneration in the brain, which is a very complex process, involves cellular replacement, but also synaptic and functional repair. Nanotechnology has been proposed as a promising tool to facilitate nervous system regeneration, with multiple mechanisms being comprehensively reviewed by [PERSON] et al. [10]. Briefly, nanoparticles may be used as carriers of drugs, growth factors, or genetic material and may exert actions on cells involved in neuroregeneration (glial cells), immune function (leukocytes), and/or circulation (endothelial cells), or may form scaffolds to support neuronal growth and modify the extracellular environment. Carbon nanotubes can be used as scaffolds for neuronal growth and neural stem cell growth and differentiation. A nanoscaffold is a three-dimensional structure composed of polymer fibers that enhances damaged cell adherence and regeneration of tissues. As the tissue grows, the scaffold is metabolized and completely cleared [9].  
Figure 2: Structure of the blood-brain barrier. Source: [PERSON] 2013. Reproduced with permission of [PERSON].  
SiRNA nanomedicines can overcome obstacles such as the BBB, particle clearance in the bloodstream, and cell entry, allowing the treatment of several brain diseases [11]. Clinical studies in childhood are needed to confirm this possibility.