Neural tissue engineering is a specific sub-field of tissue engineering. Neural tissue engineering is primarily a search for strategies to eliminate inflammation and fibrosis upon implantation of foreign substances. Often foreign substances in the form of grafts and scaffolds are implanted to promote nerve regeneration and to repair nerves of both the central nervous system (CNS) and peripheral nervous system (PNS) due to injury.
Introduction There are two parts of the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). General body functions are supervised by the central nervous system (CNS), which includes the brain and spinal cord. The PNS delivers motor signals to control body activities and receives sensory data from the CNS. The PNS It is made up of nerve fibers arranged into nerves. The PNS's autonomic nervous system (ANS), whose sympathetic and parasympathetic branches preserve homeostasis and regulate involuntary physiological functions.
The "fight-or-flight" reaction is triggered by the sympathetic nervous system (SNS), which is derived from the thoracic and upper lumbar spinal cord. It readies the body for quick reactions under pressure. The parasympathetic nervous system (PSNS), on the other hand, is derived from the brainstem and sacral spinal cord and facilitates normal physiological processes by encouraging rest and energy conservation. One of the main nerves in the PSNS, the vagus nerve, originates in the brainstem and travels throughout the body, affecting different organs. It has sensory and motor fibers. Sensory messages tell the brain what the body is doing, allowing it to maintain homeostasis and control activities. Additionally, the vagus nerve influences emotions and memory through connections to several brain regions.
Neuroimmune Interactions The immune system's role is to identify and protect the body against external chemicals and infections. It is separated into innate and adaptive immunity and consists of immune organs, cells, and active ingredients. Remarkably, under certain circumstances, a variety of non-immune cells can display immunological properties. The immune system and the neurological system, which control body processes, are interdependent. By controlling humoral chemicals on a systemic level, the central nervous system CNS affects the immune system. Sleep and other psychosocial variables can affect immunological responses. Obesity and sleep deprivation, for example, can impair immunity, and long-term stress can erode immunological responses, making people more vulnerable to infections like COVID-19. In diseases like asthma that are made worse by psychological stress or depression, neuroimmune interactions are clearly seen. The immune response can impact brain activity, and neuroendocrine hormones control the release of cytokines. Fever symptoms like drowsiness and decreased appetite are caused by proinflammatory mediators. Immune system organs get autonomic innervation from the peripheral nervous system (PNS), which facilitates specialized communication between the two systems. Comprehensive information on bidirectional crosstalk pathways is frequently lacking, despite evidence of functional links between the neurological and immune systems already in place. lymph nodes are essential components of the immune system because they serve as both collecting places for various immune cells and act as filters for dangerous chemicals. Their well-structured composition promotes efficient immune responses, protecting the body against external chemicals, infections, and malignancies. Regional innervation of lymph nodes involves complex participation from the sympathetic and parasympathetic branches of the autonomic nervous system (ANS). Furthermore, there is afferent innervation, which is in charge of immune responses in particular areas. Through the use of neuropeptides, nociceptors—specialized nerve endings that feel pain—control the immune system. Distinct nerve fibers inside lymph nodes are identified by several markers, such as TH, anti-β2-AR, ChAT, and VAChT. Studies have shown that nerve fibers originate from the hilum, travel along blood vessels, cross medullary areas, and form subscapular plexuses. Some limitations do, however, remain. These include the sparse identification of neurons and nerve fibers, the lack of a thorough examination of fine nerve fibers, the incomplete knowledge of innervation in particular regions, and the inadequate documentation in certain studies of close interactions between immune and non-immune cells and nerve fibers.
Neuroimmune interplays have possible therapeutical approaches Novel approaches focusing on neuroimmune interactions may alter the course of the disease or reduce symptoms. Targeting neuroimmune pathways is a holistic approach that seeks to affect both immune responses and brain functioning. The term "acupuncture" refers to the ancient Chinese medical technique of gently stimulating nociceptors and receptors with tiny needles inserted into certain body sites in order to treat various ailments, including pain and inflammation. The FDA-approved therapy for depression and epilepsy, vagus nerve stimulation (VNS), may also be beneficial for non-neurological conditions such rheumatoid arthritis and inflammatory bowel disease. Chemical therapies, such as peripheral nervous system (PNS) modulation, are being investigated for the treatment of infectious and inflammatory disorders, such as rheumatoid arthritis and issues associated with diabetes. Targeting tumor innervation is being explored as a potential new treatment approach. Intratumoral innervation, which involves nerves inside or around tumors, influences the biology of cancer. Peripheral neuropathy is one of the PNS-associated disorders that can be treated with immunotherapy manipulation. According to many experimental researchers, extensive clinical studies are necessary to confirm the safety, effectiveness, and regulatory approval of these experimental techniques prior to their establishment as established therapies.
Tissue Engineering The need for neural tissue engineering arises from the difficulty of the nerve cells and neural tissues to regenerate on their own after neural damage has occurred. The PNS has some, but limited, regeneration of neural cells. Adult stem cell neurogenesis in the CNS has been found to occur in the hippocampus, the subventricular zone (SVZ), and spinal cord. CNS injuries can be caused by stroke, neurodegenerative disorders, trauma, or encephalopathy. A few methods currently being investigated to treat CNS injuries are: implanting stem cells directly into the injury site, delivering morphogens to the injury site, or growing neural tissue in vitro with neural stem or progenitor cells in a 3D scaffold. Proposed use of electrospun polymeric fibrous scaffolds for neural repair substrates dates back to at least 1986 in a NIH SBIR application from Simon. For the PNS, a severed nerve can be reconnected and reinnervated using grafts or guidance of the existing nerve through a channel.
Recent research into creating miniature cortexes, known as corticopoiesis, and brain models, known as cerebral organoids, are techniques that could further the field of neural tissue regeneration. The native cortical progenitors in corticopoiesis are neural tissues that could be effectively embedded into the brain. Cerebral organoids are 3D human pluripotent stem cells developed into sections of the brain cortex, showing that there is a potential to isolate and develop certain neural tissues using neural progenitors.
Another situation that calls for implanting of foreign tissue is use of recording electrodes. Chronic Electrode Implants are a tool being used in research applications to record signals from regions of the cerebral cortex. Research into the stimulation of PNS neurons in patients with paralysis and prosthetics could further the knowledge of reinnervation of neural tissue in both the PNS and the CNS. This research is capable of making one difficult aspect of neural tissue engineering, functional innervation of neural tissue, more manageable.
CNS
Causes of CNS injury There are four main causes of CNS injury: stroke, traumatic brain injury (TBI), brain tumors, or developmental complications. Strokes are classified as either hemorrhagic (when a vessel is damaged to the point of bleeding into the brain) or ischemic (when a clot blocks the blood flow through the vessel in the brain). When a hemorrhage occurs, blood seeps into the surrounding tissue, resulting in tissue death, while ischemic hemorrhages result in a lack of blood flow to certain tissues. Traumatic brain injury is caused by external forces impacting the cranium or the spinal cord. Problems with CNS development results in abnormal tissue growth during development, thus decreasing the function of the CNS.
CNS treatments and research
Implantation of stem cells to the injury site One method to treat CNS injury involves culturing stem cells in vitro and implanting the non-directed stem cells into the brain injury site. Implanting stem cells directly into the injury site prevents glial scar formation and promotes neurogenesis originating from the patient, but also runs the risk of tumor development, inflammation, and migration of the stem cells out of the injury location. Tumorigenesis can occur due to the uncontrolled nature of the stem cell differentiation, inflammation can occur due to rejection of the implanted cells by the host cells, and the highly migratory nature of stem cells results in the cells moving away from the injury site, thus not having the desired effect on the injury site. Other concerns of neural tissue engineering include establishing safe sources of stem cells and getting reproducible results from treatment to treatment.
Alternatively, these stem cells can act as carriers for other therapies, though the positive effects of using stem cells as a delivery mechanism has not been confirmed. Direct stem cell delivery has an increased beneficial effect if they are directed to be neuronal cells in vitro. This way, the risks associated with undirected stem cells are decreased; additionally, injuries that do not have a specific boundary could be treated efficiently.
Delivery of molecules to the injury site Molecules that promote the regeneration of neural tissue, including pharmaceutical drugs, growth factors known as morphogens, and miRNA can also be directly introduced to the injury site of the damaged CNS tissue. Neurogenesis has been seen in animals that are treated with psychotropic drugs through the inhibition of serotonin reuptake and induction of neurogenesis in the brain. When stem cells are differentiating, the cells secrete morphogens such as growth factors to promote healthy development. These morphogens help maintain homeostasis and neural signaling pathways, and they can be delivered into the injury site to promote the growth of the injured tissues. Currently, morphogen delivery has minimal benefits because of the interactions the morphogens have with the injured tissue. Morphogens that are not innate in the body have a limited effect on the injured tissue due to the physical size and their limited mobility within CNS tissue. To be an effective treatment, the morphogens must be present at the injury site at a specific and constant concentration. miRNA has also been shown to affect neurogenesis by directing the differentiation of undifferentiated neural cells.
Implantation of neural tissue developed in vitro