Nerve Regeneration and Stem Cells

Kamal Seyed-Forootan1, Hamid Karimi*2, Ali-Akbar Jafarian3, Nazilla-Sadat Seyed-Forootan4 Farzad Kargarzadeh Ravari5 and Ali-Mohammad Karimi6 1Associate Professor of Plastic and Reconstructive Surgery, Faculty of Medicine, Iran University of Medical Sciences, Iran 2Professor of Plastic and Reconstructive Surgery, Faculty of Medicine, Iran University of Medical Sciences, Iran 3Associate Professor of Anesthesiology, Faculty of Medicine, Iran University of Medical Sciences, Iran 4Medical school, case western reserve university, USA 5Orthopedic surgeon, Private practice, UAE, Iran 6Iran University of Medical School, Tehran, Iran


Introduction
Nerve traumas are very common in the world and complete disruption of the nerve trunks without treatment, would result in partial regeneration and repair, partial or total loss of function [1,2]. One of the treatments for these traumas, is direct repair of disrupted nerve, which is sometimes possible. In most of the cases, it needs urgent repair, normally before 6-12 hours after the accident. Other treatment is nerve graft, but it may not diagnosed in proper time, or is not possible, sometimes there is lack of enough donor nerve graft available and sometimes it would not result in complete healing because of the nature of trauma and nature of function of that particular nerve [3].
Currently new modality for treatment of these traumas is using stem cells. We know that pluripotential stem cell from bone marrow, adipose tissue, skin, dental pulp, umbilical cord or from nerve trunks can differentiate into the Schwann cells and nerve cells with axons. It could help in regeneration of the missing nerves.
The function of these cells is supposed to be a combination of selfrenewal function, production and release of many growth factors, preparing the environment for nerve growth and to provide a closed space for axons to grow [4][5][6][7][8][9][10][11]. The difference between Central Nervous System (CNS) and Peripheral Nervous System (PNS) is the presence of Schwann cells in PNS.
In this regard the PNS with the help of Schwann cells can grow and regenerate more properly and effectively [2,12]. Harvesting the Schwann cells, culturing and using them in proper time is not easy and we may not use them in the right and proper time after the trauma (delay in treatment). So, the other way is to use stem cells.
Stem cells from bone marrow, adipose tissue, skin, dental pulp, umbilical cord and nerve tissue has been used for this purpose. The best, easiest (and cheapest) cells for cell culture and differentiation into the Schwann cells of nerve cells and axons are Bone Marrow Mesenchymal Stem Cells (BMSC) [1][2][3]. In this review article we are going to review the most recent and the most important researches about stem cells and usage of them in nerve repair.

Materials and Methods
With using two search engines, Pubmed and Embase. Following key terms were used: stem cells, embryonic stem cells, fetal stem cells, nervous regeneration, nerve regeneration, peripheral nerve regeneration, central nervous system regeneration; nerve repair; adult stem cells, Adipose derived stem cells; skin stem cells; nerve stem cells; facial nerve, dental pulp stem cells, and nerve trauma.
The search was restricted to experimental studies that used stem cell and tissue engineering for nerve repair after trauma.
All relevant papers were chosen and their data were used for the present manuscript. This review article has been written on the base of these researches.
Out of these studies, 13 were about using stem cells in nerve repair, 30 were about using BMSC, 9 were about ADSC, 17 were about MSC, 2 were about HSC, one was about skin SC, 12 were about NSC, 5 were discussed other stem cells origins. There were some studies (5 studies) about using growth factors and neurotrophic factors.

Results
We found 197 papers and some of them were not relevant, 52 papers were selected for our study. All the 52 papers were studies which meet the inclusion criteria. Different sources of stem and precursor cells were explored (bone marrow mesenchymal stem cells, adipose-derived stem cells, skin stem cells, dental pulp cells, umbilical cord cells and neural stem cells) for their potential application in the repair of nerve injuries or traumas.
Different material conduits (veins, collagen, synthetic conduits, and polyglycolic acid) were used as bridges. Immunochemistry and electrophysiology were the principal methods for analyzing.

Regeneration results of the treatment. Recent studies have
shown that stem cells can act as a promising bridge for nerve repair, considerable optimization of these therapies will be needed for them to be used in a clinical setting. Out of these studies, 13 were about using stem cells in nerve repair, 30 were about using BMSC, 9 were about ADSC, 17 were about MSC, 2 were about HSC, 2 were about skin SC, 12 were about NSC, 5 were discussed other stem cells origins. There were some studies (5 studies) about using growth factors and neurotrophic factors.

Discussion
For repairing the nerve injuries, we have three options available; direct nerve suturing, nerve graft and stem cell or Schwann cell repair [1,3]. Although some of the results of the first two options are acceptable, but none of them are excellent and always we would have some remaining defects in the nerve function.

Schwann Cells
According to numerous studies, using Schwann cells has a very good result in most of the reports. But their harvesting would result to injury to the other nerves. Time for culture of Schwann cells is somehow long, reproduction of these cells is very long and we cannot re-produce enough Schwann cells in proper time [4].

Stem Cells
The next option is to use precursor stem cells, many studies promoting using the stem cells, culture them and use them after differentiation into nerve cells or Schwann cells [4,5,[13][14][15][16].

Embryonic Cells
The embryonic cells and fetal cells are multi-potential cells that can be used for this issue, but usage of them has ethical problems and in most of the centers their usage is prohibited [6,7,35]. Therefore, the best option is adult stem cells. Adult stem cells can be harvested from Bone Marrow (BMSC), Adipose Tissues (ADSC), umbilical cord, skin, dental pulp and nerve tissues [1,3,36,37] ( Table 1). bone defects, non-unions and osteoblast reproduction [6,14,20,32].
They have also been used for reproduction of myocardial cells, kidney, liver and for insulin-producing cells of pancreas [6,14,38]. It has been postulated that BM stem cells can give neurotrophic factor gene (s) to the genome of the neurons [23]. So this is a kind of gene delivery and can induce neurogenesis [25].

Adipose-Derived Stem Cells
The

Neural Stem Cells
Neural stem cells are other option. These cells are very capable in re-producing the nerve and neural tissue, but they are difficult to harvest, the amount of harvest is not high, and it always results in damage to the donor nerves. So, in most of the trauma cases it cannot be used [1]. But the results are very good, and the surgeon can rely on the results with NSCs.

Olfactory Ensheating Cells
Olfactory ensheating cells are some sort of Schwann cells that are present both in PNS and CNS. They have neurotrophic and neuroprotective effects. They can secret cytokines. They can be used in regeneration of the nerves. These cells can bridge the defect and provide a good environment for nerve growth. Also, they can induce myelinization of the axons. So, they are very powerful in nerve repair [19,26].

Dental Pulp Progenitor Cells
Dental pulp progenitor cells can also be used for cell culture and treatment of nerve injuries. These are MSC and dedifferentiate into Schwann cells and nerve cells. It is said that they have characteristic of MSCs so can be used for repair of injured nerves [3,31]. Harvesting is easy and with abundant stem cells.

Small Embryonic-like Stem Cells
Very small embryonic-like stem cells are the cells that are normally found in bone marrow and they can migrate to the peripheral blood. They are potential stem cells for all part of the human body. They are also present in the brain. These cells are very few and can go to any part of the body and begin the tissue repair [19,26,30]. The studies about these cells are few. And it needs more researches to confirm its results.

Epiblast/Germ Line-Derived Stem Cells
Epiblast/germ line -derived stem cells are the cells that have pluripotential potential and can differentiate into many lines of cells and start to regenerate the whole organ [19].

Umbilical Cord Stem Cells
Umbilical cord stem cells (endothelial progenitor cells, hematopoietic stem cells) are a kind of MSCs and can produce the Schwann cells, neural cells and axons. Fortunately, their use has not any ethical issue [5,7]. Their results are very promising and good results have been reported.

Skin Stem Cells
Skin-derived precursor cells also have been used for wound healing and skin defects. They can also be used for regeneration of the nerves. The function of these progenitor cells is that they work according to the cell differentiation, type of delivery, number of the injected cells and differentiation into other progenitor cells [40].

Conduits
For having complete effects of stem cells during the repair of neural tissues, some authors recommend using conduits, fibrin glue, immune-modulator drugs, scaffolds, biomaterials and rehabilitation, functional electrical stimulation, and physiotherapy [2,3,11,14,32,[41][42][43]. Four mechanism has been postulated for the action of stem cells; resident stem cells, circulating stem cells, transient dedifferentiation of local paranchyma and neuroneogenesis [23,25,44]. With these mechanisms the stem cells can have a good to excellent results for nerve repair.
Although recent studies have shown that stem cells can act as a promising bridge for nerve repair, considerable optimization of these therapies will be needed for them to be used in a clinical setting. Limitations of our study were that only papers with English language have been selected. The most of studies have been done on animals and human researches are few.

Current and Future Development
Currently