Hematopoietic stem cells (HSCs) have been routinely utilised to investigate SCBT [59,60]. HSCs have a CD34 cell surface marker and are the most common type of stem cell within bone marrow [61]. HSCs can transform into progenitor cells via differentiation and can renew themselves [58,62,63]. HSC’s can also be derived from cytokine-mobilized and umbilical blood [59,60]. Blood disorders have been treated using HSCs for an extensive period, and therefore the use of the cells in SCBT is not thought to be a novel approach [60,64].

Jang et al., 2004 carried out a study to see how in vivo/in vitro HSCs behaved towards injury of the liver, since functional and phenotypic changes occur within the liver in this instance. It was found that HSCs have the potential to differentiate into liver cells when they are co-cultured with an injured liver [65]. The in vitro experiment displayed that microenvironmental cues had a hand in the transformation of HSCs into liver cells. This was found through analysis of protein expression, chromosomal studies and tissue-specific genes. A trend was noticed when HSCs were transplanted into mice who had liver injuries, it was noticed that viable hepatocytes were being introduced as the injury progressed. The restoration of liver function occurred between two to seven days after the HSC transplantation occurred. From research it can be concluded that HSCs can differentiate into functional hepatocytes and contribute towards the regeneration of an injured liver. Lagasse et al., 2000 experimental results further support this conclusion. Adult bone marrow cells were intravenously injected into a tyrosinemia type I affected FAH mouse and ultimately only HSC cells gave rise to hepatic regeneration [66].

From the research of Liu et al., 2006, liver regeneration (after partial orthotopic liver transplant) can be induced by mobilizing HSCs with granulocyte-colony stimulating factor (G-CSF). It is less difficult to conduct this treatment in a universal way. Current therapeutic procedures such as this one, have a basic concept of mobilizing, proliferating and directing stem cells to improve recovery from liver injury by a partial liver transplantation. Although stem cells can be linked to restoring liver function, it is not yet possible to define their exact action in the liver regeneration process. Tsolaki et al., 2014 reiterated this uncertainty since they also were unsure if the liver regeneration processes were triggered or mediated by the on-site HSCs and/or by the effect of the G-CSF. This further emphasises the difficulty of characterising exact root causes for hepatic regeneration. Additionally, G-CSF alongside other mobilization agents such as Plerixafor and a G-CSF+Plerixafor combination can potentially aid the reversal of chronic liver injury via unique processes. A clear observation was made that the G-CSF possessed the greatest anti-fibrotic ability when it was used for an extended period. This result further proves the link between G-CSF and its anti-fibrotic outcome [67].

Mesenchymal stem cells (MSCs) are being thoroughly investigated for the purposes of regenerative medicine as they possess distinct biological properties [68]. MSCs can be extracted from adipose tissue, umbilical cord blood and bone marrow. Since MSCs can undergo extensive expansion within in vitro conditions, this allows them to swiftly reach the desired quantity when required for in vivo therapy [69,70]. Mesenchymal stem cell (MSC) can be differentiated into hepatocytes, promote liver regeneration, inhibit liver fibrosis and induce liver apoptosis, particularly via paracrine mechanisms [71]. Dominici et al., 2006 states that it is difficult to compare results from research papers investigating MSCs due to the different procedures used for isolation, expansion and characterization of cells, which in turn hinders progress in this field of study [72]. To combat this issue, the International Society for Cellular Therapy has defined a criteria list to characterize the human MSC. Firstly, it is crucial that the MSC displays plastic adherence when stored in a standard cell culture environment. Additionally, the MSC must express (CD90, CD105, CD73) and lack expression of (HLA-DR, CD34, CD14 or CD11b, CD79alpha or CD19 and CD45) certain cell surface molecules. Finally, the MSC must have the differential ability to transform into adipocytes, chondroblasts and osteoblasts in vitro.

Though isolated MSCs all express the MSC marker profile, they are commonly diverse in both differentiation and proliferation. In vitro cultivated MSCs have excellent differentiation ability, immunoregulatory potential and can aid tissue remodelling through the emission of trophic factors. These key properties make MSCs useful in cellular therapy [73,74]. The review paper by Shiota and Itaba 2016 found that MSCs have the potential to improve liver diseases such as fatty liver disease, acute liver failure and liver fibrosis. MSCs have great versatility and, therefore, they can differentiate into hepatic lineages. In one study, HNF3β was expressed when a tetracycline-controlled system was regulated in the UE7T-13 (bone marrow derived) MSCs. This expression caused a ripple effect, since it also increased the expression of alpha-fetoprotein, epithelial cell adhesion molecules, albumin and tyrosine amino transferase genes. When tetracycline treatment was continued over 8 days, it was seen that majority of the MSCs expressed albumin. Albumin is linked to the maturing of hepatocytes. It can be concluded that MSCs are a valuable source to use in SCBT to treat liver disease [70,75].

Gao et al., 2017 carried out an investigation with (adipose-derived) MSCs and found they are useful for suppressing rejection in reduced sized liver allografts. Regulatory T cell production is stimulated by MSCs; this is key in its immunosuppressive effects. MSCs were also found to boost hepatocyte regeneration and hinder hepatocyte apoptosis, which in turn promotes liver regeneration [76]. Liu et al., 2018 explored (bone marrow-derived) MSCs and found evidence that MSC transplantation could potentially aid recovery for patients who have undergone a liver resection. Liver regeneration occurs since MSCs encourage cell proliferation/growth and MSC infusion can potentially improve lipid accumulation within the liver [77].

Embryonic stem cells (ESCs) are categorised as pluripotent stem cells that are extracted from the inner cell mass of blastocyst stage embryos. These stem cells retain the potential to self-renew indefinitely, whilst having the ability to differentiate into all cell types in the human body (as seen in Figure 5) when supplied with the relevant stimulus for differentiation [48,78]. This high degree of differentiation and unlimited potential for self-renewal, allow ESCs to be considered as an effective regenerative therapy for tissue replacement following liver disease. Tsolaki and Yannaki, 2015 discussed that ESCs have the capacity to proficiently differentiate into hepatocyte-like cells in vitro, creating cells that have some similar properties to that of mature hepatocytes [79]. For this reason, ESCs are a valuable option for studying the molecular source of hepatocyte differentiation. A recent study by Tolosa et al., 2015 identified an effective method for the differentiation of ESCs into neonatal hepatocytes that had the ability to reintroduce livers in vivo without instigating tumour development in mice [80].

However, there are some limitations to the use of ESCs in cellular therapies. The fact that the derivation of ESCs requires the destruction of blastocyst stage embryos raises ethical issues that have slowed the advancement of ESC research. Tsolaki and Yannaki, 2015 emphasise that regardless of the significant progress and development of complex differentiation methods mimicking embryonic development, ESCs extracted from hepatocyte-like cells are unable to completely function as mature adult hepatocytes [79]. Furthermore, the concern of immunological incompatibility of the transplanted cells, hinder their application as a cell replacement therapy [81].

Induced pluripotent stem cells (iPSCs) have properties comparable to those of ESCs, such as self-renewal and pluripotency, while avoiding the main issues that are associated with the use of ESCs. iPSCs can be reprogrammed from somatic cells through the use of pluripotency factors. Forbes et al., 2015 established that they do not require embryonic material (as seen in Figure 6) [78], thereby obviating ethical controversies [82]. In addition, iPSCs hold the potential to be clinically beneficial since they can be formed from autologous stem cells, which provides the opportunity for autologous use and avoiding the condition for immunosuppression [83]. In the event that iPSCs are used for the derivation of hepatocyte-like cells for treatment of a genetic liver disorder, then the autologous source would need some type of “gene surgery” before use. Nicolas et al., 2016 stated that the use of iPSCs as a cellular therapy date back to 2006, and has drastically grown in popularity as a favourable alternative to ESCs [81]. However, before considering their clinical applications, several issues need to be resolved.

iPSCs have increased expectations for regenerative medicine due to their potential to deliver personalised treatment, while their derivation from patient-specific cells overcomes the risk of rejection. Although iPSCs are an interesting alternative to ESCs, there are some issues that need to be addressed before this new form of cellular therapy can move from proof of concept to the clinic. Yu et al., 2014 suggests that an in-depth preclinical evaluation of iPSCs in appropriate large animal models is essential to make sure that treatment with iPSC-derived cells is safe and efficacious before human trials [84]. Hence, important concerns need to be addressed, which include effectiveness of the iPSC-based treatments, and long-lasting safety tolerability. It is vital that the optimal reprograming method is established by using clinically applicable standardised protocols. Cost-efficient manufacturing techniques need to also ensure that they can support the development of rapid differentiation methodologies for culturing mature cell types from iPSCs. Despite these restrictions, iPSC-derived hepatocytes still promise innovative solutions for liver cell therapy, and donor grafts extracted from iPSCs could potentially provide suitable organs for liver transplants [79].

Endothelial progenitor cells are categorised as immature endothelial cells that are located within bone marrow and peripheral blood vessels. These cells manifest from hemangioblasts and are responsible for the neovascularisation of damaged tissue in the body. However, Kwak et al., 2018 emphasises that EPCs are likely to be derived from several cell lineages, which is proven by their various surface markers [58]. According to an animal study conducted by Nakumura et al., 2007, transplantation of EPCs resulted in the intervention of liver fibrosis by successfully suppressing the activation of HSCs. These EPCs were also exhibited to stimulate hepatocyte proliferation and greater matrix metalloproteinase activity, in addition to increased secretion of growth factors [85]. Rautou, 2012 states that, in vitro, EPCs are identified by their capability to produce adherent cell populations that differentiate and multiply into endothelial lineage. When in vivo, EPCs contribute to the dynamic process of angiogenesis in ischemic sites or assist with vascular repair following damage to vessel walls [86]. Accumulating data display that these characteristics of EPCs detect a heterogeneous cluster of cells with respect to their origin, differentiation and functional properties.

SCBT has been deemed an alternative treatment for liver diseases since promising results have been found via clinical and experimental research. A range of stem cells (including HSCs, MSCs, iPSCs, EPCs, ESCs) have been explored to investigate their clinical potential and viability. Extensive research has been carried out with MSCs, which results in a broader understanding of the potential regenerative liver treatments available. Long-term efficacy is still unproven and experimental trials do not have standardized protocols, which is disadvantageous. However innovative technologies of the future are likely to resolve the current issues associated with SCBT [58]. Table 2 summarizes the derivation of stem cells that have been derived from their respective locations and additional information.