Bibliometric analysis of global scientific activity on umbilical cord mesenchymal stem cells: a swiftly expanding and shifting focus

Abstracts Several studies have focused on umbilical cord-derived mesenchymal stem cells (UC-MSCs) due to their potential therapeutic effects in a cluster of diseases. However, there has been no bibliometric analysis evaluating the evolution in UC-MSC research. Therefore, this study aimed to assess scientific activity regarding UC-MSC research. Publications on UC-MSCs were retrieved from the Science Citation Index-Expanded (SCI-E) of the Web of Science (WoS) from 1975 to 2017. Statistical analyses were performed using Excel, GraphPad Prism 5, and VOSviewer software. Comparative analyses were employed to assess contributions between different countries, institutes, and researchers. With 21.26 citations per paper, 1206 papers cited 25,517 times were included. Mainland China contributed the most with 558 papers, with the most citations (6858 times) and the highest H-index (43). South Korea ranked first for number of papers per million people and per trillion gross domestic product (GDP). Keywords were stratified into two clusters by VOSviewer software: cluster 1, “treatments and effects”; and cluster 2, “characteristics”. The average appearing years (AAY) of keywords in cluster 1 was more recent than that in cluster 2. For promising hotspots, “TNF-α” showed the latest AAY at 2014.09, followed by “migration”, “angiogenesis”, and “apoptosis”. We conclude that the number of publications has been continuously growing dramatically since 2002 and that mainland China and South Korea are the most productive regions. The focus gradually shifts from “characteristics” to “treatments and effects”. Attention should be drawn to the latest hotspots, such as “TNF-α”, “migration”, “angiogenesis”, and “apoptosis”. Furthermore, funding agencies might increase investments in exploring the therapeutic potential of UC-MSCs. Electronic supplementary material The online version of this article (10.1186/s13287-018-0785-5) contains supplementary material, which is available to authorized users.


Background
Mesenchymal stem cells (MSCs) have a significant capacity for self-renewal and differentiation. Activation of MSCs may provide avenues for regenerative medicine due to convenient isolation techniques and immune allorecognition escape [1]. MSCs encompass multipotent cells derived from bone marrow tissue, umbilical cord, and adipose tissue [2]. Comparatively, umbilical cordderived MSCs (UC-MSCs) are inexhaustible and can be harvested without any invasive medical operations [1]. In addition, UC-MSCs have been shown to have more efficient proliferation with lower immunogenicity than that of bone marrow-derived MSCs [3][4][5]. As a result, a growing number of studies have reported that UC-MSCs may be an alternative source of stem cells with promising therapeutic effects [6,7]. Research evidence proposes that UC-MSCs can potentially be used to improve cardiac regeneration [8][9][10][11], alleviate cutaneous wounds [1,12,13], treat various neurological disorders [3,14,15], and ameliorate insulin resistance in type 2 diabetes [16].
However, no bibliometric reports assessing the relevant scientific output and research trends regarding UC-MSCs have been performed. Bibliometrics can assess not only the quantity, but also the quality of publications regarding a specific field, issue, institute, or region [17]. To a certain degree, they can provide details on the development processes in a specific field [18] and can systematically estimate the research activity trends. Moreover, bibliometric analysis can provide reference proposals during government policy establishment, particularly for determining funding-orientation guidance. Until now, bibliometric analysis has been employed for estimating the research trends on dozens of fields such as intracranial aneurysms [19], retina regeneration [20], obesity [21], and spinal tuberculosis [22].
This study aimed to estimate the publication pattern of UC-MSCs worldwide. Data were obtained from the Web of Science (WoS). We systematically assessed publication distribution stratified by geography, institutions, funding agencies, journals, and more. Furthermore, we also assessed the frequency of keywords, and then employed bibliometric mapping to describe the development of UC-MSC research.

Materials and methods
Bibliometric data and search strategy A comprehensive bibliographic retrieval was performed online using the Web of Science (WoS) on 1 July 2017. This was performed on a single day to avoid daily updating bias since the database is still open.
Search keywords were referred to MESH terms from PubMed, and then the search term was used as follows: TI = *UCMSC* OR TI = *UC-MSC* OR (((TI = (umbilical cord) OR TI = (Wharton's jelly)) AND (TI = (mesenchymal stromal cell*) OR TI = (mesenchymal stem cell*))) AND Language = English. The time period of article publication was from 1 January 1975 to 1 July 2017. For manuscript types, only peer-reviewed articles and reviews were included.
Distributed details such as original countries, institutes, journals, and funding agencies were refined by WoS.

Data collection
The original data download from WoS were firstly imported into Microsoft Excel 2010, and then were verified and then assessed by two independent researchers (XZ and JZ), respectively. Any difference was unified through discussion. Finally, the following bibliometric parameters were extracted: the quantity of papers, the number of citations, and H-index [23] (https://en.wiki pedia.org/wiki/H-index).
Given the differences in aggregate economic volume and populations among countries, we calculated the publication number per million people and per trillion gross domestic product (GDP). The latest information on population and GDP was obtained from the World Bank [24] and the Central Intelligence Agency [25].

Statistical methods
The time trend of the number of publications was analyzed using a mathematical fitting curve via GraphPad Prism 5 (GraphPad Software Inc., CA, USA). The logistic growth model fðxÞ ¼ c=½1 þ a Â e ðÀbÂðxÀ2001Þ was used to model the cumulative volume of documentation due to its good fitness and ability to predict the future trends in the literature [19,26], where x represents the year and f(x) is the cumulative volume of papers by year.
The year 2001 was defined as year zero since publications were recorded starting from 2002. The inflection point of the logistic growth curve is the point in time when the publication growth rate shifted from positive to negative, and was generated by the formula T = 2001 + ln a/b [19].
The java program VOSviewer (Leiden University, Leiden, Netherlands) was used for mapping and clustering of keywords [27]. It portrayed keywords by colors and sizes of the circles [28] according to keyword occurrences in both titles and abstracts. In addition, hotspots are defined as the keywords of popular scientific fields and their frequency was calculated using VOSviewer.

Evaluation of global publications
We retrieved 1630 publications in total and only included 1206 papers in the analysis (Additional file 1). In regards to the research strength of countries (Table 1), mainland China (558 papers, 46.27%) was the most productive, followed by South Korea (160 papers, 13.27%) and the United States (136 papers, 11.28%). When adjusted by population, South Korea was on top with 3.14 papers per million people. After the adjustment by GDP, South Korea also ranked the first with 82.94 papers per trillion GDP, followed by Iran with 36.33 papers per trillion GDP.  Table 1).

Distribution of highly contributive institutes, journals, and funding agencies
The top 10 contributive institutes are listed in Fig. 1c. Seoul National University in South Korea contributed the most publications with 48 papers published, followed by the Chinese Academy of Medical Sciences (42 publications), and Medipost Co. Ltd., from South Korea (38 publications). Among the top 10 productive institutions, six of them were from China, and the other four were Korean institutes.
There were 333 articles published in the top 10 journals (28.51% of all publications; Fig. 1d). Cytotherapy ranked the first (55 articles), followed by PLoS One (48 articles) and Stem Cells and Development (44 articles).
The top 10 funding bodies are shown in Table 2, with seven funding agencies based in China. The National Natural Science Foundation of China endorsed 255 studies in this field (ranked first, 21.14%), followed by the Jiangsu Province for Outstanding Sci Tech Innovation   Table 3).

Characteristics of the top 10 UC-MSC articles
When it came to the top 10 most cited articles, there were 7398 citations (28.99%; Table 4). The study by Kern et al. [29], published in 2006, was the most cited article (1382 times) with an average citation of 115.17 per year. Among the 10 most cited articles, five were published in Stem Cells, two in Experimental Hematology, one in Blood, one in the British Journal of Haematology, and one in Haematologica.

Hotspot analysis
Keywords were extracted from titles and abstracts of 1206 studies and analyzed by VOSviewer software. Keywords appearing more than 100 times were included in the map ( Fig. 2 and Additional file 2) and were stratified into two clusters: cluster 1 (treatments and effects; Fig. 2a, left, in red), and cluster 2 (characteristics; Fig. 2a, right, in green). The most frequent keywords in cluster 1 were "effect" (447 times), "treatment" (357 times), and "transplantation" (343 times). In cluster 2, the most frequent keywords were "umbilical cord" (468 times), "marker" (309 times), and "Wharton" (277 times). Based on the different average appearing year (AAY) of keywords, VOSviewer marked keywords included in the map with different colors (Fig. 2b). Keywords in blue appeared earlier than those in yellow and red. Keywords in the "treatments and effects"-related cluster 1 appeared in more recent years than those in the "characteristics"related cluster 2. Besides UC-MSC-related keywords, "TNF-α" showed the latest AAY of 2014.09, followed by "migration" with an AAY of 2013.85, "angiogenesis" with an AAY of 2013.78, and "apoptosis" with an AAY of 2013.76 (Additional file 3).

Discussion
In the present study, we found that the number of global publications per year increased dramatically from 2002, and that the global inflection point may have occurred in 2014. A total of 1206 papers on UC-MSC research have been cited 21.26 times per paper. Mainland China was the most productive region accounting for 46.27% of papers published, with the most citations (6858 times) and the highest H-index (43). South Korea ranked the  first regarding number of papers per million people and per trillion GDP. Keywords extracted from titles and abstracts were stratified by VOSviewer software into two clusters, the relatively outdated "characteristics" cluster and the relatively new "treatments and effects" cluster. Furthermore, "TNF-α", "migration", "angiogenesis", and "apoptosis" may be the latest, promising research hotspots in this field. Prior to 2007, global publications on UC-MSC research exhibited a steady growth, whereas from 2008 onwards, a dramatic growth was witnessed. The regression growth model of cumulative amounts showed an inflection point in 2014; however, there is still a possibility that increasing trends go on longer than expected by the proposed model since function and therapeutic application of UC-MSCs might attract more research attention. Moreover, the scientific communities may pay more attention to these latest issues on UC-MSCs.
When it came to analysis by country, China and South Korea were the most productive countries in this field. We found that both China and South Korea had an overwhelmingly higher number of publications that were supported by local funding agencies and from native innovative institutions than that of other countries. In addition, on the list of top 10 researchers, six were from mainland China and the other four were from South Korea. Oh from South Korea as well as Xu and Han from China contributed the most publications and were leaders in this field. As a result, it is theorized that further publications from these regions may still have an ongoing vital role. Additionally, top scholars from the top institutes can be good choices for partnerships BM bone marrow, HUCPVC human umbilical cord perivascular cell, MSC mesenchymal stem cell, UC umbilical cord, UCB umbilical cord blood and may also have the priority for more investments and grants. Regarding journals, those listed in Fig. 2d, such as Cytotherapy, Stem Cells and Development, Stem Cell Research & Therapy, Stem Cells International, Stem Cells, and Cell Transplantation, may be the core journals of UC-MSC research publication. Further studies can be guided for submission to these journals. Subsequently, researchers may pay more attention to research published by the aforementioned journals.
Bibliometrics combined with visualized mapping has been recognized as an effective means of assessing scientific research trends regarding a specific field, both quantitatively and systematically [30]; they can also Fig. 2 The mapping on keywords of UC-MCS. a The keywords were divided into two clusters: cluster 1, "treatments and effects"; and cluster 2, "characteristics". In general, the smaller the distance between two terms, the larger the number of co-occurrences of the terms. The size of the circle represents the frequency of the keyword, with a larger circle indicating the keyword appears more frequently. b Based on the average time of appearance, keywords in blue presented earlier than those in yellow and red uncover directions of scientific research, as shown previously [22,31]. In this study, a gradual shift in terms of research focus from "characteristics" to "treatments and effects" was seen, which is in accordance with the rule of the development of the translational medicine; therefore, the scientific community appears to be interested in the therapeutic potential of UC-MSC research at present. For instance, a pilot clinical trial demonstrated that intravenous transfusion of UC-MSCs was safe and well tolerated, effectively alleviates blood glucose, and increases generation of C-peptide levels and regulatory T cells in a subgroup of type 2 diabetes mellitus patients [32]. Subsequently, those funding agencies may increase investments for these kinds of studies. To illustrate the importance of bibliometric studies, we can also find examples of its impacts in other scientific and professional communities, such as in antimicrobial resistance surveillance. Since data from major surveillance studies are not available for the whole scientific community and are limited by time and region, scholars used bibliometrics methods to compare scientometric results with data from the major surveillance network data and found that bibliometrics provided a fast and reliable global overview of a specific antimicrobial resistance [33]. As a result, bibliometric studies may provide meaningful references for research communities.
While investigating the details provided by visualized mapping, an article titled "Conversion of human umbilical cord mesenchymal stem cells in Wharton's jelly to dopaminergic neurons in vitro: potential therapeutic application for Parkinsonism" in the more recent "treatments or effects" cluster was found to be the most cited article for a total of 251 times at 20.92 citations per year (published in Stem Cells in 2006) [34]. This paper proposed that UC-MSCs can be a potential therapeutic strategy for Parkinson's disease.

Strengths and limitations
This bibliometric analysis coupled with visualized mapping can provide systematical information on UC-MSC-related research and help readers learn about the evolution of UC-MSC research with relative ease. Furthermore, since publications were assessed based on countries, institutes, and researchers, the analysis can provide relevant information for scientists and funding agencies by highlighting potential cooperative partnerships and providing investment guidance.
However, there are several limitations to this study. First, it may have missed some important research published in other languages since only English papers were retrieved and included in the analysis. Second, all studies could not be identified using one database search. Journals included in the database of Science Citation Index-Expanded (SCI-E) in WoS are described as the world's leading journals due to a rigorous selection process, and the concept of SCI was based on Bradford's law in bibliometrics, which can be applied to define a core set of journals or publications. As a result, publications included in WoS may represent studies in the discipline, and WoS provided metadata with further distribution refinement. Third, there exists a discrepancy between bibliometric analysis results and real research conditions; this is due to the database remaining open as it continuously updates studies. Moreover, the increasing trend in the number of papers published might be sustained longer than that calculated by the mathematical model.

Conclusion
The number of publications regarding UC-MSCs has been continuously growing since 2002. Mainland China and South Korea were found to be the most productive regions. Keyword focus gradually shifted from "characteristics" to "treatments and effects", meaning that those funding agencies may increase investments for exploring the therapeutic potential of UC-MSCs. It is recommended to pay closer attention to the latest promising hotspots, such as "TNF-α", "migration", "angiogenesis", and "apoptosis".