- Open Access
A microsurgical procedure for middle cerebral artery occlusion by intraluminal monofilament insertion technique in the rat: a special emphasis on the methodology
© Güzel et al.; licensee BioMed Central Ltd. 2014
- Received: 16 September 2013
- Accepted: 9 May 2014
- Published: 6 June 2014
Although there are many experimental studies describing the methodology of the middle cerebral artery occlusion (MCAO) in the literature, only limited data on these distinct anatomical structures and the details of the surgical procedure in a step by step manner. The aim of the present study simply is to examine the surgical anatomy of MCAO model and its modifications in the rat.
Materials and methods
Forty Sprague-Dawley rats were used; 20 during the training phase and 20 for the main study. The monofilament sutures were prepared as described in the literature. All surgical steps of the study were performed under the operating microscope, including insertion of monofilament into middle cerebral artery through the internal carotid artery.
After an extensive training period, we lost two rats in four weeks. The effects of MCAO were confirmed by the evidence of severe motor deficit during the recovery period, and histopathological findings of infarction were proved in all 18 surviving rats.
In this study, a microsurgical guideline of the MCAO model in the rat is provided with the detailed description of all steps of the intraluminal monofilament insertion method with related figures.
- Middle cerebral artery occlusion
- Surgical anatomy
Rat models of focal cerebral ischemia are widely used in experimental studies aiming at the elucidation of pathophysiological mechanisms of stroke and the evaluation of new therapeutic approaches in the treatment of occlusive cerebrovascular diseases [1–12].
Among the endovascular techniques of middle cerebral artery occlusion (MCAO), the suture occlusion method is the most frequent experimental paradigm that has been used over the last 20 years . The basis of this procedure consists in the blocking of the blood flow into the MCA with an intraluminal suture (nylon monofilament) inserted through one of the big arteries of the neck, as described before [6, 13–16]. If properly performed, this technique provides reproducible MCA territory infarction [4, 14, 15, 17]. It allows transient occlusion with following cerebral reperfusion by retracting of the suture and thereby, different levels of lesion severity depending on the occlusion time can be obtained [12, 18–24].
Albeit its common use, getting started with this model in research is difficult. Therefore, we provide here the detailed description of all steps of the modified intraluminal monofilament method with an array of related figures.
All surgical procedures were performed in accordance with our institutional guidelines and the German animal protection legislation, under the operating microscope (SMED-Studer Medical, Engineering-AG, Switzerland Yasargil System, VM-900) Female Sprague-Dawley rats (250 to 280 grams) were housed under 12-h light/12-h dark conditions; under temperature of 22-24°C and with food and water ad libitum. The animals were allowed to acclimatize for 2 weeks prior to experiment and were fasted overnight with free access to water, before the surgery.
The animals were anesthetized by 10 mg/kg i.p Ketamine hydrochloride (Ketamine® 10% Essex Pharma GmbH, Germany) and 5 mg/kg i.p Xylazine hydrochloride (Rompun®, Bayer AG, Germany) given intraperitoneally.
The animals were not intubated and blood gases were not monitored during the MCAO.
All procedures were in concordance with German animal law regulations. The animal protocol granted by the Regierungspraesidium Freiburg as well as the ethical commission of the Faculty of Medicine in the University of Freiburg gave ethical permission to perform the described experiments.
For temporary MCAO, reperfusion was obtained by withdrawing the suture approximately 13–15 mm after the ischemia time chosen for the experiment until resistance was felt when the tip reached the ligation of the ICA. In the present study, we chose an occlusion time of 60 minutes. The method allows reperfusion of the two distal branches of the ICA; the anterior choroidal and hypothalamic arteries [31, 35, 36], preventing the possible loss of the experimental animal, as the hypothalamic artery occlusion contributes to hyperthermia after intraluminal suture occlusion which is related to more pronounced ischemic damage and postoperative mortality .
Intraluminal suture preparation
Disposable scalpel No. 10 (Feather company, Japan), 4/0 nylon suture and 6/0 silk suture (Ethicon Inc. Deutschland), Wullstein retractors (No. 17018–11), adson forceps (No. 91106–12), MORIA forceps (Straight, No.11370-40) MORIA forceps (Curved, No. 11370-42), Micro-Mosquito (Straight, serrated, No. 13010-12), Hartman Hemostatic forceps (No. 13002–10), Student iris scissors, Straight No. 91460–11), MORIA spring scissors (Straight, No. 15396–00), 2 micro clips (curved serrefines No. 18055–01 and straight serrefines No.18055-05) Micro-clip applicator (No. 18056-14), Michel suture clips (No. 12040-02), Applying forceps for Michel suture clips (No. 12018-12), Ear punch for animal identification (No. 24210–02) were from FST (Fine Science Tools GmbH, Germany) catalogue.
The illustrations were acquired with the digital camera D2Xs (Nikon, Japan) equipped with the objective NIKKOR AF-S 300 mm f/2,8G ED VR II (Nikon, Japan).
Learning experience and potential pitfalls
In a preliminary experiment, 20 rats have been operated and 15 of them died either during the operation or within the first 24 hours (mortality rate of 67.5%). Ten rats died due to intracerebral hemorrhage as revealed by the post-mortem examination of these animals caused probably by the perforation of the ACA beyond the ostium of the right MCA during insertion of the monofilament [6, 21]. Three rats died due to bleeding from the big vessels of the neck during early stages of the operation, and two died because of cervical haematoma or haemorrhage leading to compression of the trachea, vascular and neural structures. After extensive training in the separate group (n = 20) we lost only two rats (surgical success rate was 90% (n = 18), and mortality rate was 10%). One animal died due to intracerebral haemorrhage (complication of monofilament insertion), and the other due to ICA bleeding, while we introduced the monofilament through the CCA. 18 of 20 rats survived at least four weeks. All of the surgeries in this study were performed by trained neurosurgeons with extensive micro-surgical experience and a neurosurgery-resident in the second year of the training. Nevertheless, the relative small experience with the rat extracranial vascular anatomy and intraluminal placement of the filament resulted initially in the high rate of perioperative mortality. The length of the necessary training depends clearly on the surgical experience of the investigator. Previous micro-surgical skills unequivocally facilitate a fast development of the MCAO model. In our hands the crucial modification contributing to the safe and reliable occlusion of the MCA and reproducible stroke induction within its perfusion territory was the temporary closure of the PAA preventing an erroneous insertion of the filament into the extracranial ICA branches. Apparently this maneuver has also been applied by researches introducing intraarterial catheters for experimental, intracerebral drug/cells delivery (personal communication P. Walczak/M. Janowski, Johns Hopkins University, Baltimore, USA).
Neurological evaluation of rats after MCAO 
No apparent deficit
Contralateral forelimb flexion
Decreased grip of the contralateral forelimb while tail pulled
Spontaneous movement in all directions; contralateral circling only if pulled by tail
Spontaneous contralateral circling
The neurological examination was carried out after full recovery from anesthesia. The rats were assessed for contralateral motor deficit to confirm ischemia by using a previously described scoring method (Table 1) . In the present study, all surviving animals showed clear neurological motor deficits within the first two hours after MCAO (100% percent with score 4).
During the recovery period, all surviving rats showed also forelimb flexion and contralateral forelimb paralysis, confirming the permanent damage following temporary brain ischemia .
To produce focal ischemia, the occlusion of the MCA has been the target of most investigations, because this vessel is the most commonly affected in stroke victims . This model has been first introduced by Koizumi et al. , and later modified by Longa et al. . Numerous further modifications of this method have been reported in the literature [2, 14, 15, 19, 21, 25–27]; however, the literature describing the important microsurgical hallmarks of the MCAO and identifying the critical steps and highlighting the possible pitfalls of the surgical technique is very scarce. For MCAO, the filament may be inserted through the ECA, ICA or CCA [6, 8, 14, 21, 24, 26, 27, 32, 39, 40]. Alternatively to the method we applied, MCAO is frequently produced by insertion of the monofilament through the ICA to the origin of the MCA via the ECA. This technique requires coagulation or ligation of the OA . Another significant difference between our operation technique and those described by many other authors is the thorough closure of the PPA that represents a substantial step in our operation protocol. It guarantees the insertion of the monofilament fiber directly into the MCA.
To insert the monofilament through ECA, further dissection of the ECA and its branches is required. Inserting the monofilament through CCA, we minimized the dissection of the ECA and its branches in the area of the carotid bifurcation. Moreover, ligation of CCA facilitates introduction of the monofilament and reduces active hemorrhage and hematoma formation during and after the procedure. The disadvantage of CCA ligation is to provide cerebral blood flow through anterior communication artery instead of ICA.
Along with many advantages like its simple technique, the minimal invasive nature of the procedure, low mortality and redundancy of a craniotomy , all the intraluminal suture models of MCAO share the same disadvantages: insertion of the suture occludes the entire course of the ICA, leading to obstruction of the hypothalamic artery (HA). This causes hypothalamic infarction with associated pathologic hyperthermia that confounds the results of the investigation, for instance neuroprotective drug evaluation [6, 38, 42]. Finally, the anterior choroidal artery can be occluded by the filament, while the lumen of the MCA still allows perfusion. This may cause clinical stroke signs mimicking MCAO. Some other unwanted side effects of this method are subarachnoid hemorrhage, intraluminal thrombus formation, and premature reperfusion [6, 21, 30]. When carefully carried out, sharp dissection allows a fast and easy approach to the vessels securing their protection at the same time. We suggest reducing the interventions on the vessels and their surrounding structures to a minimum, especially manipulations on the ECA and its branches. As a technique to reduce tissue damage, we strongly recommend the use of a temporary microvascular clip for the occlusion of the PPA as described before  instead of a ligation.
Ischemic stroke is a very heterogeneous disorder. In this respect, mimicking all aspects of human stroke in one animal model is not possible. Although ischemic stroke was shown clinically and histologically in this study, volume of infarcted tissue was not measured. If volume of infarcted area could be measured on histologic sections or MRI, the results might be more objective. However, the study was focused on describing the methodology of surgical MCAO, and volume measurement was not planned.
Another limitation of the study is that, some physiological data, such as blood gases and body temperature of animals, were not measured during MCAO experiment.
Intraoperative Doppler ultrasonography could be useful for measurement of cerebral blood flow, however Doppler ultrasonography was not available during the procedure unfortunately.
It is well known that there is a learning curve for a MCAO model. Therefore, before the study, 20 rats were used for training and detailed description of surgical technique. We focused on the occlusion technique and evaluated the results of MCAO on clinical and histological findings. We believe that if all steps of this method is applied correctly, the procedure is sufficient for MCAO in rats.
In conclusion, we present a modified surgical technique for intraluminal MCAO. In comparison to methods described by other authors, our procedure avoids the dividing of the omohyoid muscle. We showed that a gentle dissection and efficient distraction is sufficient to reach the relevant anatomical structures. Furthermore, we reduced the dissection of the ECA and its branches to a minimum in the area of the carotid bifurcation. In our procedure, we did not coagulate the OA but ligated it together with the ECA as this saves time and reduces tissue damage. As mentioned above, we found the microvascular clip to be an excellent way to close the PPA greatly facilitating the introduction of the filament if positioned correctly. We recommend placing it on the PPA as close as possible to the origin of this vessel from the ICA.
The presented study demonstrates that the microsurgical filament occlusion of the MCA can be easily performed in rats by the above described procedure following some intensive microsurgical training. This modified surgical approach is simple and can be followed easily by the microsurgical guidelines and landmarks provided here. This may promote experimental approaches in stroke that may ultimately advance the scientific progress in experimental, and potentially, also clinical forms of cerebrovascular diseases.
This study was supported by the Deutsche Forschungsgemeinschaft (DFG) and by the Scientific and Technical Research Council of Turkey (TUBITAK; to Aslan Guzel). UDK is supported by the Dr. Mildred-Scheel stipend by the Deutsche Krebshilfe. The authors would like to thank Mr. Andreas Kubitza for his contribution in preparing the figures and Manuela Schaetzle for secretarial assistance.
- Bayona NA, Gelb AW, Jiang Z, Wilson JX, Urquhart BL, Cechetto DF: Propofol neuroprotection in cerebral ischemia and its effects on low-molecular-weight antioxidants and skilled motor tasks. Anesthesiology 2004, 100: 1151–1159. 10.1097/00000542-200405000-00017PubMedView ArticleGoogle Scholar
- Belayev L, Alonso OF, Busto R, Zhao W, Ginsberg MD: Middle cerebral artery occlusion in the Rat by intraluminal suture; neurological and pathological evaluation of an improved model. Stroke 1996, 27: 1616–1623. 10.1161/01.STR.27.9.1616PubMedView ArticleGoogle Scholar
- David CA, Prado R, Dietrich WD: Cerebral protection by intermittent reperfusion during temporary focal ischemia in the rat. J Neurosurg 1996, 85: 923–928. 10.3171/jns.1996.85.5.0923PubMedView ArticleGoogle Scholar
- Dempsey RJ, Sailor KA, Bowen KK, Tureyen K, Vemuganti R: Stroke-induced progenitor cell proliferation in adult spontaneously hypertensive rat brain: effect of exogenous IGF-1 and GDNF. J Neurochem 2003, 87: 586–597. 10.1046/j.1471-4159.2003.02022.xPubMedView ArticleGoogle Scholar
- Dogan A, Rao AM, Baskaya MK, Rao VL, Rastl J, Donaldson D, Dempsey RJ: Effects of ifenprodil, a polyamine site NMDA receptor antagonist, on reperfusion injury after transient focal cerebral ischemia. J Neurosurg 1997, 87: 921–926. 10.3171/jns.1997.87.6.0921PubMedView ArticleGoogle Scholar
- Gerriets T, Stolz E, Walberer M, Muller C, Rottger C, Kluge A: Complications and pitfalls in rat stroke models for middle cerebral artery occlusion: a comparison between the suture and the macrosphere model using magnetic resonance angiography. Stroke 2004, 35: 2372–2377. 10.1161/01.STR.0000142134.37512.a7PubMedView ArticleGoogle Scholar
- Gorgulu A, Kins T, Cobanoglu S, Unal F, Izgi NI, Yanik B: Reduction of edema and infarction by Memantine and MK-801 after focal cerebral ischaemia and reperfusion in rat. Acta Neurochir (Wien) 2000, 142: 1287–1292. 10.1007/s007010070027View ArticleGoogle Scholar
- Pena-Tapia PG, Diaz AH, Torres JL: Permanent endovascular occlusion of the middle cerebral artery in Wistar rats: a description of surgical approach through the internal carotid artery. Rev Neurol 2004, 39: 1011–1016.PubMedGoogle Scholar
- Serteser M, Ozben T, Gumuslu S, Balkan S, Balkan E: Lipid peroxidation in rat brain during focal cerebral ischemia: prevention of malondialdehyde and lipid conjugated diene production by a novel antiepileptic, lamotrigine. Neurotoxicology 2002, 23: 111–119. 10.1016/S0161-813X(02)00018-9PubMedView ArticleGoogle Scholar
- Tatlisumak T, Takano K, Carano RA, Miller LP, Foster AC, Fisher M: Delayed treatment with an adenosine kinase inhibitor, GP683, attenuates infarct size in rats with temporary middle cerebral artery occlusion. Stroke 1998, 29: 1952–1958. 10.1161/01.STR.29.9.1952PubMedView ArticleGoogle Scholar
- Williams AJ, Bautista CC, Chen RW, Dave JR, Lu XC, Tortella FC, Hartings JA: Evaluation of gabapentin and ethosuximide for treatment of acute nonconvulsive seizures following ischemic brain injury in rats. J Pharmacol Exp Ther 2006, 318: 947–955. 10.1124/jpet.106.105999PubMedView ArticleGoogle Scholar
- Zhang Y, Wang L, Li J, Wang XL: 2-(1-Hydroxypentyl)-benzoate increases cerebral blood flow and reduces infarct volume in rats model of transient focal cerebral ischemia. J Pharmacol Exp Ther 2006, 317: 973–979. 10.1124/jpet.105.098517PubMedView ArticleGoogle Scholar
- Koizumi J, Yoshida Y, Nakazawa T, Ooneda G: Experimental studies of ischemic brain edema, I: a new experimental model of cerebral embolism in rats in which recirculation can be introduced in the ischemic area. Jpn J Stroke 1986, 8: 1–8. Jpn 10.3995/jstroke.8.1View ArticleGoogle Scholar
- Longa EZ, Weinstein PR, Carlson S, Cummins R: Reversible middle cerebral artery occlusion without craniectomy in rats. Stroke 1989, 20: 84–91. 10.1161/01.STR.20.1.84PubMedView ArticleGoogle Scholar
- Smrcka M, Otevrel F, Kuchtickova S, Horky M, Juran V, Duba M, Graterol I: Experimental model of reversible focal ischeamia in the rat. Scr Med (BRNO) 2001, 74: 391–398.Google Scholar
- Uluç K, Miranpuri A, Kujoth GC, Aktüre E, Baskaya MK: Focal cerebral ischemia model by endovascular suture occlusion of the middle cerebral artery in the Rat. J Vis Exp 2011, (48):e1978. doi:10.3791/1978Google Scholar
- Ma J, Zhao L, Nowak TS Jr: Selective, reversible occlusion of the middle cerebral artery in rats by an intraluminal approach: Optimized filament design and methodology. J Neurosci Methods 2006, 156: 76–83. Abstract-Pubmed 10.1016/j.jneumeth.2006.02.006PubMedView ArticleGoogle Scholar
- Chu K, Kim M, Jung KH, Jeon D, Lee ST, Kim J, Jeong SW, Kim SU, Lee SK, Shin HS, Roh JK: Human neural stem cell transplantation reduces spontaneous recurrent seizures following pilocarpine-induced status epilepticus in adult rats. Brain Res 2004, 1023: 213–221. 10.1016/j.brainres.2004.07.045PubMedView ArticleGoogle Scholar
- Maier CM, Ahern K, Cheng ML, Lee JE, Yenari MA, Steinberg GK: Optimal depth and duration of mild hypothermia in a focal model of transient cerebral ischemia: effects on neurologic outcome, infarct size, apoptosis, and inflammation. Stroke 1998, 29: 2171–2180. 10.1161/01.STR.29.10.2171PubMedView ArticleGoogle Scholar
- Menzies SA, Hoff JT, Betz AL: Middle cerebral artery occlusion in rats: a neurological and pathological evaluation of a reproducible model. Neurosurgery 1992, 31: 100–106. 10.1227/00006123-199207000-00014PubMedView ArticleGoogle Scholar
- Schmid-Elsaesser R, Zausinger S, Hungerhuber E, Baethmann A, Reulen HJ: A critical reevaluation of the intraluminal thread model of focal cerebral ischemia: evidence of inadvertent premature reperfusion and subarachnoid hemorrhage in rats by laser-Doppler flowmetry. Stroke 1998, 29: 2162–2170. 10.1161/01.STR.29.10.2162PubMedView ArticleGoogle Scholar
- Traystman RJ: Animal models of focal and global cerebral ischemia. ILAR J 2003, 44: 85–95. 10.1093/ilar.44.2.85PubMedView ArticleGoogle Scholar
- Walker WF Jr, Homberger DG: Anatomy and Dissection of the Rat. New York: W. H.Freeman and company; 1997:25–26.Google Scholar
- Zausinger S, Westermaier T, Plesnila N, Steiger HJ, Schmid-Elsaesser R: Neuroprotection in transient focal cerebral ischemia by combination drug therapy and mild hypothermia: comparison with customary therapeutic regimen. Stroke 2003, 34: 4526–4532.Google Scholar
- Arvidsson A, Collin T, Kirik D, Kokaia Z, Lindvall O: Neuronal replacement from endogenous precursors in the adult brain after stroke. Nat Med 2002, 9: 963–970.View ArticleGoogle Scholar
- Doerfler A, Forsting M, Reith W, Staff C, Heiland S, Von Schabitz WR, Kummer R, Hacke W, Sartor K: Decompressive craniectomy in a rat model of “malignant” cerebral hemispheric stroke: experimental support for an aggressive therapeutic approach. J Neurosurg 1996, 85: 853–859. 10.3171/jns.1996.85.5.0853PubMedView ArticleGoogle Scholar
- Kokaia Z, Zhao Q, Kokaia M, Elmer E, Metsis M, Smith ML: Regulation of brain-derived neurotrophic factor gene expression after transient middle cerebral artery occlusion with and without brain damage. Exp Neurol 1995, 136: 73–88. 10.1006/exnr.1995.1085PubMedView ArticleGoogle Scholar
- Krinke GJ: The Laboratory Rat. Handbook of Experimental Animals Series. London: Academic Press; 2000:257–259.Google Scholar
- Marcin R: Comparative cranial anatomy of rattus norvegicus and proechimys trinitatus. Undergraduate honors theses. [Newman Library web site] April 3, 2000. Available at: , Accessed 22 September 2007 http://www.baruch.cuny.edu/library/honorstheses/pdf/RichardMarcin.pdf
- He Z, Yamawaki T, Yang S, Day AL, Simpkins JW, Naritomi H: Experimental model of small deep infarcts involving the hypothalamus in rats: changes in body temperature and postural reflex. Stroke 1999, 30: 2743–2751. 10.1161/01.STR.30.12.2743PubMedView ArticleGoogle Scholar
- Fisher M, Tatlisumak T: Use of animal models has not contributed to development of acute stroke therapies: con. Stroke 2005, 36: 2324–2325. 10.1161/01.STR.0000179039.76922.e8PubMedView ArticleGoogle Scholar
- Forsting M, Reith W, Schabitz WR, Heiland S, Von Kummer R, Hacke W, Sartor K: Decompressive craniectomy for cerebral infarction. An experimental study in rats. Stroke 1995, 26: 259–264. 10.1161/01.STR.26.2.259PubMedView ArticleGoogle Scholar
- Mimura T, Dezawa M, Kanno H, Yamamoto I: Behavioral and histological evaluation of a focal cerebral infarction rat model transplanted with neurons induced from bone marrow stromal cells. J Neuropathol Exp Neurol 2005, 64: 1108–1117. 10.1097/01.jnen.0000190068.03009.b5PubMedView ArticleGoogle Scholar
- Takano K, Tatlisumak T, Bergmann AG, Gibson DG 3rd, Fisher M: Reproducibility and reliability of middle cerebral artery occlusion using a silicone-coated suture (Koizumi) in rats. J Neurol Sci 1997, 153: 8–11. 10.1016/S0022-510X(97)00184-6PubMedView ArticleGoogle Scholar
- Garcia JH, Liu KF, Ye ZR, Gutierrez JA: Incomplete infarct and delayed neuronal death after transient middle cerebral artery occlusion in rats. Stroke 1997, 28: 2303–2309. 10.1161/01.STR.28.11.2303PubMedView ArticleGoogle Scholar
- Li F, Omae T, Fisher M: Spontaneous hyperthermia and its mechanism in the intraluminal suture middle cerebral artery occlusion model of rats. Stroke 1999, 11: 2467–2470.Google Scholar
- Barber PA, Hoyte L, Colbourne F, Buchan AM: Temperature-regulated model of focal ischemia in the mouse: a study with histopathological and behavioral outcomes. Stroke 2004,35(7):1720–1725. 10.1161/01.STR.0000129653.22241.d7PubMedView ArticleGoogle Scholar
- Ardehali MR, Rondouin G: Microsurgical intraluminal middle cerebral artery occlusion model in rodents. Acta Neurol Scand 2003, 107: 267–275. 10.1034/j.1600-0404.2003.00010.xPubMedView ArticleGoogle Scholar
- Aspey BS, Cohen S, Patel Y, Terruli M, Harrison MJ: Middle cerebral artery occlusion in the rat: consistent protocol for a model of stroke. Neuropathol Appl Neurobiol 1998, 24: 487–497. 10.1046/j.1365-2990.1998.00146.xPubMedView ArticleGoogle Scholar
- Dittmar M, Spruss T, Schuierer G, Horn M: External carotid artery territory ischemia impairs outcome in the endovascular filament model of middle cerebral artery occlusion in rats. Stroke 2003, 34: 2252–2257. 10.1161/01.STR.0000083625.54851.9APubMedView ArticleGoogle Scholar
- Laing RJ, Jakubowski J, Laing RW: Middle cerebral artery occlusion without craniectomy in rats: which method works best? Stroke 1993, 24: 294–298. 10.1161/01.STR.24.2.294PubMedView ArticleGoogle Scholar
- Virtanen T, Jolkkonen J, Sivenius J: Re. External carotid artery territory ischemia impairs outcome in the endovascular filament model of middle cerebral artery occlusion in rats. Stroke 2004, 35: 3–10.Google Scholar
- Kawamura S, Yasui N, Shirasawa M, Fukasawa H: Rat middle cerebral artery occlusion using an intraluminal thread technique. Acta Neurochir (Wien) 1991, 109: 126–132. 10.1007/BF01403007View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.