Fermat's Library | Human and nonhuman primate meninges harbor lymphatic vessels that can be visualized noninvasively by MRI annotated/explained version.

Although the immune-brain connection has been studied and hypothesi...

Lymphatic vessels are responsible for conducting lymph between diff...

The immune connection to neurological diseases has long been hypoth...

A lymph vessel is a thin tube that carries lymph (lymphatic flu...

The blood–brain barrier (BBB) is a highly selective semipermeable m...

Paolo Mascagni (January 25, 1755 – October 19, 1815) was an Italian...

*For correspondence: reichds@

ninds.nih.gov

†

These authors contributed

equally to this work

Competing interest:

See

page 13

Funding: See page 12

Received: 19 June 2017

Accepted: 01 September 2017

Published: 03 October 2017

Reviewing editor: Heidi

Johansen-Berg, University of

Oxford, United Kingdom

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Human and nonhuman primate meninges

harbor lymphatic vessels that can be

visualized noninvasively by MRI

Martina Absinta

1†

, Seung-Kwon Ha

1†

, Govind Nair

, Pascal Sati

Nicholas J Luciano

, Maryknoll Palisoc

, Antoine Louveau

, Kareem A Zaghloul

Stefania Pittaluga

, Jonathan Kipnis

, Daniel S Reich

Translational Neuroradiology Section, National Institute of Neurological Disorders

and Stroke, National Institutes of Health, Bethesda, United States;

Hematopathology Section, Laboratory of Pathology, National Cancer Institute,

National Institutes of Health, Bethesda, United States;

Center for Brain

Immunology and Glia, Department of Neuroscience, School of Medicine, University

of Virginia, Charlottesville, United States;

Surgical Neurology Branch, National

Institute of Neurological Disorders and Stroke, National Institutes of Health,

Bethesda, United States

Abstract Here, we report the existence of meningeal lymphatic vessels in human and nonhuman

primates (common marmoset monkeys) and the feasibility of noninvasively imaging and mapping

them in vivo with high-resolution, clinical MRI. On T2-FLAIR and T1-weighted black-blood imaging,

lymphatic vessels enhance with gadobutrol, a gadolinium-based contrast agent with high

propensity to extravasate across a permeable capillary endothelial barrier, but not with

gadofosveset, a blood-pool contrast agent. The topography of these vessels, running alongside

dural venous sinuses, recapitulates the meningeal lymphatic system of rodents. In primates,

meningeal lymphatics display a typical panel of lymphatic endothelial markers by

immunohistochemistry. This discovery holds promise for better understanding the normal

physiology of lymphatic drainage from the central nervous system and potential aberrations in

neurological diseases.

DOI: https://doi.org/10.7554/eLife.29738.001

Introduction

Recent reports (Aspelund et al., 2015; Louveau et al., 2015b) described the existence of a network

of true lymphatic vessels within the mammalian dura mater that runs alongside blood vessels, nota-

bly the superior sagittal and transverse sinuses. The dural lymphatic vessels display typical immuno-

histochemical markers that identify lymphatic vessels elsewhere in the body. They provide an

alternate conduit for drainage of immune cells and cerebrospinal fluid (CSF) from the brain, beyond

previously described pathways of flow: via arachnoid granulations into the dural venous sinuses, and

via the cribriform plate into the ethmoid region (

Weller et al., 2009). Although early reports, based

on injections of India ink into the cisterna magna of the rat, suggested that the dural pathway

accounts for only a minority of the drainage (

Kida et al., 1993), the more recent studies

(

Aspelund et al., 2015; Louveau et al., 2015b), which are based on injections of fluorescent tracers

and in vivo microscopy, indicate that the dural system may be substantially more important for drain-

age of macromolecules and immune cells than previously realized.

Whether a similar network of dural lymphatics is present in primates remains unknown. Moreover,

noninvasive visualization of the dural lymphatics – a necessary first step to understanding their

Absinta et al. eLife 2017;6:e29738. DOI: https://doi.org/10.7554/eLife.29738 1 of 15

SHORT REPORT

normal physiology and potential aberrations in neurological diseases – has not been reported. We

therefore verified pathologically the existence of a dural lymphatic network in human and nonhuman

primates (common marmoset monkeys) and evaluated two magnetic resonance imaging (MRI) tech-

niques that might enable its visualization in vivo. First, the T2-weighted fluid-attenuation inversion

recovery (T2-FLAIR) pulse sequence, which is the clinical standard for detecting lesions within the

brain parenchyma, is highly sensitive to the presence of gadolinium-based contrast agents in the

CSF (

Mamourian et al., 2000; Mathews et al., 1999; Absinta et al., 2015). Second, ‘black-blood’

imaging sequences, which are typically used for measurement of vascular wall thickness or detection

of atherosclerotic plaque, are tuned to darken the contents of blood vessels (even when they contain

a gadolinium-based contrast agent), but in the process the images may highlight vessels with other

contents and flow properties (

Mandell et al., 2017). For comparison, we also acquired a postcon-

trast T1-weighted Magnetization Prepared Rapid Acquisition of Gradient Echoes (MPRAGE) MRI

sequence, which is widely implemented for structural brain imaging and depicts avid enhancement

of dura mater and blood vessels, but which would not be expected to discriminate lymphatic

vessels.

Results and discussion

Cerebral blood vessels have a highly regulated blood-brain barrier, protecting the neuropil from

many contents of the circulating blood. Under physiological conditions, the blood-brain barrier pre-

vents gadolinium-based chelates in standard clinical use from passing into the Virchow-Robin peri-

vascular spaces and parenchyma, so that these structures do not enhance on MRI. On the other

hand, dural blood vessels lack a blood-meningeal barrier, enabling leakage of circulating fluids and

small substances, including gadolinium-based compounds. This explains the thin, though often

incomplete, dural enhancement that is seen on conventional T1-weighted MRI scans under

eLife digest How does the brain rid itself of waste products? Other organs in the body achieve

this via a system called the lymphatic system. A network of lymphatic vessels extends throughout

the body in a pattern similar to that of blood vessels. Waste products from cells, plus bacteria,

viruses and excess fluids drain out of the body’s tissues into lymphatic vessels, which transfer them

to the bloodstream. Blood vessels then carry the waste products to the kidneys, which filter them

out for excretion. Lymphatic vessels are also a highway for circulation of white blood cells, which

fight infections, and are therefore an important part of the immune system.

Unlike other organs, the brain does not contain lymphatic vessels. So how does it remove waste?

Some of the brain’s waste products enter the fluid that bathes and protects the brain – the

cerebrospinal fluid – before being disposed of via the bloodstream. However, recent studies in

rodents have also shown the presence of lymphatic vessels inside the outer membrane surrounding

the brain, the dura mater.

Absinta, Ha et al. now show that the dura mater of people and marmoset monkeys contains

lymphatic vessels too. Spotting lymphatic vessels is challenging because they resemble blood

vessels, which are much more numerous. In addition, Absinta, Ha et al. found a way to visualize the

lymphatic vessels in the dura mater using brain magnetic resonance imaging, and could confirm that

lymphatic vessels are present in autopsy tissue using special staining methods.

For magnetic resonance imaging, monkeys and human volunteers received an injection of a dye-

like substance called gadolinium, which travels via the bloodstream to the brain. In the dura mater,

gadolinium leaks out of blood vessels and collects inside lymphatic vessels, which show up as bright

white areas on brain scans. To confirm that the white areas were lymphatic vessels, the experiment

was repeated using a different dye that does not leak out of blood vessels. As expected, the signals

observed in the previous brain scans did not appear.

By visualizing the lymphatic system, this technique makes it possible to study how the brain

removes waste products and circulates white blood cells, and to examine whether this process is

impaired in aging or disease.

DOI: https://doi.org/10.7554/eLife.29738.002

Absinta et al. eLife 2017;6:e29738. DOI: https://doi.org/10.7554/eLife.29738 2 of 15

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Figure 1. MRI-visualization of dural lymphatic vessels in human and nonhuman primates. In both species, conventional post-gadobutrol coronal T1-

weighted MRI is unable to discriminate lymphatic vessels due to diffuse physiological enhancement of the dura (arrows) and blood vessels, including

the superior sagittal sinus and straight sinus (arrows). On post-gadobutrol coronal T2-FLAIR and subtraction images, the dura does not enhance, and

Figure 1 continued on next page

Absinta et al. eLife 2017;6:e29738. DOI: https://doi.org/10.7554/eLife.29738 3 of 15

Short report Human Biology and Medicine Neuroscience

physiological conditions (Figure 1), as well as its abnormal diffuse or localized thickening in a variety

of pathological conditions (

Smirniotopoulos et al., 2007; Antony et al., 2015). Using high-resolu-

tion (in-plane resolution 270  270 mm or finer) T2-FLAIR and T1-weighted black-blood MRI images,

obtained after the intravenous injection of standard FDA-approved contrast material (gadobutrol),

we were able to visualize the collection of interstitial gadolinium within dural lymphatic vessels (maxi-

mum apparent diameter ~1 mm) in 5/5 human healthy volunteers and 3/3 common marmoset mon-

keys (

Figure 1). Our results suggest that in the dura, similar to many other organs throughout the

body, small intravascular molecules extravasate into the interstitium and then, under a hydrostatic

pressure gradient, collect into lymphatic capillaries through a loose lymphatic endothelium

(

Sharma et al., 2008).

To further test this hypothesis, meningeal lymphatics were also assessed using a second gadolin-

ium-based contrast agent, gadofosveset, a blood-pool contrast agent suitable for angiography

(

Lauffer et al., 1998). Gadofosveset binds reversibly to serum albumin, increasing its molecular

weight from 0.9 to 67 kDa. Under physiological conditions, albumin has a low transcapillary

exchange rate into the interstitial compartment, estimated to be on the order of 5% per hour, which

explains the propensity of gadofosveset to remain within blood vessels (

Richardson et al., 2015). In

both species, gadofosveset did not reveal dural lymphatics, especially on T1-black blood images

(

Figure 2 and Figure 2—figure supplement 1). As expected, on T1-weighted MPRAGE images,

gadofosveset provided superior intravascular enhancement, in both meningeal and parenchymal

blood vessels, compared to gadobutrol (Figure 2).

On 3D-rendering of subtraction MRI images (

Videos 1–2, Figure 1—figure supplement 1), dural

lymphatics are seen running parallel to the dural venous sinuses, especially the superior sagittal and

straight sinuses, and alongside branches of the middle meningeal artery. The topography of the

meningeal lymphatics fits with the previously described network in rodents as well as our neuropath-

ological data (

Figures 3 and 4). It is worth noting that the lymphatics visualized by MRI are large

slow-flow lymphatic ducts, whereas blind-ending and small lymphatic capillaries, clearly seen by his-

topathology (Figure 3 and Figure 3—figure supplement 1), are unlikely to be revealed by MRI. The

induction of experimental autoimmune encephalomyelitis (EAE) did not affect detection of dural lym-

phatic vessel in either of the two animals that we tested (not shown).

To support our in vivo imaging results, we further investigated the existence and topography of

lymphatics in coronal and longitudinal sections of human and marmoset dura mater. To accomplish

this, we tested a variety of putative lymphatic endothelial markers and found that selective double

immunostaining for D2-40 podoplanin/CD31 and for PROX1/CD31 was the most effective strategy

in discriminating lymphatic vs. venous blood vessels in dura samples – a challenging task since lym-

phatics sprout from transdifferentiation of venous endothelium (

Ny et al., 2005; Yaniv et al., 2006;

Srinivasan et al., 2007; Aspelund et al., 2014; Lowe et al., 2015) and persistently share some

endothelial markers. A branched network of lymphatics was clearly seen within the dura mater. On

D2-40 podoplanin/CD31 double staining, we identified a total of 93 human dural lymphatics; most

were collapsed, explaining the large range of maximum transverse diameters (range = 7–842 mm,

mean = 125 mm, standard deviation = 161 mm). The density of dural lymphatics was higher around

the venous sinuses than in more lateral areas of the dura, and higher within the meningeal layer than

the periosteal layer of the dura. As expected, red blood cells were not seen within lymphatics. In

marmosets, direct comparison between in vivo MRI and histopathology was performed

(

Absinta et al., 2014; Guy et al., 2016; Luciano et al., 2016). As shown in Figure 4 and Figure 4—

figure supplement 1, the three dural vessels detected on coronal postcontrast T2-FLAIR and on

subtraction images colocalize with three clusters of dural cells expressing the full panel of lymphatic

Figure 1 continued

lymphatic vessels (red arrows), running alongside the venous dural sinuses and within the falx cerebri, can be appreciated. Numbers refer to minutes

after the intravenous administration of gadobutrol.

DOI: https://doi.org/10.7554/eLife.29738.003

The following figure supplement is available for figure 1:

Figure supplement 1. 3D rendering of human dural lymphatics.

DOI: https://doi.org/10.7554/eLife.29738.004

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Figure 2. Gadobutrol vs. gadofosveset in MRI-visualization of dural lymphatic vessels. Coronal T1-weighted black-blood images were acquired after

intravenous injection of two different gadolinium-based contrast agents (31 min after gadobutrol and 42 min after gadofosveset), during two MRI

sessions separated by one week. Dural lymphatics (red arrows in magnified view boxes) were better discerned using gadobutrol (standard MRI contrast

agent, which readily enters the dura) compared to gadofosveset (serum albumin-binding contrast agent, which remains largely intravascular) and were

localized around dural sinuses, middle meningeal artery, and cribriform plate (white arrows). Notably, the choroid plexus (white arrows) enhanced less

Figure 2 continued on next page

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endothelial markers (LYVE-1, D2-40 podoplanin, PROX1, COUP-TFII) and CCL21, a chemokine impli-

cated in lymphatic transmigration.

In the ongoing debate about the precise localization of lymphatics within the meninges (either

completely within the dura or ‘shared’ between the dura and the arachnoid) (

Kipnis, 2016;

Raper et al., 2016), our pathological data clearly show that at least some lymphatics are contained

entirely within the dura (

Figures 3 and 4). On limited evaluation, we were unable to visualize lym-

phatics within the leptomeninges, but additional dedicated studies, ideally with nonconventional tis-

sue-preparation methods, are warranted to fully explore this possibility. A comprehensive map of

the meningeal lymphatic network would have implications for unraveling the ways in which the men-

ingeal lymphatics participate in waste clearance and in immune cell trafficking within the central ner-

vous system (

Louveau et al., 2015a; Kipnis, 2016; Raper et al., 2016).

In inflammatory pathological conditions, cellular migration toward the dural lymphatics might be

profoundly enhanced by specific signaling and lymphatic plasticity (

Alitalo et al., 2005; Kim et al.,

2012

; Stacker et al., 2014 ). Noteworthy in this context are clusters of extravascular CD3+ lympho-

ocytes and CD68+ phagocytic meningeal macrophages that we observed in the dura of several mul-

tiple sclerosis autopsies (not shown), confirming intense immune cell trafficking and communication.

Indeed, without proper normative comparison, we cannot rule out that the extensive presence of

small lymphatics observed in multiple sclerosis dura samples is the result of inflammation-mediated

lymphoangiogenesis. On the other hand, lymphatic dysfunction might impair waste clearance in neu-

rodegenerative diseases and aging, in line with the recently captured deposition of b-amyloid in

human dura in elderly people (

Kovacs et al., 2016).

Differently from experiments implementing

injections of tracers within brain structures, here

we aimed primarily to image dural lymphatic ves-

sels in human and nonhuman primates, but we

could not prove whether dural lymphatic vessels

drain immune cells, CSF, or other substances

from the brain to deep cervical lymph nodes, nor

could we assess any link with the glymphatic sys-

tem (

Iliff et al., 2012; Xie et al., 2013;

Iliff et al., 2013). Such an analysis would proba-

bly require injection of specific MRI-detectable

tracers and acquisition of MRI time-series (not

thoroughly explored in the current work), an

important future research direction for the non-

human primate work.

Overall, our data clearly and consistently

demonstrate the existence of lymphatic vessels

within the dura mater of human and nonhuman

primates. Together with recent studies in

rodents, our results show that the meningeal

lymphatic system is evolutionarily conserved in

mammals and confirm, after exactly two centu-

ries, what the Italian anatomist Paolo Mascagni

speculated were lymphatic vessels at the surface

of human brain (

Mascagni and Bellini, 1816).

Figure 2 continued

with gadofosveset than gadobutrol, whereas meningeal and parenchymal blood vessels (both veins and arteries) did not enhance with any contrast

agent and appeared black. On conventional T1-weighted MPRAGE images, meningeal and parenchymal blood vessels enhanced with both contrast

agents, more clearly with gadofosveset.

DOI: https://doi.org/10.7554/eLife.29738.005

The following figure supplement is available for figure 2:

Figure supplement 1. MRI visualization of dural lymphatic vessels in a healthy marmoset with different contrast agents.

DOI: https://doi.org/10.7554/eLife.29738.006

Video 1. 3D-rendering of dural lymphatics (green) in a

47 year old woman from skull-stripped subtraction T1-

black-blood images (horizontal view, 180 degrees, 7

frames/minute).

DOI: https://doi.org/10.7554/eLife.29738.014

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The ability to image the meningeal lymphatics

noninvasively immediately suggests the possibil-

ity of studying potential abnormalities in human

neurological disorders.

Materials and methods

Approvals

We carried out human studies under a protocol

(

NCT02504840) approved by NIH Institutional

Review Board. Informed consent was obtained

from all participants. Formalin-fixed human

brains were attained at autopsy after obtaining

consent from the next of kin. Animal studies

were performed under a protocol approved by

the Institutional Animal Care and Use

Committee.

Human imaging

We studied five healthy volunteers (three

women, age range 28–53 years) and obtained

scans on a 3-tesla MRI unit (Skyra, Siemens

Healthcare, Erlangen, Germany), using the body

coil for radiofrequency transmission and a 32-

element phased array coil for reception.

Prior to injection of gadolinium-based contrast agent, the following high-resolution MRI sequen-

ces were collected:

1. Whole-brain T1-Magnetization Prepared Rapid Acquisition of Gradient Echoes (MPRAGE, sag-

ittal 3D turbo-fast low angle shot [TFL] sequence, acquisition matrix 256  256, isotropic reso-

lution 1 mm, 176 slices, repetition time [TR]/echo time [TE]/inversion time [TI]=3000/3/900 ms,

flip angle 9, acquisition time 5 min 38 s);

2. Limited T2-weighted Fluid Attenuation Inversion Recovery (FLAIR, coronal 2D acquisition over

the superior sagittal sinus, field-of-view 256  256, 22 slices, reconstructed in-plane resolution

0.25 mm x 0.25 mm, 42 contiguous 3 mm slices, TR/TE/TI = 6500/93/2100 ms, echo train

length 17, bandwidth 80 Hz/pixel, acquisition time 5 min), optimized for detection of gadolin-

ium-based contrast agent in the subarachnoid space (

Absinta et al., 2015);

3. Black-blood scan (coronal 2D acquisition, Sampling Perfection with Application optimized Con-

trasts using different flip angle Evolution [SPACE] sequence, field-of-view 174  174, matrix

320  320, reconstructed in-plane resolution 0.27  0.27 mm, 64 contiguous 0.5 mm sections,

TR/TE = 938/22 ms, echo train length 35, bandwidth 434 Hz/pixel, acquisition time 7 min 50

s). A series of 2 or three overlapping coronal acquisitions were acquired to cover most of the

cerebral hemispheres;

4. Whole-brain T2-FLAIR scan (coronal 3D acquisition, SPACE sequence, field-of-view 235  235,

matrix 512  512, reconstructed in-plane resolution 0.46  0.46 mm, 176 1 mm sections, TR/

TE/TI = 4800/354/1800 ms, nonselective inversion pulse, echo-train length 298, bandwidth 780

Hz/pixel, acceleration factor 2, acquisition time 14 min);

5. Whole-brain T1-SPACE (axial 3D acquisition, acquisition matrix 256  256, isotropic resolution

0.9 mm, 112 sections, TR/TE = 600/20 ms, flip angle 120, echo-train length 28, acquisition

time 10 min).

We repeated these acquisitions after injection of gadobutrol (Gadavist, Bayer HealthCare, Whip-

pany, NJ) in all five participants. In 2 of the participants, we also repeated the entire protocol prior

to and following intravenous injection of gadofosveset (

Lauffer et al., 1998) (Ablavar, Lantheus

Medical Imaging, N Billerica, MA). Injections followed the manufacturer’s suggested dosing (0.1

mmol/kg for gadobutrol, 0.03 mmol/kg for gadofosveset).

Video 2. 3D-rendering of dural lymphatics (green) in a

6.7-year-old common marmoset from skull-stripped

subtraction T2-FLAIR images (horizontal view, 180

degrees, 7 frames/minute).

DOI: https://doi.org/10.7554/eLife.29738.015

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Figure 3. Histopathology of human dural lymphatic vessels. (A–B) Whole-mount and coronal views of the human dura mater before sampling for

histological analysis. The dotted line in A shows where the superior sagittal sinus runs within the dural layers. (C) Coronal section of human dura stained

with H and E to highlight anatomical features of interest, including the falx cerebri and the dural venous sinus. Note the distortion of the dura after

paraffin embedding in comparison to B. (D, F, G) Within the dura mater, lymphatic and blood vessels can be differentiated using double

Figure 3 continued on next page

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Marmoset imaging

We studied three healthy adult common marmosets (Callithrix jacchus) (one female, two males, age

range 4–11 years). After the baseline MRI, experimental autoimmune encephalomyelitis (EAE) was

induced in 2 marmosets with 0.2 mg of fresh-frozen human white matter homogenate as previously

described (

Gaita

n et al., 2014). Marmosets were placed in a sphinx position within the magnet, and

scans were obtained on a 7-tesla MRI unit (Avance AVIII, Bruker BioSpin, Billerica, MA, USA). Data

acquisition was performed in transmit-only/receive-only mode using a homemade linear birdcage

coil (120 mm inner diameter) as a radiofrequency transmitter and a homemade, 8-channel radio-fre-

quency surface-array receiver coil assembly placed over the head of the animal. Prior to injection of

contrast agent, we collected:

1. Whole-brain T1-weighted MPRAGE scan (coronal 3D acquisition, Modified Driven Equilibrium

Fourier Transform [MDEFT] sequence, in-plane voxel size 0.15 mm x 0.15 mm, 36 contiguous 1

mm sections, TR/TE/TI = 12.5/4/1200 ms, flip angle 12 degrees, 2 segments of 1800 ms, acqui-

sition time 7 min);

2. Whole-brain T2-FLAIR scan (coronal 2D acquisition, Rapid Acquisition with Relaxation

Enhancement [RARE] sequence, voxel size 0.15 mm x 0.15 mm, 36 contiguous 1 mm sections,

TR/TE/TI = 10,000/36/2500 ms, flip angle 90–180 degrees, 2 averages, acquisition time 13

min).

We performed the same scans following intravenous injection of gadobutrol and gadofosveset, in

two different MRI sessions. Injections used single (0.1 mmol/kg for gadobutrol, 0.03 mmol/kg for

gadofosveset) or triple the recommended human dosing (0.3 mmol/kg for gadobutrol, 0.09 mmol/

kg for gadofosveset).

Image processing

Scanner-generated DICOM images were converted into NIFTI files for postprocessing. Using MIPAV

software (

https://mipav.cit.nih.gov), precontrast scans were rigidly registered to postcontrast scans,

and subtraction images were created for anatomical identification of dural lymphatic vessels. 3D

skull-stripped subtraction images were then imported into Osirix software for maximum intensity

projection (MIP) 3D rendering. The same postprocessing was performed for scans acquired after

gadofosveset injection, and a direct comparison between the two gadolinium-based contrast agents

was made.

Neuropathological evaluation of human and primate dural lymphatic

vessels

Neuropathological evaluation focused on human and marmoset brain dura mater samples. Multiple

human dura samples were obtained from 2 formalin-fixed brains (60- and 77-year-olds with long-

standing progressive multiple sclerosis) and from a 33-year-old with refractory epilepsy undergoing

anterior temporal lobectomy (

Figure 3—source data 1). Primate samples were obtained from 3

adult common marmosets (2 with EAE) [Figure 4—source data 1]. After general anesthesia and

transcardial perfusion of 4% paraformaldehyde, marmoset brains were extracted and stored in 10%

Figure 3 continued

immunostaining for PROX1 (a transcription factor involved in lymphangiogenesis, nuclear staining) and CD31 (a vascular endothelial cell marker). (E, H–

K) Similarly, lymphatic and blood vessels can be differentiated using immunohistochemical (E, H, I) and immunofluorescent (J, K) double staining for

podoplanin (D2-40, endothelial membrane staining) and CD31. Red blood cells are seen within blood vessels, but not within lymphatic vessels. Insets

(F–I) were rotated relative to the original figures in D and E. Abbreviations: H and E: hematoxylin and eosin; LV: lymphatic vessels; BV: blood vessels.

DOI: https://doi.org/10.7554/eLife.29738.007

The following source data and figure supplements are available for figure 3:

Source data 1. Table of human tissue sampling.

DOI: https://doi.org/10.7554/eLife.29738.010

Figure supplement 1. Histopathology of human dural lymphatic vessels.

DOI: https://doi.org/10.7554/eLife.29738.008

Figure supplement 2. Positive and negative control staining for lymphatic vessels.

DOI: https://doi.org/10.7554/eLife.29738.009

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