Impact Factor 3.915 | CiteScore 3.7
More on impact ›

Editorial ARTICLE

Front. Cardiovasc. Med., 31 July 2020 |

Editorial: Where Is Waldo: Contextualizing the Endothelial Cell in the Era of Precision Biology

  • National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, MD, United States

Many readers may recall Waldo, the North American version of the original character of “Where is Wally?” introduced in 1987 by British author Martin Handford. The book challenged readers to identify—in post-cards depicting crowded scenes in various locales—a solitary, distinctively equipped world traveler carrying situationally appropriate implements: a walking stick, camera, or skis. As readers of the book “sequels” became more astute, Waldo was recognizable in the context of ever more complex and grander scenes.

Like Waldo, the Endothelial Cell (EC) Is Hiding in Plain Sight Everywhere

When viewed in various histological “landscapes” ECs may appear sparse due to their peculiar organizing principle into a single cell-thin layer, the endothelium, the contiguous inner lining of all blood and lymphatic vessels, large and small, distributed everywhere in the body. Covering thousands of square meters, ECs, the third most abundant human cell type (1), strategically interface with all tissues, acting as master body-wide integrators and local mediators of tissue survival and function (2). To this day, the endothelium escapes clinical detection, the “out of sight, out of mind” challenge is a likely reason for still underestimating the EC's importance in health and disease. Definitive proof of the EC's very existence had depended on reaching single cell (SC) level resolution through Electron Microscopy (EM) and the discovery of the pan-EC-specific organelle, the Weibel-Palade body (3). Recent SC technologies extract multi-dimensional information about sparse cells in complex mixtures, thus ready to tackle the EC “scarcity” challenge.

As Observers Have Grown Astute, Just Like Waldo, Ecs Become Recognizable in Ever More Complex Scenarios

The EC makes frequent, yet diverse appearances in different tissue contexts. Extreme examples of EC specialization are found in the key vascular-functional units of different organs, e.g., the “continuous” EC essential for the tight blood-brain barrier vs. the “fenestrated” EC essential for the kidney's filtration function.

Knowledge about ECs has grown through many different types of inquiry. Some investigations have provided “wide and shallow” information, e.g., averaging responses from bulk, mixed populations of ECs cultured in vitro or harvested in situ. Early “narrow and deep” EM interrogations of ECs reporting context-dependent structural and functional heterogeneity (4) were initially questioned as new technology artifacts. While EC heterogeneity is currently believed essential for the function or dysfunction of every tissue (5), the dichotomy between ECs having both body-wide shared and distinctive local variations of structural-functional characteristics remains challenging. The new SC technologies summarized by Chavkin and Hirschi, probing both deeply and widely to extract single EC comprehensive profiles from among more abundant cell assortments, are overcoming both the “sparse” and the “diverse” EC appearances. They should permit us to recapitulate, reconsider, and add new dimensions to the known EC biology. Indeed, Xiang et al. and Feng et al. unravel ECs' exquisite local adaptations based on vessel type or tissue context. Computational tools comparing molecular repertories gathered by various SC technologies and across species, such as those described by Chavkin and Hirschi and Xiang et al., will increase recognition of key molecular features of the EC's functional heterogeneity.

Like Waldo, Ecs Hold Important Contextual Information, and Reveal Their Precise Location

When Waldo is seen carrying his skis, snow is likely close by! Likewise, exquisitely equipped to adapt, in space and in time, to support each tissue's metabolic and functional needs, ECs have inextricable relations to their natural context and are sensitive reporters of neighborhood's workings, metabolic state, and stresses (6) and Xiang et al., and signal transition to dysfunction or aging (7). Precise characterization of the human EC repertoire central to specialized vascular functional units of different organs, including the kidney, brain, lung, or muscle, stands to be of special translational and clinical value.

The potential of using vasculature as a road map to report the location of every SC in the body is described by Weber et al. The concept is based on the known oxygen-dependence of every mammalian cell, thus its proximity to ECs, and known organ-specific vascular hierarchies. The vasculature provides a built-in solution for a Common Coordinate Framework (CCF), a central challenge to developing high-resolution maps of tissues, organs, or the entire body.

ECs' expression of positional information is already used by cancer therapeutic agents delivered to organ-specific EC “zip codes” (8), identified by phage display (9). Ever more precise EC positional information derived from SC analyses suggests they are fitting “cellular landmarks” for a GPS-like precise positioning when developing SC-resolution tissue maps, even if sites of sampling were unknown. For instance, individual ECs were placed along the brain vasculature, while walking on the endothelium from the arterial to venous side via capillaries (10), and within specific zones of the kidney (6) or the lymph nodes by Xiang et al. The EC's closest cellular neighbors may be identifiable by exploiting molecular fingerprints of their interactions (11). We hypothesize that positioning of other SC within the vital <100 microns radius 3D-map centered around a recognizable EC “zip code” may be inferred from the SC expression of hypoxia-driven gene gradients.

Precision Biology and the EC

The new era of “Precision Biology,” combining high content investigative technologies and computational analyses including SC methods, reveals comprehensive cellular profiles in their natural contexts and in action, allowing researchers to overcome the “scarcity” and “heterogeneity” challenges specific to cells such as the EC. Zeroing in on the ECs—apparently sparse yet omnipresent, recognizable yet diverse—through the use of SC methods will pay off in essential knowledge about their cellular neighborhoods, including revealing the underpinnings and states of key functional vascular-tissue units specific to different organs such as the brain, kidney, or heart. Precision EC biology will contribute to the development of a variety of new targeted clinical diagnostics and interventions, “Waldo's” important contribution to the future of precision medicine.

Author Contributions

ZG has conceived and written the editorial and is solely responsible for the content of this article.

Conflict of Interest

ZG is a US Federal employee.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


The author wishes to thank the current contributors to Single Cell Analysis: Advancing Vascular Biology Research Issue for providing the articles cited in this editorial.


EC, endothelial cell; SC, single cell; EM, electron microscopy; CCF, common coordinate framework.


1. Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. (2016) 14:e1002533. doi: 10.1371/journal.pbio.1002533

PubMed Abstract | CrossRef Full Text | Google Scholar

2. Aird WC. Phenotypic heterogeneity of the endothelium: I. Structure, function, and mechanisms. Circ Res. (2007) 100:158–73. doi: 10.1161/01.RES.0000255691.76142.4a

CrossRef Full Text | Google Scholar

3. Weibel ER, Palade GE. New cytoplasmic components in arterial endothelia. J Cell Biol. (1964) 23:101–12. doi: 10.1083/jcb.23.1.101

PubMed Abstract | CrossRef Full Text | Google Scholar

4. Ghitescu L, Galis Z, Simionescu M, Simionescu N. Differentiated uptake and transcytosis of albumin in successive vascular segments. J Submicrosc Cytol Pathol. (1988) 20:657–69.

PubMed Abstract | Google Scholar

5. Aird WC. Phenotypic heterogeneity of the endothelium: II. Representative vascular beds. Circ Res. (2007) 100:174–90. doi: 10.1161/

PubMed Abstract | CrossRef Full Text | Google Scholar

6. Dumas SJ, Meta E, Borri M, Goveia J, Rohlenova K, Conchinha N, et al. Single-cell RNA sequencing reveals renal endothelium heterogeneity and metabolic adaptation to water deprivation. J Am Soc Nephrol. (2020) 31:118–38. doi: 10.1681/ASN.2019080832

PubMed Abstract | CrossRef Full Text | Google Scholar

7. Kovacic JC, Dimmeler S, Harvey RP, Finkel T, Aikawa E, Krenning G, et al. Endothelial to mesenchymal transition in cardiovascular disease: JACC state-of-the-art review. J Am Coll Cardiol. (2019) 73:190–209. doi: 10.1016/j.jacc.2018.09.089

PubMed Abstract | CrossRef Full Text | Google Scholar

8. Narasimhan K. Zip codes: deciphering vascular addresses. Nat Med. (2002) 8:116. doi: 10.1038/nm0202-116

PubMed Abstract | CrossRef Full Text | Google Scholar

9. Arap W, Haedicke W, Bernasconi M, Kain R, Rajotte D, Krajewski S, et al. Targeting the prostate for destruction through a vascular address. Proc Natl Acad Sci USA. (2002) 99:1527–31. doi: 10.1073/pnas.241655998

PubMed Abstract | CrossRef Full Text | Google Scholar

10. Vanlandewijck M, He L, Mae MA, Andrae J, Ando K, Del Gaudio F, et al. A molecular atlas of cell types and zonation in the brain vasculature. Nature. (2018) 554:475–80. doi: 10.1038/nature25739

PubMed Abstract | CrossRef Full Text | Google Scholar

11. Nitzan M, Karaiskos N, Friedman N, Rajewsky N. Gene expression cartography. Nature. (2019) 576:132–7. doi: 10.1038/s41586-019-1773-3

PubMed Abstract | CrossRef Full Text | Google Scholar

Keywords: endothelial cell, endothelium, single cell analysis, endothelial cell heterogeneity, precision biology

Citation: Galis ZS (2020) Editorial: Where Is Waldo: Contextualizing the Endothelial Cell in the Era of Precision Biology. Front. Cardiovasc. Med. 7:127. doi: 10.3389/fcvm.2020.00127

Received: 13 May 2020; Accepted: 19 June 2020;
Published: 31 July 2020.

Edited and reviewed by: Masanori Aikawa, Harvard Medical School, United States

Copyright © 2020 Galis. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Zorina S. Galis,