Lymphatic vessels are best known as the body’s drainage network, collecting interstitial fluid, ferrying immune cells, and absorbing dietary fats. In most textbook accounts, they are supporting infrastructure rather than active participants in organ construction. The new study, published in the journal Genes & Development, challenges that view. Led by senior author Guillermo Oliver, the Thomas D. Spies Professor of Lymphatic Metabolism at Northwestern, the research demonstrates that cardiac lymphatic vessels send molecular signals to multiple types of heart cells, actively shaping how the growing heart regulates its own size.

The work builds directly on an earlier discovery from the Oliver laboratory, which showed that lymphatic vessels in the heart secrete a protein called Reelin. Reelin is famous in neuroscience, where it orchestrates the layered migration of neurons in the developing brain, but its role in the heart was a surprise. In that earlier work, the team found that Reelin promotes cardiac growth by stimulating the proliferation of cardiomyocytes, the contractile muscle cells that make up the bulk of the heart. When cardiomyocytes divide during embryonic development, the heart enlarges; when they stop dividing, as happens shortly after birth in mammals, that growth largely ceases.

The new study reveals that Reelin’s influence extends well beyond heart muscle. Using single-cell RNA sequencing, a technique that profiles gene expression in thousands of individual cells at once, combined with genetically engineered mouse models in which lymphatic-derived Reelin was eliminated, the researchers mapped a signaling pathway that connects the cardiac lymphatics to the epicardium, the thin layer of tissue that envelops the heart’s outer surface. The epicardium is far more than a protective membrane. During development it serves as a source of fibroblasts, coronary vascular cells, and growth factors, making it essential for normal cardiac morphogenesis.

What the team observed in mice lacking lymphatic-derived Reelin was striking. These hearts developed significant defects in the epicardium and lost specific populations of fibroblasts, the cells that build and maintain the heart’s structural scaffold. The investigators also documented a reduction in the number of epicardial cells expressing two critical cardiac factors: WT1, a transcription factor with well-established roles in epicardial biology and cardiac progenitor function, and insulin-like growth factor-1, or IGF-1, a potent growth factor that drives tissue expansion. In other words, removing a single lymphatic signal disrupted an entire regulatory network governing cardiac growth.

“We recently identified the cardiac lymphatic vasculature as an unexpected regulator of heart size during embryonic development,” Oliver said. “Here, by combining single-cell RNA sequencing with loss-of-function mouse models, we uncover another novel and unexpected critical regulatory network also participating in this process.” According to Oliver, the findings show that Reelin is not merely a muscle-cell proliferation signal but is essential for the maintenance and expansion of other cardiac cell types, including the epicardium and fibroblasts, and for sustaining the expression of WT1 and IGF-1, which together are responsible for the growth of the heart.

To test whether the mechanism holds in human tissue, the researchers turned to laboratory-grown human epicardioids, three-dimensional organoids that recapitulate key features of the developing heart’s outer lining. These mini-hearts, cultivated from human cells, allowed the team to probe Reelin’s effects in a controlled system that is more representative of human biology than mouse tissue alone. When the organoids were treated with Reelin protein, the expression of multiple epicardial markers increased, indicating that the protein promoted epicardial cell fate. This result supports the idea that Reelin directly influences epicardial maintenance during development and can even induce epicardial characteristics in the adult heart.

That last point carries significant implications for regenerative medicine. The mammalian heart loses most of its regenerative ability shortly after birth, and the epicardium, so active during embryogenesis, becomes largely dormant in the adult. Yet this dormant layer retains latent developmental programs that can be partially reawakened after injury, such as a heart attack, when the epicardium thickens and re-expresses genes like WT1. The Northwestern findings suggest that the beneficial effects of lymphatic vessels during adult cardiac injury may be partly mediated by Reelin reactivating the dormant epicardium, potentially leading to more efficient repair. If researchers could one day harness or mimic that signal, they might enhance the heart’s limited self-repair capacity after a myocardial infarction, the leading cause of death worldwide.

Timing appears to be everything. Oliver noted that Reelin expression in cardiac lymphatics declines progressively after birth and becomes nearly undetectable by day seven of life in mice. That postnatal decrease coincides precisely with the window in which newborn mice lose their ability to regenerate heart tissue, a phenomenon first described in landmark studies showing that a one-day-old mouse can fully repair its heart after surgical injury, while a seven-day-old mouse cannot. “This postnatal decrease coincides with the cessation of cardiac regeneration in newborn mice, suggesting that loss of Reelin may contribute to the arrest of these processes,” Oliver said. The correlation does not prove causation, but it provides a compelling lead for scientists trying to understand why the regenerative capacity of the heart switches off so early in life.

Beyond regeneration, the work may illuminate the origins of congenital heart defects, which affect roughly one percent of live births and often involve faults in the very cell types and signaling pathways the study identifies. If lymphatic-derived signals are required for proper epicardial maintenance, fibroblast development, and growth factor expression, then disruptions in cardiac lymphatic development could plausibly contribute to structural heart malformations, a hypothesis the authors suggest is worth investigating in future work.

Oliver and his colleagues have also proposed a broader and provocative conceptual model: that lymphatic vessels may function as organ-wide monitoring systems that help determine when growth is complete. As lymphatics grow into a developing organ, they are uniquely positioned to sample functional features of the tissue around them, including interstitial fluid pressure, the accumulation of metabolic waste, and tissue stiffness, all of which change as an organ matures. “What we propose is that it’s possible that lymphatics function as organ-level quality control systems,” Oliver said. In this view, the vessels continuously assess the organ’s condition and relay instructions: keep growing, keep proliferating, until the tissue reaches an appropriate state, and then stop.

If that model proves correct across organ systems, it would reframe the lymphatic vasculature from a passive drainage network into an active, instructive component of organogenesis, with Reelin as one of its molecular messengers. It would also connect two fields that have historically developed in parallel, lymphatic biology and cardiac developmental biology, around a shared question of size control. The technical achievements underpinning the study, from single-cell transcriptomics that resolved the cross-talk between lymphatic endothelial cells, epicardial cells, and fibroblasts, to the generation of human epicardioids that allowed direct functional testing, illustrate how modern tools are enabling biologists to dissect these multicellular conversations with unprecedented precision.

Much remains to be determined. The study was conducted in mice and in lab-grown organoids, and translating the findings to human therapy will require years of additional research, including work to establish whether Reelin signaling can be safely and effectively modulated in the injured adult heart. Still, the central message is clear and conceptually striking: the heart’s blueprint for growth is not written in muscle cells alone. It is co-authored by the lymphatic vessels threading through the organ, which whisper instructions to the epicardium, sustain the fibroblasts that build its framework, and keep the growth factors flowing until the job is done. Understanding that conversation, the Northwestern team argues, could ultimately give science new insight into how the heart responds to and repairs itself after a heart attack, turning a developmental curiosity into a potential therapeutic frontier.

Subject of Research: The role of lymphatic-derived Reelin signaling in regulating epicardial maintenance and cardiac growth during heart development

Article Title: Study reveals a new way the developing heart controls its growth

Article References: Study reveals a new way the developing heart controls its growth. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cardiac lymphatics, Reelin, epicardium, heart development, cardiomyocyte proliferation, fibroblasts, WT1, IGF-1, organoids, regeneration, congenital heart defects, single-cell RNA sequencing

The heart is an organ with an unforgiving relationship to its blood supply. Cardiac muscle works continuously and demands a constant delivery of oxygen, which means the coronary vasculature must grow in careful proportion to the muscle it serves. Too little vessel growth starves the tissue; too much creates a disorganized, inefficient network. During development, and again after injury, the heart must therefore solve a problem that engineers would recognize immediately: building the right amount of blood vessel, and knowing precisely when to stop. The new study suggests that the zebrafish heart solves this problem with a dedicated cellular control system, one that had gone unnoticed until now because it only becomes prominent under specific conditions of growth and repair.

The research is rooted in a question that has animated the field for years. Zebrafish possess a remarkable ability that mammals largely lack: after significant injury to the heart, they can regenerate the damaged muscle almost completely, restoring function rather than forming a permanent scar. Understanding how they accomplish this has become a major strategy for identifying the regenerative programs that might, in principle, be reawakened in human patients. Dr. Jingli Cao, associate professor of cell and developmental biology at Weill Cornell Medicine and a member of its Cardiovascular Research Institute, has spent much of his career tracing this process. As a postdoctoral fellow at Duke University, he determined that the epicardium, the thin layer of cells that envelops the heart, provides both the signals and the cells needed to regenerate heart tissue in zebrafish.

The epicardium has long been viewed as more than passive packaging. In response to injury, it activates a specific population of progenitor cells that give rise to the various cell types involved in regenerating heart muscle and coronary blood vessels. But a deeper puzzle remained: what directs these precursor cells to specialize into one fate or another? To answer it, Dr. Cao’s team, including graduate student Björn Perder and postdoctoral fellow Dr. Yu Xia, catalogued the genes that switch on in these cells when the heart is injured. Among the activated genes they identified a master regulator called scxa, a transcription factor that steers a subset of epicardial progenitor cells toward a fate that had never been characterized before.

That fate turned out to be a novel perivascular cell type. Once specified by scxa, these cells migrate to the coronary vessels and surround them, taking up a position that immediately suggested a regulatory role. Perivascular cells in general are known to modulate vessel stability, maturation, and remodeling, so the location of the new cell type was a strong clue. The team found that these cells express a particular type of collagen, and that this collagen does more than provide structural support. The cells process the collagen molecule to release a small protein fragment, a derived signal that ultimately curbs further vessel expansion. In other words, the very cells that accompany the growing vasculature carry within them the molecular message that tells the network when enough is enough.

Dr. Cao framed the significance of this arrangement in practical terms. During development, or when the heart is damaged, the organism needs to build the right amount of blood vessel, and it needs to know when to stop, he noted. The newly discovered system provides that layer of control. This kind of negative feedback is a recurring theme in biology, but identifying the specific cell type and the specific signaling molecule responsible in the heart gives researchers a concrete handle on a process that has been difficult to study. It also suggests that disorders of vascular growth in the heart, whether insufficient vessel formation after injury or excessive, disorganized growth, might be traceable to failures of this regulatory circuit.

Perhaps the most intriguing aspect of the study is what it reveals about timing. The researchers had previously shown that when regions of the heart experience low oxygen levels, a condition known as hypoxia, signals from the epicardium coordinate the growth of heart muscle and coronary vessels so that the two develop in step with each other. The new findings illuminate a plausible mechanism for how this coordination happens. Low oxygen appears to act as an environmental cue that temporarily activates scxa, which in turn directs a subset of epicardial progenitors toward the perivascular fate. These cells then surround the coronary vessels and may help regulate their development and remodeling. Hypoxia, in this model, is not merely a stress signal but an instruction: it tells the epicardium that the tissue is outpacing its blood supply and that vascular support cells are needed now.

This oxygen-sensing logic has an elegant developmental rationale. As heart muscle proliferates, the growing tissue inevitably pushes local oxygen levels down, because diffusion cannot keep pace with expansion. By using hypoxia as the trigger for scxa activation, the heart ensures that vessel-supporting cells are generated exactly where and when the muscle is growing fastest. The same mechanism likely operates after injury, when the damaged tissue becomes hypoxic and the epicardium mounts its regenerative response. If the system works as the study suggests, the perivascular cells and their collagen-derived braking signal would form a self-correcting loop: hypoxia recruits the cells, the cells support vessel growth, and the vessels restore oxygen delivery, at which point the signal to expand fades.

The translational implications are considerable, though the researchers are careful about the distance between zebrafish and humans. Humans do carry a related gene, SCX, but no one yet knows whether human epicardial cells use it in the same way as their zebrafish counterparts. There is a cautionary wrinkle: after an injury such as a heart attack in mammals, SCX is switched on in cardiac fibroblasts, the cells that promote scar tissue formation. In mammals, then, the same regulatory gene appears to be associated with fibrosis rather than regeneration. Dr. Cao suggests that in the future it may be possible to use SCX to reactivate human epicardial cells and steer them toward generating the cells and signals that promote heart regeneration, redirecting the gene’s activity away from scarring and toward repair.

Toward that end, Dr. Cao’s laboratory is working with cultured human epicardial cells and cardiac organoids, laboratory-grown miniature tissue models, to develop ways of using activated cells to repair damaged regions of the heart. The vision is a kind of biological bandage: a living material that could be applied to an injured site and induce controlled growth of both heart muscle and vessels, rebuilding the tissue in a coordinated fashion rather than leaving a non-contractile scar. Dr. Cao is quick to temper expectations, noting that he does not believe a single magic factor controls regeneration. But, as he put it, the team has discovered one more factor, one more mechanism that could eventually contribute to turning on epicardial cells and repairing human hearts. The work was supported by the New York State Stem Cell Science program, the Louis and Rachel Rudin Foundation, the Belgian American Educational Foundation, the American Heart Association, and the National Institutes of Health, reflecting the broad institutional bet that the zebrafish heart still has many secrets worth stealing.

Subject of Research: Discovery of a novel scxa-specified perivascular cell type that regulates coronary vascular development in the regenerating zebrafish heart

Article Title: Newly discovered cell helps shape blood vessel growth in the heart

Article References: Newly discovered cell helps shape blood vessel growth in the heart. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: zebrafish, heart regeneration, epicardium, perivascular cells, coronary vessels, scxa, hypoxia, collagen, Weill Cornell Medicine, Nature Communications, cardiac progenitor cells, vascular development