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