Early human embryonic cells may be vulnerable to SARS-CoV-2 infection – Image for illustrative purposes only (Image credits: Unsplash)
Riverside, California — Researchers at the University of California, Riverside have uncovered evidence that cells forming the earliest layers of human embryos may face significant vulnerability to SARS-CoV-2, the virus behind COVID-19. Their work, published this week, highlights how these nascent tissues interact with virus-like particles in lab models, shedding light on potential risks during the critical first weeks of pregnancy.[1][2] This discovery arrives amid ongoing questions about long-term effects from maternal infections, urging closer scrutiny of developmental outcomes.
A Lab Model Unlocks Early Development Secrets
Scientists turned to a “disease-in-a-dish” approach to probe stages of human growth that remain elusive in live pregnancies. Ann Song, a doctoral student in molecular, cell, and systems biology, and her advisor Prue Talbot differentiated human embryonic stem cells into key early cell types, including those mimicking the ectoderm, mesoderm, and endoderm.[1] They then exposed these models to SARS-CoV-2 pseudoparticles, harmless mimics that glow upon cell entry to reveal infection patterns.
This method allowed precise measurement of susceptibility without ethical concerns tied to actual embryos. Multiple cell types took up the particles, confirming broad potential access for the virus. Yet variation emerged clearly, with some layers proving far more receptive than others. The technique built on prior stem cell research but marked a novel focus on post-implantation stages, roughly weeks one through four.
Ectoderm Stands Out in Infection Trials
Ectodermal cells, precursors to the skin and nervous system, displayed exceptional vulnerability. Song reported that these cells proved about 23 times more susceptible than undifferentiated embryonic stem cells and six times more so than mesodermal cells, which form muscles and organs.[1] Such disparity underscored a targeted tropism, where the virus favored outer-layer progenitors.
Mesodermal and endodermal counterparts showed lower uptake, though still notable. Undifferentiated stem cells served as a baseline, highlighting how differentiation amplified risks for certain lineages. These results aligned with the pseudoparticles’ design, which replicated viral entry without replication, ensuring safety in the controlled setting. The findings appeared in Frontiers in Cell and Developmental Biology.
Key Biological Drivers of Vulnerability
Several cellular traits explained the ectoderm’s heightened risk. Elevated TMPRSS2 protease activity cleaved viral proteins to enable fusion, a primary entry route. Talbot noted, “Together, these factors create conditions that strongly favor viral attachment and entry and highlight how certain early cell types may be particularly permissive to infection.”[1]
A thinner glycocalyx, the sugar shield on cell surfaces, further eased access to ACE2 receptors, the virus’s main docking site. Ectoderm cells exploited dual pathways – membrane fusion and endocytosis – doubling opportunities compared to other types. These features combined to boost infection efficiency dramatically. Receptor expression alone did not dictate outcomes; structural and enzymatic elements played decisive roles.[2]
Ramifications for Pregnancy and Beyond
The ectoderm’s role in brain and neural formation raises alarms for early pregnancy infections. Potential disruptions could lead to neurodevelopmental delays or structural issues, though real-world transmission remains unproven. Song emphasized, “Our findings highlight urgent need for clinical studies to monitor the long-term neurological and developmental health of infants born to mothers who were infected with COVID-19 during early pregnancy.”[1]
Prior clinical data has linked maternal COVID-19 to varied fetal outcomes, but gaps persist in pre-implantation effects. This lab evidence suggests biological plausibility for vertical transmission risks. Weeks one to four represent a black box in human studies, amplifying the work’s value. Talbot added, “This study gives us a clearer picture of how SARS-CoV-2 can interact with human cells at the earliest stages of life.”[1]
What Matters Now: While lab models show susceptibility, they do not confirm embryo infections in vivo. Clinical follow-ups on exposed pregnancies hold the key to translating these insights into actionable health guidance.
Path Forward Amid Lab Limitations
Investigators stressed the study’s foundational nature. Pseudoparticle assays proved infection potential but sidestepped full viral replication or maternal-fetal barriers. No direct evidence emerged of embryos contracting SARS-CoV-2 in pregnancies, only that early cells could permit entry under ideal conditions.
Funding from the Tobacco-Related Disease Research Program supported the effort, reflecting broader interests in cellular vulnerabilities. Future steps include refining models and pursuing cohort studies on COVID-exposed infants. Talbot concluded that further research would clarify real-world implications. These results invite heightened vigilance in pregnancy care, even as COVID-19 recedes from acute headlines.