Early embryos develop… and eliminate threats

By Joan Roncero-Carol and Esteban Hoijman

What is this?

This video shows the outer epithelial layer (cyan) of an early zebrafish embryo actively engulfing Escherichia coli bacteria (yellow).

How was this taken?

This video was obtained using confocal microscopy of a zebrafish blastula (5 hpf) immediately after challenge with mCherry-expressing E. coli. The plasma membranes of epithelial cells were visualized by injecting GPI-GFP mRNA at the 1-cell stage.

Is this relevant for development?

Embryos are exposed to environmental bacteria, which can adversely affect normal development. We observed that embryos actively destroy phagocytosed bacteria, and blocking their ability to clear bacteria impairs embryonic development. These findings suggest that early bacterial clearance is a critical defense mechanism that protects the embryo during its most vulnerable stages.

Why should people care about this?

Because this is the earliest known example of an immune-like defense in development. Although developmental biologists primarily focus on how embryos develop, the influence of their biological environment is often overlooked. Not just in fish, but in mammals as well. For example, at the site where mammalian blastocysts hatch for implantation, they become exposed to the uterine cavity. This environment is prone to bacterial infections, which have been linked to infertility. Since these embryos have yet to form their immune cells, they were long thought to be defenseless against infection. Importantly, we detected clearance of these pathogenic bacteria by both mouse and human embryos. Therefore, we show that innate immunity against bacteria is already active before implantation, mediated by epithelial cells that trigger a comprehensive immune gene program. This finding opens a new perspective on how life protects itself from its very foundations.

How would you explain this to an 8-year-old?

Our bodies fight germs that make us sick with special helpers called immune cells. These cells are really good at catching and destroying germs to keep us healthy. But when we are tiny and developing inside our mom, we don’t have those immune cells yet. We found that other cells we have when we’re so small can still catch and eat germs to keep us safe. It’s like having an early team of protectors before the immune cells arrive, even before our organs are made. This happens at the very beginning of development, when we first meet other living things, like bacteria.

Where can people find more about it?

If you want to learn more about this research, please visit:

https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(25)00208-2

https://www.embryobioimaging.com

_{Images and videos from Cell Host Microbe. 2025;33:1106-1120.e8.}

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