December 5, 2024 — Zebrafish embryos actively control their hatching timing which has significant evolutionary implications, offering new insights into neurobiology, survival strategies, and environmental adaptation in vertebrates, according to Hebrew University of Jerusalem researchers.

According to the study published in Science, this discovery uncovers a previously hidden neural circuit that governs one of the most crucial life-stage transitions and demonstrates how fish embryos, far from being passive, possess the ability to actively control their own hatching process.

The process of hatching is a pivotal moment for all egg-laying species. Emerging too soon or waiting too long can mean certain death for a newborn animal, unprepared to navigate the challenges of the outside world. From breathing to evading predators, survival hinges on perfect timing. Remarkably, this timing is dictated by the embryo itself – but until now, the mechanism behind this event was unknown.

Dr. Matan Golan of the Hebrew University Robert H. Smith Faculty of Agriculture, Food, and Environment and the Volcani Institute discovered that fish embryos initiate hatching through a signal from their brain: a neurohormone called Thyrotropin-Releasing Hormone (TRH). TRH travels via the bloodstream to a specialized gland, triggering the release of enzymes that dissolve the egg wall, allowing the embryo to break free. This critical neural circuit for hatching forms just before the event and disappears shortly thereafter. Without TRH, embryos are unable to release the enzymes, resulting in their death inside the egg.

In addition to its evolutionary insights, this research underscores the remarkable ability of embryos to make decisions that directly affect their survival, offering a deeper understanding of the intricate interplay between neurobiology and environmental adaptation.

Looking ahead, the researchers plan to explore how TRH and other neuroendocrine factors influence hatching in other species.

The research paper titled “A transient neuro-hormonal circuit controls hatching in fish” is now available here.

Researchers:

Deodatta S. Gajbhiye^1,2, Genevieve L. Fernandes^1,2, Itay Oz^1,2, Yuni Nahmias^1,2 and Matan Golan^1,2

Institutions:

1. Department of Aquaculture and Poultry, Institute of Animal Sciences, Agricultural Research Organization, Volcani Center
2. Department of Animal Sciences, The Robert H. Smith Faculty of Agriculture, Food, and Environment, The Hebrew University of Jerusalem