In an effort to navigate ethical boundaries, Israeli stem cell scientist Jacob Hanna has pioneered the development of synthetic “headless embryos” — lab-grown structures that mimic early human development without involving sperm, eggs, or fertilization. These models, derived entirely from stem cells, spontaneously organize into embryo-like forms, raising profound scientific and moral questions. In May of last year, Hanna was detained at a U.S. border checkpoint during a routine inspection. When asked about biological materials, the focus shifted to “embryos.” The question was no longer just about what he carried, but what it meant. Just weeks earlier, a Harvard researcher had been arrested for transporting frog embryos and detained in Louisiana. Though Hanna was not carrying any lab samples, the situation highlighted a growing legal and ethical gray zone: his work involves synthetic embryo models — entities that resemble real embryos but are not created through natural reproduction. Hanna, a leading figure at the Weizmann Institute of Science, has declined direct interviews. Yet over the past three years, MIT Technology Review has tracked his research through presentations, conferences, and ethics panels. What emerges is the portrait of a scientist operating at the frontier of biology — pushing boundaries with techniques that have cleared institutional review but provoke intense debate. His company, Renewal Bio, founded in 2022 with investor Omri Amirav-Drory of NFX, claims to have advanced its synthetic models to at least 28 days of development — possibly even approaching day 40, when rudimentary eyes and limb buds begin to form. While Renewal Bio has not responded to requests for comment, a scientist connected to the company suggests the work may be nearing that stage. At such points, the models resemble early human fetuses in complexity. At a conference in Cambridge, Massachusetts, Hanna openly stated his ambition to generate models reaching day 30, 40, or even day 70 — stages where the embryo would be comparable in size and development to a fetus in its third month of pregnancy. To avoid crossing ethical lines, Hanna proposes genetically modifying these models to prevent the formation of heads, brains, or hearts. “No brain means no consciousness. No consciousness means not a person. So no moral issue — just a collection of organs,” he has argued. The drive behind this work is both scientific and commercial. Human life begins as a single fertilized cell capable of self-organizing into a complete body. For decades, scientists have sought to harness this potential. In the 1990s, researchers began isolating stem cells from five-day-old embryos, creating a source of pluripotent cells that could become any cell type. But despite decades of research, stem cell therapies have delivered only limited clinical success — currently able to generate just a handful of functional cell types. Hanna’s vision is to use synthetic embryos to generate young, healthy cells — such as liver tissue or even eggs — for regenerative medicine. In Europe, Dawn Bio is testing “blastoid” models — synthetic structures that can trigger a positive pregnancy test when placed on uterine tissue, offering new insights into IVF failure. Meanwhile, patent offices in the U.S. and Europe are flooded with applications for these new life-like systems, as universities and companies race to claim ownership. The technology is not just theoretical. In 2022, Renewal Bio secured $9 million in funding and acquired rights to Weizmann Institute patents. The company’s progress is supported by a specialized bioreactor system developed in Israel — a rotating container filled with serum that sustains the growing embryo models. Despite the technical challenges — such as the lack of a circulatory system, which limits size and survival — the field is expanding. In March, a Stanford-based team published a provocative essay in MIT Technology Review introducing the concept of “bodyoids” — lab-grown, self-organizing human-like structures that could one day serve as sources of organs or test subjects for drugs. One author, bioethicist Henry Greely, admitted the idea unsettled him, but he signed the piece because he believed it was too real to ignore. Another co-author, biologist Hiro Nakauchi, revealed he had been drawn into a hidden network of biotech entrepreneurs working on synthetic embryos, artificial wombs, and even body replacement. He was approached by a longevity entrepreneur with a plan to transplant an aging human head onto a cloned young body — a project allegedly operating on a Caribbean island. These developments have drawn concern from the International Society for Stem Cell Research (ISSCR). In June, a committee led by Amander Clark, former ISSCR president and a UCLA fetal development expert, issued a statement warning that some groups are exploring the creation of viable embryos in artificial systems. The group reiterated that any research aiming to grow embryos to a point of potential viability is “unsafe and unethical” and must be banned — regardless of purpose. There is currently no legal framework governing synthetic embryo models. The 2006 U.S. “Fetal Farming Prohibition Act” bans the creation of human fetuses for organ harvesting, but it only applies to pregnancies and uterine development — not to lab-grown models in artificial containers. Similarly, the widely accepted “14-day rule” — which limits the culture of natural human embryos — does not apply to synthetic models, as they are not defined as “embryos” under most laws. Hanna argues that the moral cost is justified: “Growing a model for a month — long enough to generate key blood cells — is ethically acceptable. At that stage, it lacks personhood. It’s a tool for saving lives.” Though the idea of synthetic embryos, head transplants, and artificial wombs sounds like science fiction, it is increasingly becoming the foundation of real biotech ventures. From Colossal’s de-extinction projects to Conception’s efforts to grow human eggs — backed by Sam Altman — the line between speculative fiction and biological reality is blurring. The question remains: who will set the rules? And when do we stop building new life — even in a lab — and start asking what it means to be human?