The challenge of studying human development shortly after implantation has been restricted due to the ethical and technical difficulties connected with in-utero growth post-implantation. Yet, recent breakthroughs have been made using mouse naïve embryonic stem cells (ESCs) to develop embryo-like structures. Extending this method, scientists have now successfully developed complete human embryo models using unmodified human naïve ESCs. These models mimic almost all the known features of post-implantation human embryos up to 14 days after fertilization.

In a revolutionary step, researchers at the Weizmann Institute have cultivated an entity resembling a 14-day-old human embryo using stem cells, without resorting to sperm, eggs, or a womb. Remarkably, this "embryo model" exuded hormones that registered a positive result on a pregnancy test in a laboratory setting.

These embryo models offer a promising and ethically sound method to probe into the earliest phases of human existence. The transition from a vague cluster of cells post-fertilization to a recognizable entity during a prenatal scan undergoes significant transformations, which, unfortunately, are inadequately understood despite being vital as it's a period that often results in miscarriages or birth defects.

The novelty of this research, shared in the Nature journal, lies in its comprehensive replication of primary structures seen in early embryos. This achievement, creating a vivid representation of a 14-day-old human embryo, is unprecedented.

Initiating with naïve stem cells, which can morph into any body tissue, they used chemicals to guide the cells into four primary types present during an embryo's earliest stages. Astonishingly, around 1% of these cultivated cells spontaneously organized into a structure reminiscent of a human embryo. Simply providing the right environment allows the cells to autonomously arrange themselves.

These models matured until they resembled 14-day post-fertilization embryos, a significant juncture in numerous countries that often marks the limit for standard embryo studies.

Through these models, scientists hope to elucidate cellular differentiation processes, observe initial organ formation stages, and gain insights into inherited or genetic conditions. Notably, the study indicates that certain embryo parts won't develop unless enclosed by early placenta cells. This research could even aid in enhancing IVF success rates.

However, this method's present 99% failure rate demands refinement, points out Prof Robin Lovell Badge from the Francis Crick Institute, who otherwise praises the study.

In conclusion, while these models showcase immense potential, using them for pregnancy remains firmly off the table, with ethical, legal, and practical reasons making it an impossibility.

Sources

Weizmann Institute of Science

• Human embryo models grown from stem cells

Nature

• Complete human day 14 post-implantation embryo models from naïve ES cells

BBC News

• Scientists grow whole model of human embryo, without sperm or egg