On a larger scale, these and other techniques can be used to grow tissues and organs to order. To achieve this, it is necessary to first understand how those tissues develop naturally, and research is under way on a host of body parts, including the immune system, heart,¹⁵ muscle,¹⁶ breasts¹⁷ and skin.¹⁸ Replicating the three-dimensional structures of organs and how they change over time will be difficult, but bioprinting will enable this,¹⁹ and AI is also likely to help optimise tissue engineering.²⁰

Tissue engineers have worked for decades to develop “scaffolds” on which tissues can be grown.²¹ Originally made from synthetics, these are increasingly made from biological materials such as human collagen,²² biological films²³ and decellularised matrices.²⁴ It is vital to understand how the developing tissues interact with these scaffolds.²⁵

A key challenge is to ensure that implanted cells and tissues do not cause harm. It is becoming possible to “cloak” the cells by adding and then overexpressing a set of immune genes, enabling the introduced cells to survive long-term.²⁶ Another promising precaution is a “safety switch”²⁷ that enables the cells to be quickly killed if they prove harmful²⁸ — or, preferably, multiple safety switches for redundancy.²⁹

Organoids have been developed for many body systems, including the intestines,³² kidneys,³³ bone marrow,³⁴ cartilage³⁵ and neurons.³⁶ It has also proved possible to create “gastruloids” that mimic some of the earliest stages of embryonic development.³⁷ Furthermore, organoids can be used to simulate the growth of tumours, shedding light on the internal mechanisms of cancers.³⁸

To create more realistic organoids, it will be necessary to add in more cells and complex tissues. For example, living organs have a blood supply, which affects their development and function.³⁹ Researchers are now developing ways to create vascularised organoids,⁴⁰ but the next generation may also need a lymphatic system and other additions.

Ultimately, organoids could be used to create replacement tissues and organs, serving as an alternative to organ donation. However, at present the culturing of organoids is inefficient, and it is difficult to upscale them to model more complex tissues. For clinical purposes, it will be necessary to validate organoids against “real” tissues or organs, and this is challenging because the necessary in-vitro data is frequently unavailable. One useful benchmark is the Human Cell Atlas⁴¹: however, for testing organoids it would be necessary to create an atlas showing how cells and tissues change during development.

As a result, organoids are not ready to be widely used clinically.⁴² Improvements may emerge from integrating organoids with the related field of organ-on-a-chip,⁴³ and from the use of AI to optimise the construction.⁴⁴

Many unanswered foundational questions in biology — the mechanisms of cell differentiation⁴⁸ and embryo implantation,⁴⁹, for example — could be answered through the study of model embryos.⁵⁰⁵¹

By revealing the processes governing embryonic development, model embryos could enable the development of treatments for genetic diseases and certain forms of infertility. Considerable challenges must be overcome if this is to come to fruition: at present, it is difficult to make model embryos that are both high-fidelity and controllable.⁵² Model embryos would also benefit from a degree of standardisation.⁵³ Digital twins of model embryos may enable us to improve our methods here.

Model embryos pose challenging ethical and regulatory questions because they blur the line between cell cultures and human beings. At present, researchers often deliberately simplify their model embryos: this ensures they stay within regulations, at the cost of limiting the insights that can be gleaned. The UK created new rules to guide model embryo research in 2024,⁵⁴ but these will surely not be the last word on the matter. It may be necessary to reconsider legal norms around the beginnings of development⁵⁵ and to create distinct regulatory systems for model embryos.⁵⁶ Such regulations should also anticipate potential downstream developments, such as growing viable embryos from stem cells, and even “ectogenesis” — growing a human fetus entirely outside the womb.⁵⁷

The project builds on previous work to create simpler synthetic genomes,⁶⁰ beginning with viruses and bacteria,⁶¹ and to “upload” these artificial genomes into living organisms.⁶² Following early proof-of-concept work, researchers have begun creating synthetic genomes that are heavily modified from the originals.⁶³

Synthesising and booting up entire genomes remains challenging, however.⁶⁴ Consequently, many steps in the workflow will need to be optimised before a complete human genome is synthesised. Likewise, it will be necessary to obtain a fuller understanding of the functions of the “natural” human genome⁶⁵ if researchers are to safely design a synthetic version.

In parallel with the technical developments, public dialogue and governance is essential. The public’s expectations will be shaped by early proof-of-concept research, as will industry interest. Regulators will need to develop new frameworks to govern synthetic human genomes, especially if they are used therapeutically or for human enhancement.