From egg to adult: insights into human embryonic development

South Korean scientists have revealed new insights into the process of human embryonic development using large-scale, whole-genome sequencing of cells and tissues from adult humans. Their study, published in the journal Nature, is said to be the first to analyse somatic mutations in normal tissue across multiple organs within and between humans.

How a human develops from a single fertilised egg to a fully grown adult is a fundamental question in biomedical science — but due to the ethical challenges of performing studies on human embryos, the details of this process remain largely unknown. To overcome these issues, the research team — led by the Korea Advanced Institute of Science and Technology (KAIST) — took a different approach.

The researchers analysed genetic mutations in cells taken from adult human post-mortem tissue. Specifically, they identified mutations that occur spontaneously in early developmental cell divisions. These mutations, also called genomic scars, act like unique genetic fingerprints that can be used to trace the embryonic development process.

The team’s study, which looked at 334 single-cell colonies and 379 tissue samples from seven recently deceased human body donors, is believed to be the largest single-cell, whole-genome analysis carried out to date. The researchers examined the genomic scars of each individual in order to reconstruct their early embryonic cellular dynamics, with the results revealing several key characteristics of the human embryonic development process.

Firstly, mutation rates are higher in the first cell division, but then decrease to approximately one mutation per cell during later cell division. Secondly, early cells contributed unequally to the development of the embryo in all informative donors, for example, at the two-cell stage, one of the cells always left more progeny cells than the other. The ratio of this was different from person to person, implying that the process varies between individuals and is not fully deterministic.

The researchers were also able to deduce the timing of when cells begin to differentiate into individual organ-specific cells. They found that within three days of fertilisation, embryonic cells began to be distributed asymmetrically into tissues for the left and right sides of the body, followed by differentiation into three germ layers, and then differentiation into specific tissues and organs.

“It is an impressive scientific achievement that, within 20 years of the completion of human genome project, genomic technology has advanced to the extent that we are now able to accurately identify mutations in a single-cell genome,” said KAIST’s Professor Young Seok Ju. “This technology will enable us to track human embryogenesis at even higher resolutions in the future.”

The techniques used in the study could also be used to improve our understanding of rare diseases caused by abnormalities in embryonic development, and to design new precision diagnostics and treatments for patients.

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