Scientists Unravel the Mysteries of Human Embryogenesis and Evolution
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Human life starts from a fertilized egg. One of the most important questions in science is how a fertilized egg develops into a complicated organism with about 200 cell types and 36 important organs. Humans have 25,000 genes. During human development, different genes should be expressed at the right time and right place. Setting the order of gene expression is called as “the Programming of Gene Expression”. Computer software is coded by computer language. Similarly, one of the language for the Programming of Gene Expression is called as “Chromatin Accessibility”.  


Recently, LIU Jiang’s group in Beijing Institute of Genomics, Chinese Academy of Sciences, cooperating with CHEN Zijiang’s group in Center for Reproductive Medicine, Shandong University and LIU Jianqiao’s group in Guangzhou Medical University, has revealed how the “programming language” directs gene expression pattern in human early embryos. This work has been published in Cell on March 8th. 


Normally, each cell in our body can express more than 10,000 genes. However, the first 2 days after fertilization is a very special period during human early embryogenesis. During this period, almost no genes are expressed in human embryos. Until now, it remains unknown how human genomes become active and start expressing new genes in early embryos.  


Traditionally, it needs a million cells to investigate chromatin accessibility landscape. LIU Jiang and his colleagues established a method by using only 50 cells to study it. They find a key molecular OCT4 which plays crucial roles during zygotic genome activation.  


During animal evolution, some genes are originated in the beginning of life on earth, and then these genes are very old genes. In contrast, some genes are originated in mammals, and then these genes are young genes. Specially, some genes are originated only in human, and then these genes are the youngest genes.  

Interestingly, scientists find that older genes usually start the expression at early embryonic stages, and younger genes usually start the expression at later stages. The reason is that the switch controlling the expression of older genes is turned on at earlier embryonic stage.  


This study also finds an interesting result associating with human evolution. DNA mutations can drive human evolution. Transposons are a class of DNA elements, which can jump from one position to another in genome. The jump of transposons can introduce DNA mutation. In human genomes, there are a limited number of transposon have the activity of mobility. Scientists finds that these transposons usually only have activity in early embryos, but not in differentiated tissues. Mutations generated in early embryos can higher change to pass into germ line, and then pass to the next generations. Therefore, activation of transposon in early embryos will have high impact on human evolution.  


LIU Jiang ’s group is focus on embryonic development. They have already published 3 articles in Cell journal. Their previous studies find the rule how DNA methylome is inherited in mammals and fishes. The discovery in this study will open the gate in understanding the regulation of gene expression during human embryogenesis, which lets China in the leading position in the research of human development. This study may benefit the assisted reproductive technology in future.  



The dynamic landscape of open chromatin during early human embryogenesis provides a rich resource and insights into a zygotic genome activation (Image by LIU Jiang's group) 



Prof. LIU Jiang