A team of British and German scientists has managed for the first time to observe what happens in a human embryo between 16 and 19 days after fertilization, during a key stage of its development called gastrulation.
They managed to make a detailed cellular and molecular examination of an ethically donated embryo after a voluntary interruption of pregnancy, and were able to observe which cells originate, when they do so or where they come from.
A long-awaited sample
The study, which was published in the journal Nature, has been carried out with a single specimen, a healthy male embryo, between 16 and 19 days old. At this stage, the embryos are very small (just over a millimeter long) and very difficult to collect.
They assure that the most difficult part of the process was to obtain the donation of the sample, which they waited patiently.
“This is the only sample from such an early stage of development in the more than five years that we have received samples from the HDBR (Human Development Biology Resource),” says Shankar Srinivas , lead author of the study, from the University of Oxford.
“Our study represents a missing link , so to speak, in our understanding of the progression from the fertilized egg to the more mature cell types in the body,” he says.
The stage studied: gastrulation of the embryo
The development phase analyzed corresponds to that of the third week of gestation after fertilization (week 5 of pregnancy), which is known as Carnegie stage 7 , at which time the embryo gastrulation occurs.
Embryonic development exceeds a biological line at 14 days. From that moment on, one of the most important stages of the embryo’s formation begins, which ensures its biological individualization. Until then, the embryos can divide in two or merge.
This process is known as gastrulation and lasts until the 21st. During that week there is an explosion of diversity in the cells aimed at specializing. In fact, this process lays the foundation for the formation of the various types of cells in the body.
The embryo takes the form of a simple flattened disc giving rise to three germ layers (ectoderm, mesoderm and endoderm) with progenitor cells that will become the various tissues and organs that characterize the human being. It is at this stage that many miscarriages and birth defects can occur.
What happens in the embryo in the third week of life?
Thanks to the sequencing of single- cell RNA, the work has provided a detailed description of the types of cells present that develop and diversify at different rates. But the activity of the genes they express and how they change over time during the gastrulation process was also observed, making comparisons with in vitro experimental models that gave similar results.
The researchers detected primordial germ cells ( precursor cells that give rise to eggs or sperm) – although they could not specify where. The work also shows that at this very early stage the human embryo has various types of blood cells , including primitive red blood cells .
In addition, they discovered that the cellular specification of the nervous system had not yet begun at this stage of development, when there were already signs of it in the mouse. “We do not see any neurons in this phase, which indicates that the embryo does not yet have the necessary apparatus to receive sensations or other inputs,” says Srinivas, “something to keep in mind when we review the ’14-day rule’ in relation to with the culture of human embryos ”.
In Babies and more | Pregnancy week by week: from conception to birth
