A previously unknown type of cell that self-destructs within days after
formation as part of a quality control procedure to protect the growing
fetus has been found by researchers looking at gene activity data of the
early human embryo. The discoveries shed light on what transpires during the
earliest phases of life following fertilization, which may one day aid to
enhance IVF or regenerative medicine therapies.
An international team of experts, including those from the University of
Bath, found in a recent study that was published on June 20, 2023 in PLoS
Biology, suggests that our initial development in the womb may be rather
different from what we have previously imagined.
We all began as only one cell, the fertilized egg, even though mature
humans are made up of billions of cells. This splits into two cells, which
divide into four cells, which divide into eight cells, and so on. The cells
eventually begin to specialize in their task. Some will be diverted to form
the placenta while others will become the embryo, like trains diverted to
various terminal stations.
embryonic cell that self-destructs
A fourth of the cells didn't fit the profile of any of the recognized cell
categories (pre-embryo, pre-placenta, etc.), according to analysis of
previously published data on gene activity of each individual cell from
5-day-old embryos.
Further research revealed that these cells possessed what are known as
"Young transposable elements" or "jumping genes." These are renegade DNA
components that may replicate and re-insert themselves into human DNA,
frequently causing harm in the process.
The presence of the cells with proteins resulting from the jumping genes
was established by staining of embryos by project colleagues in Spain.
A bit later along in time, the scientists discovered that their offspring
experience both DNA damage and programmed cell death.
quality assurance procedure
The researchers hypothesize that this mechanism resembles a type of quality
control, selecting cells in favor of the best.
Dr. Zsuzsanna Izsvák, co-senior author and an authority on mobile DNA at
the Max Delbrück Center, stated: "Humans, like other species, engage in a
never-ending cat-and-mouse game with these hazardous jumping genes.
"While we make every effort to inhibit these jumping genes, they remain
active in some cells very early in development, perhaps because our genetic
defenses cannot evolve quickly enough."
Professor Laurence Hurst, co-lead author and director of the Milner Centre
for Evolution at the University of Bath, said: "The embryo is better off
removing these cells and not allowing them to become part of the developing
baby if a cell is damaged by the jumping genes or any other type of error
such as having too few or too many chromosomes.
Natural selection is understood to favor certain organisms over others. The
survival of the fittest appears to be taking place within embryos as well,
although this time amongst almost identical cells. It appears that we have
discovered a new weapon in our armory to combat these dangerous genetic
components.
Professor Laurence Hurst, Milner Centre for Evolution at the University of
Bath, is a co-lead author.
fighting new genetic threats with old genetic foes
Contrarily, the single-cell data revealed that the inner cell mass, or ICM,
or the crucial cells that will develop into the embryo, do not include
jumping genes but rather express a virus-like gene known as human endogenous
virus H. This supports a developing pattern in which we deploy our old
genetic foes to battle our new ones, which helps suppress the youthful
jumping genes in the inner cell mass.
The authors contend that an overly sensitive quality control procedure
might result in the death of the entire embryo. This may help to explain why
some abnormalities that affect our ability to recognize harm to developing
embryos are also linked to infertility.
