University of Cambridge and EMBL-EBI researchers have revealed that differences in gene expression begin emerge earlier in human development than originally thought. According to the Cambridge and EMBL teams, genetic differences arrive as early as the second day after the completion of fertilization. These four cell-stage embryos consist of four “blastomeres” that appear identical in size and shape. However, even at these early stages, these four blastomeres are already beginning to display subtle differences in gene expression.
Fertilization of an egg (oocyte) by a sperm is a multistep process that begins with the contact of the sperm with the jelly layer that surrounds the egg (zona pellucida), and the acrosomal reaction of the sperm, contact of the egg and sperm membranes, followed by fusion of the egg and sperm membranes, egg activation, disassembly of the sperm and remodeling of the sperm and egg pronuclei, contact of the sperm and egg pronuclei, and culminating in the initiation of the first mitotic division. The first cell division or “cleavage” occurs approximately 24 hours after the initiation of fertilization, and forms the two-cell embryo. The next cleavage occurs about 12 hours later, and the blastomeres initially divide synchromously (at the same time), but eventually divide asynchronously (at different times). During these early cleavages of the zygote, special embryonic cell cycles and include S phases and M phases that alternate without any intervening G1 or G2 phases. Therefore individual cell volume decreases. About day 4, the embryo is a solid ball of 16-20 cells with peripheral cells flattened against the zona pellucida, and compaction occurs forming a cavity that leads to the next blastocyst stage, which is a large free-floating ball of stem cells.
At first, the blastomeres of the early embryo are “totipotent,” which means that each blastomere can potentially divide and grow and produce every single cell of the whole body and the placenta. After compaction, two cell populations emerge that include, round, slow-dividing cells in the center and fast-growing flatter cells on the outside. The central cells of the inner cell mass have a “pluripotent” status, which means that they can generate the cells of the whole body, but not the placenta. However, the point during development at which cells begin to show a preference for becoming a specific cell type is unclear.
At this point, the new study, which was published in the journal Cell, presents rather convincing data that even as early as the four-cell embryo stage, the cells are indeed different.
The EMBL/Cambridge teams utilized the latest sequencing technologies to model embryo development in mice and examined the activity of individual genes at a single cell level. This analysis showed that some genes in each of the four blastomeres showed distinct genetic signatures. The expression of one gene in particular, Sox21, differed the most between cells. Sox21 is part of the so-called “pluripotency network.” The pluripotency network consists of a cascade of genes that are essential both in culture (in vivo) and in vitro (in the organism) for early development and maintenance of pluripotency. The EMBL/Cambridge teams discovered that when the activity of Sox21 was reduced, the activity of a master regulator that directs cells to develop into the placenta increased.
“We know that life starts when a sperm fertilizes an egg, but we’re interested in when the important decisions that determine our future development occur,” says Professor Magdalena Zernicka-Goetz from the Department of Physiology, Development and Neuroscience at the University of Cambridge. “We now know that even as early as the four-stage embryo – just two days after fertilization – the embryo is being guided in a particular direction and its cells are no longer identical.”
Dr John Marioni of EMBL-EBI, the Wellcome Trust Sanger Institute and the Cancer Research UK Cambridge Institute, adds: “We can make use of powerful sequencing tools to deepen our understanding of the molecular mechanisms that drive development in individual cells. Because of these high-resolution techniques, we are now able to see the genetic and epigenetic signatures that indicate the direction in which early embryonic cells will tend to travel.”
This research tends to diffuse one of the arguments embryonic stem cell proponents use to justify the destruction of human embryos. Namely, the early human embryos consist of cells that are all the same and have no interactions with each other. The embryo is, then, not an individual organism, but a collection of many potential organisms that eventually becomes as unified organism. This turns out to be incorrect, since the cells of the early blastomere are not all equivalent. Instead, the blastomeres are interacting with each other and using these interactions to figure what kind of cells their progeny will form. This is the hallmark of an entity with a unified purpose that has a distinct goal. Folks, that sounds like a unified organism. It is simply young.