Scientists have traced adult pluripotent stem cells back to the embryonic stage

Images illustrating the methodical process that was used to turn single cells of the embryo into a red color for the purpose of this study. Credit: Julian Kimura

Stem cells are a wonderful discovery in the field of biology. They have the ability to repair, heal, replace, and rejuvenate broken or damaged cells. In the majority of animals, including humans, these cells are only able to regenerate the specific kind of cell to which they are assigned. Therefore, hair stem cells are solely capable of producing hair. Only intestines can be generated from intestinal stem cells. The ability to regenerate nearly any cell type that has been lost is referred to as whole-body regeneration. This ability is shared by several invertebrate species that are very distantly related to one another and have stem cell populations that are pluripotent in adult animals.

Even though adult pluripotent stem cells, also known as aPSCs, can be discovered in a wide variety of animal species (including sponges, hydras, planarian flatworms, acoel worms, and certain species of sea squirts), the exact mechanism by which these cells are generated is unknown in any of these organisms.

Researchers from the Department of Organismic and Evolutionary Biology at Harvard University have published a new study in the journal Cell in which they describe how they discovered the cellular mechanism and molecular pathway responsible for the formation of aPSCs in the acoel worm known as Hofstenia miamia.

H. miamia, also referred as the three-banded panther worm, is capable of complete regeneration by the use of aPSCs that are called “neoblasts.” Chop this organism into pieces, and each piece will develop into a new body, complete with a mouth, a brain, and everything in between. Professor Mansi Srivastava, the senior author, was in charge of collecting H. miamia in the field many years ago because of the fact that it has the potential to regenerate. After arriving back in the laboratory, H. Miami began to generate a large number of embryos that were easy to investigate.

In a recent work, Srivastava and his co-author and postdoctoral researcher Lorenzo Ricci created a methodology for transgenesis in H. miamia. A process known as transgenesis is one in which something that is not ordinarily present in the genome of an organism is brought there by other means. Through the use of this technology, lead author Julian O. Kimura (Ph.D. ’22) was able to investigate his hypothesis regarding the formation of these stem cells.

According to Kimura, the existence of pluripotent stem cells in the adult body is one common property among species that are able to regenerate. These cells are involved for remaking missing body parts when the animal is injured. Kimura felt that by understanding how animals like H. miamia make these stem cells, they could better understand what provides certain animals the ability to regenerate.

The expression of a gene known as Piwi is one of the characteristics that brings together all of these different stem cell types found in adult animals. However, nobody has been able to find out how these stem cells are produced in the first place in any species up to this point. According to Srivastava, they’ve largely been examined in the context of adult animals, and he went on to say, and in certain species they know a little bit about how they could be operating, but they don’t know how they are formed.

Since the researchers were aware that worm larvae possess aPSCs, they reasoned that the cells must have been produced during embryogenesis. Through the process of transgenesis, Ricci was able to generate a line that, once embryo cells were implanted with the protein Kaede, enabled the cells to emit a luminous green light. Because Kaede is photo-convertible, the green color may be changed to red by shining a laser beam with a particular precise wavelength on it. This will cause the green to change color. After that, you may use a laser to irradiate the cells, which will cause the individual green cells that make up the embryo to become red.

Using genetically modified organism with photo-conversion is a very unique innovation that the researchers invented in the lab to figure out the destinies of embryonic cells, said Srivastava. They devised this method to figure out the fates of embryonic cells.  Kimura used this approach to conduct lineage tracing by letting the embryos develop normally and observing what occurred.

Kimura observed the progression of the embryo from a single cell to numerous cells as it underwent development. The first stages of cell division are characterized by a phenomenon known as stereotyped cleavage. This phenomenon indicates that all embryonic cells divide in precisely the same way. As a result, all cells may be identified and examined in the same manner. This gave rise to the concept that each and every cell may serve a specific function in the body. At the eight-cell stage, for instance, it’s possible that the cell in the top left corner creates one type of tissue while the cell in the bottom right corner makes another type of tissue.

Kimura created a complete map of the embryo’s potential outcomes at the eight-cell stage by methodically doing photo-conversion on each of the cells of the early embryo. This allowed him to define the function of each cell. After that, he followed the cells of the worm as it matured into an adult while it continued to carry the red tagging. Kimura was able to pinpoint the precise location in each embryo where each cell was carrying out its function by performing the laborious task of repeatedly tracking each individual cell over a large number of embryos.

At the sixteen-cell stage of embryo development, he discovered a highly unique pair of cells that gave rise to cells that seemed to be neoblasts. These cells were located in the embryo. Discovering cells that simply resembled neoblasts in looks was not concrete proof that they truly were neoblasts; the researchers needed to show that they functioned like neoblasts as well. This really excited the researchers, but there was still the likelihood that neoblasts were emerging from different sources in the early embryo, not just the two pairs found at the sixteen-cell stage.

To be absolutely confident, Kimura included this specific group of cells, which he referred to as 3a/3b in H. miamia, on trial. Cells need to exhibit all of the characteristics associated with stem cells in order for them to be considered neoblasts. Are the offspring of those cells responsible for the formation of new tissue during the process of regeneration? The researchers did find that the offspring of those cells alone were responsible for the formation of new tissue throughout the process of regeneration.

The amount of gene expression in stem cells is another one of their defining properties. Stem cells have to have hundreds of genes expressed in order to be considered stem cells. Kimura utilized a sorting machine that segregated the red cells from the green cells in the progeny that had been created with 3a/3b glowing in red and all other cells glowing in green. This was done so that he could identify whether or not 3a/3b possessed this trait. After that, he used a method called single-cell sequencing to investigate the question of which genes are being expressed in red cells as opposed to green cells. These findings provided further evidence that, on a molecular level, stem cells could be identified in only the progeny of the 3a/3b cells and not in the progeny of any other cell.

According to Kimura, it was convincing proof of the fact that they located the biological source of the stem cell line in the system.  However, and this is very significant, understanding the biological source of stem cells now provides a mechanism to collect the cells as they mature and specify what genes are involved in producing them.

Kimura created a massive dataset on embryonic development at the single-cell level, describing which genes were being expressed in all of the cells in embryos from the beginning to the end of development. This dataset included information about which genes were being expressed. He permitted the 3a/3b cells that had been transformed to continue their development for a short while longer, but not all the way to the hatchling stage. After that, he made use of the sorting technique to collect these cells. Kimura was able to precisely determine which genes were being selectively expressed in the lineage of cells that make up the stem cells as a result of doing this.

The research discloses a group of genes that might be highly essential regulators for the development of stem cells. The study reveals a set of genes that could be very important.  This is relevant across species since homologues of these genes play critical functions in human stem cells.

According to Srivastava, Julian began in the lab hoping to research how stem cells are produced in the embryo, and it is an astounding scenario that when he graduated he had found it out.

The researchers intend to continue their investigation into the mechanism of how these genes are acting in the stem cells of Hofstenia miamia. This will assist them in answering the question of how nature acquired a method to produce and retain pluripotent stem cells. By gaining an understanding of the molecular regulators of aPSCs, researchers will be able to compare these processes across species, which will provide light on the evolution of pluripotent stem cells throughout animal species.


Mansi Srivastava. (2022). Embryonic origins of adult pluripotent stem cells, CellDOI: 10.1016/