Different cell types in the body, such as cells of the heart, liver, blood, or sperm have unique characteristics that help them serve specific roles in the body. This specificity is in most cases hard-wired in the cell, or in other words, a heart or liver cell cannot on its own begin to function like a blood cell. All the specialized body cells are generated at the embryonic stage of development from primordial cells, referred to as stem cells.
In what is considered a breakthrough in predetermined cell specificity, researchers at the University of Pennsylvania, working with their counterparts at the University of Texas, in the United States have induced blood cells in marmosets (a relatively small monkey species) to revert back to their earlier stem cell status. They then coaxed those stem cells to take on the characteristics of sperm precursors.
In a recent publication that detailed their step-by-step process of rewiring cells, the researchers said the findings from the study on marmoset cells opens new possibilities for studying primate biology and developing novel assisted reproductive technologies like in vitro gametogenesis — a process of generating sperm or eggs (germ cells), in a laboratory dish — similar to how in vitro fertilization involves the generation of an embryo outside the human body.
Scientists have known how to generate functional sperm and egg from induced pluripotent stem cells in mice, but mouse germ cells are very different from human germ cells. Now by studying marmosets, whose biology more closely resembles that of humans, scientists have better knowledge on germ cell generations in people.
For their research, the scientists first began by learning more about germ cell precursors in marmoset embryos. They found that these early-stage cells, known as primordial germ cells (PGCs), bore certain molecular markers that could be tracked over time. Performing single-cell RNA sequencing on these cells revealed that PGCs expressed genes characteristic of early-stage germ cells and those related to epigenetic modifications, which regulate gene expression. PGCs did not, however, express genes known to be turned on later in the process of germ cell development, when precursor cells migrate to the ovaries or testes to complete their maturation.
Their findings were consistent with the notion that marmoset germ cells undergo a reprogramming process that ‘turns off’ certain markers and allows PGCs to proceed through the stages of germ cell development. The patterns the researchers observed in marmoset cells closely resembled what has been found in both humans and other monkey species but were distinct from those of mice, providing researchers with another reason for using marmoset as a valuable model for reproductive biology studies.
The team then set about trying to reconstitute the process of development artificially, in the lab. The first step was to transform blood cells into induced pluripotent stem cells (iPSCs) — cells that retain the ability to give rise to a number of other cell types.
After much trial and error and applying lessons learned from mouse, human, and other investigations, the team landed upon a strategy that enabled them to generate and sustain stable cultures of iPSCs. The next step involved moving from iPSCs to germ cell precursors. Once again, considerable experimentation went into developing the protocol for this transformation that eventually enabled them to induce nearly 40 percent of their culture material to take on characteristics of germ cell precursors.
In a final stage of the study, the research team coaxed these lab-grown cells to take on the characteristics of later-stage germ cells. Based on a method established through earlier trials on human cells, they cultured the cells with mouse testicular cells over the course of a month. The result was a successful growth with some cells beginning to turn on genes associated with later-stage sperm cell precursors.
Developing new approaches to study the marmoset has enabled the scientific community to make use of the species as an important research model in studies on development of the reproductive system, pursuing studies of normal and abnormal development, as well as fertility.
The team also pointed out that studies on marmosets were invaluable as any clinical application of assisted reproductive technology like in vitro gametogenesis raises a lot of ethical, legal, and safety concerns. Having a good preclinical model in the form of marmosets to explore is important before we move to human clinical translation.
