Advancements in infertility research have been hindered by the complexity of understanding the human reproductive cycle in animal models. Additionally, the non-availability of germ cells, due to its formation in the very early stages of the embryonic development (8-10 weeks), has led to the scarcity of these cells for research purposes. Now, researchers at the Stanford University School of Medicine have devised a method to proficiently transform human embryonic stem cells into germ cells, in vitro. The study, which is claimed to provide the first evidence of creating functional germ cells in the laboratory, could offer a valuable insight into understanding the early stages of gametogenesis, and germ cell development and differentiation. The findings of the research are published in the recent issue of the journal Nature.
Renee Reijo Pera, Director of the Center for Human Embryonic Stem Cell Research and Education, Stanford University, California, and co-workers, quantified and isolated primordial germ cells from male and female embryonic stem cells (derived from excess IVF embryos), using a germ cell reporter gene. Subsequent to the isolation of primordial germ cells, RNA silencing technique was performed by blocking and over-expressing certain male infertility-associated genes that were identified by the researchers in their earlier study. The genes belonged to the Deleted in Azoospermia (DAZ) family.
Three DAZ gene family members, DAZ, DAZL, and BOULE, were investigated to identify their role in the development of germ cells from stem cells. The three genes were unique in that they coded for germ cell-specific cytoplasmic RNA-binding proteins and not DNA transcription factors. The researchers noted the following findings on silencing and over expression of these genes:
• DAZL functions in the early stages of germ cell development, and is crucial for the transformation of embryonic stem cells into germ cells
• In contrast, DAZ and BOULE function in mature germ cells to promote later stages of meiosis and haploid gamete development
Over expression of DAZ, DAZL, and BOULE, collectively, helped to produce haploid cells that expressed proteins present in a mature sperm. It was observed that the effect of the DAZ family genes on embryonic stem cells differed depending on whether the cells were isolated from male or female embryos. Increased expression of BOULE elevated the putative germ cell proportion from 2 to 12% only in female cells lines, suggesting its major role in the development of female germ cells. The findings emphasize the importance of DAZL, DAZ and BOULE in modulating primordial germ cell and gamete formation.
The researchers intend to follow a similar approach for optimizing egg production from embryonic stem cells, and are also probing to determine whether reprogrammed adult cells (also known as induced pluripotent cells) could be used to create germ cells.
Earlier studies carried out on human embryonic stem cells have shown the potential to generate primordial germ cells and gamete-like cells; the exact mechanism of which was unclear. Now, with the ability to generate functional germ cells from human embryonic stem cells, the current study provides a window of opportunity for researchers to closely monitor these in vitro-derived germ cells for clues on the genesis of chromosomal disorders, birth defects, unexplained infertility, and fetal disability.
References
1. Scientists turn stem cells into precursors for sperm, eggs. Press Release. Stanford School of Medicine. Last accessed November 12, 2009.
2. NIH-Funded Researchers Transform Embryonic Stem Cells Into Human Germ Cells. Press Release. National Institutes of Health. Last accessed November 12, 2009.
3. Kee K, Angeles VT, Flores M, Nguyen HN, Reijo Pera RA. Human DAZL, DAZ and BOULE genes modulate primordial germ-cell and haploid gamete formation. Nature. 2009 Nov 12;462(7270):222-5.
