Full-fledged oocytes from follicle granulosa cells were first obtained

Researchers from China using a chemical cocktail transformed granulosa cells into functional eggs of mice. After fertilization, the oocytes successfully developed into healthy offspring, showing no differences from mice born naturally.

Follicles are the main functional unit of the ovary and are composed of an oocyte, an immature egg, which is surrounded by granulosa cells. Studies have shown that granulosa cells not only play a decisive role in the development of the follicles, but also have plasticity, which exhibits properties similar to stem cells.

“The thing about in vitro fertilization is that they only use the oocyte for the procedure”, – says senior author of an article, published in Cell Reports, Lin Liu from the College of Life Sciences at Nankai University. “After the egg retrieval, the granulosa cells in the follicle are discarded. It got us thinking, what if we can utilize these granulosa cells? Since every egg has thousands of granulosa cells surrounding it, if we can induce them into pluripotent cells and turn those cells into oocytes, aren’t we killing two birds with one stone?”

Usually, granulosa cells, after removal from the follicles, are directed along the path of cell death or differentiation. Liu and his team developed a chemical “cocktail” with a Rho kinase inhibitor (ROCK) and crotonic acid to create chemically induced pluripotent stem cells (CiPSCs) from granulosa cells.

The research team used a ROCK inhibitor to prevent cell death and promote cell proliferation.

In their previous work, the authors found that crotonic acid activates the embryo genes, which are expressed at the stage of two cells, in particular, the Zscan4 gene. Together with a number of chemicals, it promotes the acquisition of a pluripotency property similar to embryonic stem cells.

This was confirmed by the expression of pluripotency markers Oct4-GFP, Sox2 and Nanog, in particular, methylation levels of their gene promoters. Also one of the important indicators was the reactivation of the second X chromosome. Cells successfully differentiated into ectoderm, mesoderm, and endoderm.

“It’s a surprising result”, – says Liu. “The competency of induced pluripotent germline is usually lower than embryonic stem cells. Germline competency is crucial for germline cells to transfer genetic information to the next generation. With the co-formulation of Rock inhibitor and crotonic acid, it’s not only more efficient, but the quality also increased.”

Scientists also tested another cocktail, consisting of a ROCK inhibitor and vitamin C. They treated pluripotent germline stem cells with it to improve follicle development and stimulate meiosis. Meiosis is a special type of cell division that results in the formation of gametes – germ cells. The germ line cells and oocytes obtained from granulosa cells possessed high genomic stability and successfully produced offspring with normal fertility.

“We can consistently manipulate the concentration and treatment time of these small chemicals”, – says Liu.

Compared to traditional methods of stem cell induction, such as transfection, reprogramming somatic cells by introducing transcription factor genes, chemical treatment provides more precise control of the process.

“Transfection method may have a higher risk of genetic instability.”

“This is the first time we turned granulosa cells into oocytes, it is a crucial and interesting work in developmental and reproductive biology”,  – he says. “But implementing this research to humans from mice still has a long way to go. I think it has more prospect in preserving fertility and endocrine function, than in treating infertility.”