Science Advances: The early stages of sex determination in mice and humans

During embryonic development, two different genetic signals determine whether the embryo's original gonads will become testicles or ovaries, and thus whether the embryo will develop into male or female
.
The interruption of this process leads to disorders of sexual development, characterized by a mismatch
between chromosomes, gonads (ovaries or testes) and genital anatomy that determine sex.
This incompatibility can take many forms, such as unclear genitals or a combination
of male and female physiological characteristics.
The disease is called Disorders of Sexual Development (DSD) and has a prevalence of 1 in 4500 newborns
.

A significant challenge in gender reversal research is the lack of in vitro systems to model and study the variants
found in individuals with DSD.
A study published today by researchers at Bar-Ilan University provides a solution to this challenge by developing new tools to create somatic/supporting cells of gonads that will be able to mimic stem cells
from DSD individuals in a dish.
This for the first time facilitated the ability to
begin studying DSD pathology in laboratory petri dishes in human-related settings, rather than through mouse models.

The research was led by Dr Nitzan Gonen from Bar-Ilan University's Goodman School of Life Sciences and the Institute for Nanotechnology and Advanced Materials, in collaboration
with the Francis Crick Institute in the UK and the Institut Pasteur in France.

About a year ago, an article published in Science by Yoshino et al.
showed that stem cells can be used to make both egg progenitor cells and ovarian somatic cells
.
When the researchers combined the two types of cells, they were able to create a healthy and fertile embryo
through these artificial eggs.
Dr.
Gonen's research demonstrates the initial stages of a similar idea, but this time on
male cells.

The male testicles contain germ cells that develop into sperm cells from puberty and throughout life
.
These cells are surrounded by somatic cells (non-germ cells)
that support germ cell function and development.

In the current study, Dr.
Gonen and researchers at the Francis Crick Institute differentiated mouse embryonic stem cells into early somatic cells
in the testicles.
They compared the cells they created to real testicular cells and showed through RNA sequencing that the two were very similar
.
The advantage of using murine-derived stem cells first is that it can be properly compared
with real gonadal cells isolated from embryonic gonads.
It is very difficult, if not impossible, to do this with human cells and embryos because the equivalent number of human embryos is embryos in the seventh week of pregnancy, and at this stage, obtaining embryos after abortion is challenging
.

Dr.
Gonen's partner at the Institut Pasteur, Dr.
Anu Bashamboo, and her lab, took this approach and demonstrated that it works
on human stem cells in a very similar way.
The different combinations of X and Y chromosomes determine the sex
of the embryo.
The researchers extracted three types of human cells — male cells (XY), female cells (XX), and cells from DSD individuals with inverse sex (XY was female at birth).

They showed that somatic cells produced by XX and XY differed from each other, while cells from gender-reversed individuals were in the middle, closer to females than males
.
However, when the mutation/mutation in DSD single cells was corrected with CRISPR genome editing technology, the cells resumed the behavior
of typical XY cells.

The creation of this model of the human cell with sex reversal opens the door
to understanding where the sex determination process went wrong in many other unexplained cases of DSD and what exactly changes occur in the cells of individuals with DSD.

"I think this study offers the possibility of generating a variety of different gonadal cell types, such as support cells for germ cells, or Leydig cells that secrete testosterone," Dr.
Gonen said
.
Combining somatic support cells with germ cells will allow us to create a "mini-testicle in a dish" to better understand cases of DSD and infertility
.
Hopefully, in the future, we will be able to use this technology to produce functional sperm in the laboratory, allowing infertile men to have a biological child
.
”

"Being able to understand the reasons behind differences in sexual development is often of great value
to affected individuals and their families.
In addition, this is often important
when deciding on possible clinical treatments.
for example, potentially preservation, maintenance or restoration of fertility"
.