How an egg cell's 'operating manual' sets the stage for fertility

Date:
October 8, 2020
Source:
Carnegie Institution for Science
Summary:
Scientists have revealed in unprecedented detail the genetic
instructions immature egg cells go through step by step as they
mature into functionality. Their findings improve our understanding
of how ovaries maintain a female's fertility.



FULL STORY ========================================================================== Recently published work from Carnegie's Allan Spradling and Wanbao Niu
revealed in unprecedented detail the genetic instructions immature egg
cells go through step by step as they mature into functionality. Their
findings improve our understanding of how ovaries maintain a female's fertility.


==========================================================================
The general outline of how immature egg cells are assisted by specific
ovarian helper cells starting even before a female is born is well
understood. But Spradling and Niu mapped the gene activity of thousands
of immature egg cells and helper cells to learn how the stage is set
for fertility later in life.

Even before birth, "germ" cells assemble a finite number of cell
clusters called follicles in a female's ovaries. Follicles consist of an immature egg cell and some "helper" cells, which guide the egg through
its maturation process. It is from a follicle that a mature egg cell
bursts during ovulation.

"Follicles are slowly used up during a female's reproductive lifespan
and menopause ensues when they run out. Understanding what it takes
for follicles to form and develop successfully, helps us learn how
damaged genes or adverse environmental factors, including a poor diet,
can interfere with fertility," explained Spradling. "By documenting
the follicle's genetic operating manual, problems in egg development
that might lead to birth defects -- as a result of mutations or due to
bad nutrition -- can be better understood and reduced." Spradling and
Niu sequenced 52,500 mouse ovarian cells at seven stages of follicle development to determine the relative expression of thousands of genes
and to characterize their roles.

The study also illuminated how mammalian ovaries produce two distinct
types of follicles and Spradling and Niu were able to identify many
differences in gene activity between them.

The first, called wave 1 follicles, are present in the ovary even before puberty. In mice, they generate the first fertile eggs; their function in humans is poorly understood, but they may produce useful hormones. The
second type, called wave 2 follicles, are stored in a resting state but
small groups are activated to mature during a female's hormonal cycle,
ending in ovulation.

The findings help clarify each type's roles.

Spradling and Niu's work and all its underlying data were published by Proceedings of the National Academy of Sciences.

"We hope our work will serve as a genetic resource for all researchers
who study reproduction and fertility," concluded Spradling.

This work was funded by the Howard Hughes Medical Institute.


========================================================================== Story Source: Materials provided by
Carnegie_Institution_for_Science. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Wanbao Niu, Allan C. Spradling. Two distinct pathways of
pregranulosa
cell differentiation support follicle formation in the mouse ovary.

Proceedings of the National Academy of Sciences, 2020; 117 (33):
20015 DOI: 10.1073/pnas.2005570117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201008124429.htm

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