Ovogonias

Journal of Reproduction and Development
, Vol. 56, No. 6, 2010, 10-071H
—Review—
Growth of Primordial Oocytes in Neonatal and Adult Mammals
Mohammad MONIRUZZAMAN1) and Takashi MIYANO1)
1)Graduate School of Agricultural Science, Kobe University, Kobe 657-8501, Japan
Abstract.
Mammalian ovaries are endowed with a huge number of small oocytes (primordial oocytes) in primordial
follicles.A small number of primordial oocytes start to grow, while others remain quiescent. Little is known about the
mechanism regulating the activation of primordial oocytes. Recently, we found that primordial follicles in mature cows
and prepubertal pigs took longer to initiate growth in xenografts compared with those in neonatal animals. We think
that primordial oocytes in adult mammals aredifferent from those in neonatal mammals. In this review, we summarize
the results regarding the activation of primordial oocytes in neonatal and adult ovaries of different species and propose
a model in which ovaries of neonatal mammals contain a mixed population of both quiescent and activated primordial
oocytes, while almost all primordial oocytes are quiescent in adult females. The dormancy ofprimordial oocytes may
be required to reserve the non-growing oocyte pool for the long reproductive life in mammals. FOXO3 is considered
one of the molecules responsible for the dormancy of primordial oocytes in adult ovaries. These quiescent primordial
oocytes are activated, perhaps by certain mechanisms involving the interaction between stimulatory and inhibitory
factors, to enter thegrowth phase.
Key words:
Adult, Neonatal, Oocyte growth, Ovary, Primordial follicle
(
J. Reprod. Dev. 56:
559–566, 2010)
n fetal ovaries, germ cells known as oogonia proliferate mitot-
ically and become oocytes that enter the meiotic cell cycle,
and the oocytes then become arrested at prophase I. At around
birth in rodents and during mid-gestation in humans and pigs,
oocytes (15–20 μm indiameter in rodents and 30 μm in diameter in
humans and pigs) are enclosed by a single layer of flattened follic-
ular epithelial cells (granulosa cells; Fig. 1). This unit consisting of
an oocyte and granulosa cells is called the primordial follicle. The
oocytes in primordial follicles are called “primordial oocytes” [1].
After formation of primordial follicles, some of the primordialoocytes begin to grow, although most spend months or years in the
quiescent state. Activation of primordial oocytes causes the trans-
formation of their surrounding granulosa cells to a cuboidal shape
[2]. The follicles at this stage are called primary follicles that con-
tain growing oocytes. The granulosa cells proliferate and become
multilayered to form secondary follicles. As folliclesdevelop
through the primary, secondary and tertiary (antral) stages, they
gain successive layers of granulosa cells, oocytes increase in size
towards 70–75 μm in rodents and 120–125 μm in humans, cows
and pigs and theca cell layers surround the follicles. Finally, a
large fluid-filled antral cavity (follicular antrum) is formed, and the
follicle is called a tertiary or antral follicle.The time point at which first activation of primordial oocytes
occurs depends on the species. In mice, growth of a portion of
oocytes begins within several days after birth [3, 4], whereas in
ruminants and primates, growth begins before birth [5, 6]. The
ovaries of neonatal piglets contain a large number of primordial
follicles and also a small population of developing follicles that
containgrowing oocytes. Primordial follicles represent more than
95% of the total follicular proportion in porcine ovaries at 2 weeks
after birth [7, 8]. The ovaries of prepubertal and adult females pos-
sess numerous growing and fully-grown oocytes in the various-
sized follicles as well as a number of non-growing primordial
oocytes as the future stock. These non-growing oocytes are
thought...