primordial and primary follicles in normal and irradiated mouse ovaries. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Mouse; Ovary; Primary follicle; Primordial follicle; Radiation
1. Introduction
A primordial follicle is the smallest follicle within the ovary and contains an oocyte Ž
. surrounded by a single layer of flattened somatic cells Meredith and Doolin, 1997 . The
formation of the primordial follicles occurs around the time of birth in rodents Ž
. Hirshfield, 1991 . By unproved mechanisms, the primordial follicles initiate growth into
primary, and consequently, secondary stages before acquiring an antral cavity. Except for a few primordial follicles, most of them remain as resting, quiescent, and nongrow-
ing pool of follicles that will be progressively depleted throughout the reproductive life span of the female. Most of the ovarian follicles undergo a degenerative process called
Ž .
atresia during the reproductive life in mammals Byskov, 1978 . However, the precise mechanism of follicular atresia has not been elucidated yet. The atretic follicles include
such morphological characteristics as gradual pyknosis of granulosa cell nuclei, reduc- Ž
tion in granulosa cell proliferation and breakdown of the basement membrane Hirsh- .
field and Midgley, 1978; Braw and Tsafriri, 1980 . However, most of the research works regarding the follicular atresia have been focused on the growing follicles. In the
mammalian ovary, two major stages of cell degeneration can be distinguished: the degeneration of germ cells, defined as attrition, which accounts for the main loss of
Ž .
oocytes Beaumont and Mandl, 1962 and occurs prenatally; and the follicular degenera- Ž
tion, defined as atresia, which occurs during postnatal reproductive life Kaipia and .
Hsueh, 1997 . Apoptosis, a regulated form of cell death, is a physiological process essential for the
Ž .
normal tissue homeostasis Kaipia and Hsueh, 1997
in the absence of immune Ž
. surveillance Kerr et al., 1994 . Ovarian follicular degeneration or atresia is a hormon-
ally controlled apoptotic process, whereby the degenerating follicles are eliminated in a Ž
. coordinated fashion Hsueh et al., 1994 . It is now accepted that pyknosis of granulosa
Ž cells is an apoptotic process Hughes and Gorospe, 1991; Billig et al., 1993; Hurwitz
. and Adashi, 1993; Gougeon, 1996 . One of the atretogenic stimuli that could accelerate
Ž .
the follicular atresia was g-radiation Kim et al., 1999 . In both normal tissues and tumors, apoptosis not only occurs spontaneously but can also be induced by irradiation
Ž .
Ž .
Hendry and West, 1997 . Radiation induced cell apoptosis Hendry and West, 1997 Ž
. and impaired the ovarian functions Chapman, 1982 . It was reported that primordial
oocytes of rats and mice were more sensitive to radiation than oocytes in the growing Ž
. follicles Ataya et al., 1995 . However, the radiation sensitivities between the resting and
nongrowing primordial and the activated primary follicles were not morphologically demonstrated. Moreover, the morphological changes of the primordial and primary
follicles occurred at natural and artificial degeneration processes were also not depicted yet.
Therefore, the present experiments have been performed to analyze the morphologi- cal features of the normal and the degenerating primordial and primary follicles in the
natural and the irradiated mouse ovary.
2. Materials and methods
