control mouse ovary was 81.3. This ratio decreased down to 80.0, 75.0, 45.5 and 33.3 at 0, 3, 6 and 12 h after irradiation, respectively.
4. Discussion
Even though it has been widely accepted that radiation had a detrimental effect on the ovarian follicles in mammals, the reports concerning the changes of morphological
characteristics of primordial and primary follicles induced by radiation were quite limited. In the present experiment, the changes of morphological characteristics of
primordial and primary follicles caused by radiation were investigated.
Ž .
Erickson 1966 reported that primordial follicles were depleted as a consequence of either oocyte attrition or initiation of growth. The result of the present experiment
showed that g-radiation had effects of oocyte attrition, as well as the induction of apoptosis of granulosa cells in the primordial and primary follicles. There were three
kinds of degenerating patterns of primordial and primary follicles. First, follicles, which were recognized by the presence of the apoptotic oocytes had intact granulosa cells.
Second, follicles had one or more apoptotic granulosa cells with relatively intact oocyte membrane. And, finally, follicles with both of the above characteristic changes were
observed. Apoptotic granulosa cells and defected oocytes were easily recognized by the dark-stained nuclei. The most characteristic morphology of the degenerated primordial
follicles was the lack of oocytes with two or more malformed granulosa cell-like cells. These malformed cells were placed in the basement membrane of the degenerated
primordial follicles. The fact that the morphological changes were induced in a short time period after irradiation indicates that the primordial follicles have considerably
greater susceptibility to ionizing radiation than primary follicles. In this regard, there is a great possibility that primary follicles, which are not degenerated, grow to secondary
follicles. Also, it is thought that granulosa cells in a follicle are categorized into two groups. One type is radioresistant, and the other, radiosensitive. It can be thought that
the radiation sensitivities between the resting and nongrowing primordial and the activated primary follicles were different.
Ž .
Ratts et al. 1995 reported that there were numerous primordial follicle-like struc- tures in bcl-2 deficient mouse ovaries. In the present study, the irradiated mouse ovary
contained numerous oocyte-deficient primordial follicles at 12 h after irradiation. In comparison, apoptotic granulosa cells and oocytes of the primordial follicles were shown
until 6 h after irradiation. The degeneration of primordial follicles caused by g-radiation showed an acute progression. Nearly all the primordial follicles were degenerated within
12 h after irradiation.
It was reported that there were no adequate morphological markers of degeneration in Ž
. primordial follicles Edwards et al., 1977 . However, in the present study, the primordial
follicular degeneration was induced by g-irradiation and the morphological changes of primordial follicles with time lapse were marked and easily recognized by the presence
of the apoptotic granulosa cells or degenerating oocytes, or both. Radiation could acutely give rise to the apoptotic degeneration of follicular cells in the present
experimental scheme. The possible explanation of such an acute follicular degeneration
lies on the high radiation dose used in this study, at which 80 of the irradiated animals will die within 30 days.
The present study provides morphological clues for the microscopical identification of the degenerating primordial and primary follicles. The result indicates that the
degeneration of primordial follicles goes much faster than that of primary follicles after irradiation and that the primordial follicles have higher sensitivity to the ionizing
radiation than the primary follicles. In any case, the pattern of degeneration is one of the
Ž . Ž .
following: 1 apoptosis of one or more granulosa cells with relatively intact oocyte, 2 Ž .
apoptosis of oocyte with intact follicle cells, or 3 apoptotic degenerations of both cells.
5. Conclusion
