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Open Access Research article

Unlaid Xenopus eggs degrade by apoptosis in the genital tract

Sho Iguchi1, Tetsushi Iwasaki2, Yasuo Fukami12 and Alexander A Tokmakov2*

Author affiliations

1 Graduate School of Science, Rokko dai 1-1, Nada, Kobe, 657-8501, Japan

2 Research Center for Environmental Genomics, Kobe University, Rokko dai 1-1, Nada, Kobe, 657-8501, Japan

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Citation and License

BMC Cell Biology 2013, 14:11  doi:10.1186/1471-2121-14-11

Published: 3 March 2013

Abstract

Background

In several species with external fertilization, including frogs, laid unfertilized eggs were found to die by apoptosis outside of the animal body. However, there is no apparent reason for the externally laid eggs to degrade by this process, considering that apoptosis developed as a mechanism to reduce the damaging effect of individual cell death to the whole organism.

Results

Here, we demonstrate that a number of eggs are retained in the genital tract of the African clawed frog Xenopus laevis after gonadotropin-induced ovulation. The majority of these eggs exit meiotic arrest within 24 hours of hormone administration. Subsequently, post-meiotic eggs die in the frog genital tract by a well-defined apoptotic process. The hallmarks of egg degradation include prominent morphological changes, cytochrome c release, caspase activation, increase in ADP/ATP ratio, progressive intracellular acidification, egg swelling and all-out proteolysis of egg proteins. The sustained presence of post-apoptotic eggs in the genital tract of ageing frogs evidenced age-associated worsening of apoptotic clearance.

Conclusions

The direct observation of egg degradation in the Xenopus genital tract provides a clue to the physiological relevance of frog egg apoptosis. It works to eliminate the mature unlaid eggs retained in the animal body after ovulation. Our findings establish egg apoptosis as a major physiological process accompanying ovulation in frogs.

Keywords:
Apoptosis; Unlaid eggs; Maturation; Ovulation; Meiotic exit; Xenopus laevis; Genital tract