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J Reprod Dev
2018 Feb 27;641:1-6. doi: 10.1262/jrd.2017-100.
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Postovulatory cell death: why eggs die via apoptosis in biological species with external fertilization.
Tokmakov AA
,
Sato KI
,
Stefanov VE
.
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Spawned unfertilized eggs have been found to die by apoptosis in several species with external fertilization. However, there is no necessity for the externally laid eggs to degrade via this process, as apoptosis evolved as a mechanism to reduce the damaging effects of individual cell death on the whole organism. The recent observation of egg degradation in the genital tracts of some oviparous species provides a clue as to the physiological relevance of egg apoptosis in these animals. We hypothesize that egg apoptosis accompanies ovulation in species with external fertilization as a normal process to eliminate mature eggs retained in the genital tract after ovulation. Furthermore, apoptosis universally develops in ovulated eggs after spontaneous activation in the absence of fertilization. This paper provides an overview of egg apoptosis in several oviparous biological species, including frog, fish, sea urchin, and starfish.
Fig. 1. Progression of oocytes and eggs in oviparous species with external fertilization.
Fig. 2. Calcium-dependent and -independent spontaneous egg activation. Mature ovulated eggs at the second meiotic arrest can be spontaneously activated by calcium-dependent (A) or -independent (B) mechanisms. (A) Egg activation by extracellular or intracellular stimuli evokes calcium release from intracellular stores of the endoplasmic reticulum (ER), leading to MAPK inactivation and exit from meiotic arrest. A gradual increase in intracellular calcium, which occurs due to disruption of calcium homeostasis in aging eggs (dotted line), may eventually elicit the same effect. (B) Alternatively, a gradual decrease in MAPK pathway and/or CSF activity, below the threshold level necessary to maintain meiotic arrest, can cause meiotic exit in the absence of a calcium signal. The asterisk denotes the time of exit from the second meiotic arrest.
Fig. 3. Calcium-dependent meiotic exit in activated Xenopus eggs. Molecular components of CSF and MPF are colored pink and green, respectively. The factors involved in egg activation are shown in orange. CSF and MPF are embedded in a positive feedback loop (red arrows). Calcium-mediated egg activation signals stimulate APC/C, resulting in consecutive degradation of cyclin B, inactivation of MPF, destabilization of Mos protein, loss of CSF activity, and exit from meiotic metaphase II arrest.