Arrest at Metaphase of Meiosis I in Starfish Oocytes in the Ovary is Maintained by High CO₂and Low O₂Concentrations in Extracellular Fluid

Abstract

During the spawning process in starfish, oocytes are arrested at metaphase of meiosis I (MI) within the ovary, and reinitiate meiosis only after they have been released into the seawater. However, this arrest does not occur if the ovary is removed from the animal. As the pH of the coelomic fluid is buffered by CO2/H(+)/HCO3(-), we investigated the involvement of gas concentrations in MI arrest. In vivo, the CO2 level in the coelomic fluid was high (∼1.5% vs. 0.04% in air) and the O2 level was low (0.1-1.0% vs. ∼20% in air). When these gas conditions were reproduced in isolated coelomic fluid or seawater, ovarian oocytes arrested at MI, just as in vivo. Isolated oocytes from the ovary required the similar high CO2 and low O2 level to remain arrested in MI and had an intracellular pH of ∼6.9. Intracellular pH increased to ∼7.3 when oocytes were transferred to seawater equilibrated with air, a condition that mimics that of spawning. We used ammonium acetate to clamp intracellular pH at different levels and found that MI arrest occurred when intracellular pH was ∼6.9. Our results support the idea that high CO2 and low O2 in the ovarian environment lead to low intracellular pH and MI arrest, while spawning into the seawater with low CO2 and high O2 results in high intracellular pH and release from MI arrest. The biological significance of MI arrest is that oocytes are spawned into seawater at the optimal physiological state of MI when the least polyspermy occurs.

Block of CDK1‐dependent polyadenosine elongation of Cyclin B mRNA in metaphase‐i‐arrested starfish oocytes is released by intracellular pH elevation upon spawning

“…One critical difference between these stages is pH i : MI‐arrested oocytes have a pH i of 6.7–6.9 whereas spawned oocytes that possess poly(A)‐elongated transcripts (Fig. , spawned at MI [in vivo]) have a pH i of 7.0 or more (Moriwaki et al, ). Therefore, we hypothesize that the key step required for polyadenylation is pH‐dependent.…”

“…The pH i of isolated oocytes stimulated in seawater containing 1‐MA increases to ∼7.3 without metaphase arrest because seawater contains a very low concentration of dissolved CO 2 , as conditions outside the body are normoxic. Therefore, MI arrest does not occur in oocytes maturing in vitro at higher pH i (Moriwaki et al, ).…”

Oocyte Maturation in Starfish

“…In the coelomic fluid of living animals, the concentration of dissolved CO 2 is~1.0%, which is 20 times higher than that in normal air [25]. As CO 2 can easily enter the plasma membrane, high CO 2 levels would decrease the pHi.…”

“…As CO 2 can easily enter the plasma membrane, high CO 2 levels would decrease the pHi. The pHi of ovarian oocytes in vivo has been estimated to be~6.7, by studying oocytes that are incubated in seawater, where the gas conditions are similar to those of the coelomic fluid of living animals [25]. Under in vivo gas conditions, the 1-MA increases pHi, but the relatively high dissolved CO 2 in the ovary limits the rise to~6.9.…”

Two p90 Ribosomal S6 Kinase Isoforms Are Involved in the Regulation of Mitotic and Meiotic Arrest in Artemia

“…This ensures that oocytes are released into the ovisac (uterus) in an optimal physiological state, which reduces the risk of insemination failing, such as occurs in polyspermy (6). The E3 ubiquitin ligase anaphase-promoting complex/cyclosome can degrade the components of CSF, after which meiosis resumes (4).…”