Ascidians are sessiles marine oganisms (tunicates/urochordates) the eggs of which develop into a tadpole stage that resembles the vertebrate embryo. Mature oocytes of the ascidian Phallusia mammillata are shed arrested in metaphase of meiosis I, before the first polar body is extruded. Fertilization reinitiates meiosis which completion depends on transient increases of the intracellular calcium concentration ([Ca++]c). Indeed, these oscillatory changes of [Ca++]c along with changes in the plasma membrane permeability during the meiotic cell cycle displayed by Phallusia eggs have been reported in a wide variety of animals ranging from protostomes (molluscans, nemerteans, polychaetes) to deuterostomes (ascidians, mammals). In Phallusia, the spermatozoide entering the egg triggers a strong [Ca++]c transient which crosses the entire egg as a wave starting from the point of sperm entry. This activation wave is immediately followed by a first train of [Ca++]c oscillations (n=1 to 5) at the end of which the first polar body is extruded. After a lagtime, a second train of calcium waves (n=12 to 20) occurs and they are all emanating from a point source (the calcium pace-maker) located at the vegetal pole in a subcortical region. At the end of this second train of oscillations, the second polar body extrudes approximately 25 minutes after fertilization.
The aim of the work in the BioCurrents Research Center was to study the metabolic activation of the egg during fertilization and meiosis by measuring the oxygen consumption of a single egg with the self-referencing oxygen-sensitive probe developed in this laboratory. By measuring oxygen fluxes while monitoring the calcium waves, we characterized a burst of oxygen consumption which seems to be one of the earliest events triggered by the sperm-egg fuion, it lasts 4-5 minutes and corresponds to the activation wave and the first train of oscillations. After a lagtime, a second transient increase in oxygen consumption initiates and peaks with the second train of [Ca++]c oscillations, this increase is terminated when the second polar body extrudes. In all cases, these two peaks of oxygen consumption were of biggest amplitude compared to the oxygen consumption seen after meiosis and during cleavage.
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