Membrane junctions in Xenopus eggs: their distribution suggests a role in calcium regulation

The onset of activation responsiveness during maturation coincides with the formation of the cortical endoplasmic reticulum in oocytes of Xenopus laevis

Immature, Stage VI oocytes of Xenopus laevis fail to activate (i.e., to propagate a cortical reaction and elevate a fertilization envelope) when pricked or exposed to A23187. We determined the times during maturation when immature oocytes treated with progesterone in vitro developed the capacity to respond to pricking and to ionophore. Responsiveness to ionophore first appears at about 3.5-4.5 hr after progesterone treatment; all oocytes are activated by 8-9 hr after progesterone. The capacity to respond to pricking appears about 1.0-1.5 hr after first signs of ionophore responsiveness. We examined the cortical endoplasmic reticulum (CER) by TEM to determine whether the morphology of this component could be correlated with the development of responsiveness during maturation. Fully mature oocytes exhibit an extensive CER that (1) forms a "shell" around most cortical granules, (2) appears to interconnect cortical granules, and (3) forms junctions with the plasma membrane. The CER-plasma membrane junctions are especially obvious in preparations of isolated cortex. The elaborate CER is not present in immature oocytes. It first appears during maturation of progesterone-treated oocytes at 4.5-5.0 hr, coincident with the time when maturing oocytes develop their responsiveness to ionophore and to pricking. This temporal correlation is consistent with the hypothesis that the CER is one of the components required for regulation of intracellular free calcium in oocytes.

Sources of calcium in egg activation: a review and hypothesis

A careful reanalysis of the literature indicates that the initial mechanism of activation in sea urchin eggs is remarkably similar to the mechanism established in medaka eggs: i.e., sea urchin eggs are activated by a qualitatively and quantitatively similar calcium explosion; one which is propagated in a wave sustained by the calcium-stimulated release of calcium from internal sources. These sources are probably in the endoplasmic reticulum. An exhaustive survey of the literature reveals that a wide variety of other activating eggs in the vertebrate line also exhibit secretory waves which are propagated at about 10 microns/sec, and can thus be assumed to reflect the same basic mechanism. Activating protostome eggs on the other hand do not exhibit such waves. This and other systematic differences from deuterostomes suggest that unlike deuterostome eggs, protostome eggs are primarily activated by calcium ions which enter the cytosol from the medium, and do so in response to depolarization of the egg's plasma membrane.

Roles of calcium and pH in activation of eggs of the medaka fish, Oryzias latipes

Unfertilized eggs of the medaka fish (Oryzias latipes) were injected with pH-buffered calcium buffers. Medaka egg activation is accompanied by a transient increase in cytoplasmic free calcium (Gilkey, J. C., L. F. Jaffe, E. B. Ridgway, and G. T. Reynolds, 1978, J. Cell Biol., 76:448-466). The calcium buffer injections demonstrated that (a) the threshold free calcium required to elicit the calcium transient and activate the egg is between 1.7 and 5.1 microM at pH 7.0, well below the 30 microM reached during the transient, and (b) buffers which hold free calcium below threshold prevent activation of the buffered region in subsequently fertilized eggs. Therefore an increase in free calcium is necessary and sufficient to elicit the calcium transient, and the calcium transient is necessary to activate the egg. Further, these results are additional proof that the calcium transient is initiated and propagated through the cytoplasm by a mechanism of calcium-stimulated calcium release. Finally, a normal calcium transient must propagate through the entire cytoplasm to ensure normal development. Unfertilized eggs were injected with pH buffers to produce short-term, localized changes in cytoplasmic pH. The eggs were then fertilized at various times after injection. In other experiments, unfertilized and fertilized eggs were exposed to media containing either NH4Cl or CO2 to produce longer term, global changes in cytoplasmic pH. These treatments neither activated the eggs nor interfered with the normal development of fertilized eggs, suggesting that even if a natural change in cytoplasmic pH is induced by activation, it has no role in medaka egg development. The injected pH buffers altered the rate of propagation of the calcium transient through the cytoplasm, suggesting that the threshold free calcium required to trigger calcium-stimulated calcium release might be pH dependent. The results of injection of pH-buffered calcium buffers support this conjecture: for a tenfold increase in hydrogen ion concentration, free calcium must also be raised tenfold to elicit the calcium transient.