Germinal vesicle nucleoplasm and intracellular pH requirements for cytoplasmic maturity in oocytes of the prosobranch mollusk Patella vulgata

Localization and quantification of cyclin A and B mRNA during the embryonic development of Patella vulgata

As the first five cleavages of the Patella vulgata embryo are synchronous, they are well suited to determine the mRNA level of cyclin A and B genes in an embryo. During the third and fourth cleavage cycle the quantity of A and B mRNA is regulated in a cell-cycle-dependent way, reaching a high level between cleavages and a lower level just after mitosis. This implies that transcription of the cyclin genes occurs before the overall transcription increases directly after the fifth cleavage. During the first cleavages cyclin A and B mRNA is localized in distinct parts of the cytoplasm. Between two successive cell devisions it is found as a crescent-shaped domain at the peripheral side of the nucleus. At cytokinesis it is present between two separating nuclei and at newly formed cell membranes. At the fifth cleavage this localization disappears. Changes in the expression pattern of cyclin A and B may be expected after the fifth cleavage, when the first cells become arrested in cell division and differentiate. The mechanism causing cell division arrest of these primary trochoblasts is still unknown. Cell division arrest caused by the absence of cyclin A and/or B mRNA could be conditional for further differentiation. However, a decrease in cyclin A and B mRNA level in the trochoblasts is not detectable until 4 h after their last division. Later in development no cyclin A and B mRNA can be detected in these cells, whereas cyclin A and B mRNA is present in other cells of the embryo. Thus, the absence of cyclin A and B mRNA in primary trochoblasts, and in the later differentiating secondary and accessory trochoblasts is not obligatory for cell division arrest or cell differentiation.

Activation of polyphosphoinositide metabolism at artificial maturation of Patella vulgata oocytes

The metabolism of polyphosphoinositides (PPI) has been investigated during the meiosis reinitiation of the oocytes of a prosobranch mollusk, the limpet Patella vulgata. Meiosis reinitiation which leads to germinal vesicle breakdown (GVBD) and metaphase-1 spindle formation was artificially induced by treating the prophase-blocked oocytes with 10 mM NH4Cl, pH 8.2. This treatment, which results in a rise in intracellular pH, triggered a general increase in polyphosphoinositide synthesis. Determinations of phosphorus content showed that maturation induced a 30 to 50% increase in both phosphatidylinositol (PI) and phosphatidylinositol-1 monophosphate (PIP) concentrations. Incorporations of 32PO4 and [3H]inositol have been measured in three classes of polyphosphoinositides: PI, PIP, and phosphatidylinositol 4,5-bisphosphate (PIP2). By comparing incorporation rates of the radiolabeled precursors into PPI before and after meiosis reinitiation, we found that artificial maturation by ammonia induced a 50-fold increase in the turnover of these lipids. No significant burst of inositol 1,4,5-trisphosphate (IP3) was observed after maturation. We suggest that modifications in PPI metabolism occurring at maturation of Patella oocytes might ensure the formation of an important stock of PPI that would be available for the profuse production of IP3, the messenger responsible for the Ca2+ signal at fertilization.

Sperm nuclear transformations consist of enlargement and condensation coordinate with stages of meiotic maturation in fertilized Spisula solidissima oocytes

Rates of sperm nuclear expansion were measured and correlated with processing of the maternal chromatin in synchronous populations of fertilized surf clam (Spisula solidissima) oocytes fixed at regular intervals following insemination and stained with the DNA fluorochrome Hoechst 33342. Sperm nuclei expanded in four distinct phases each temporally coordinate with events of meiotic maturation: germinal vesicle stage (phase A), germinal vesicle breakdown (phase B), polar body formation (phase C), and female pronuclear development (phase D). Sperm nuclei were essentially unchanged during phase A (rate = 0.1 micron2/min, enlarged during phases B (rate = 8.2 microns2/min) and D (rate = 6.2 microns2/min), and condensed during phase C (rate = -1.9 micron2/min). Sperm nuclear enlargement during phase D was significantly less in polyspermic and polygynic zygotes. The effects of various treatments (temperature, microtubule disruption, pH alterations, and metabolic and protein synthesis inhibitions) which perturbed sperm nuclear enlargement and meiotic processing of the maternal chromatin indicated that the two processes are coupled and may be linked by common regulatory agents.