Date of Award
Doctor of Philosophy (PhD)
In the sea urchin egg, metabolic activation, DNA synthesis and the chromosome cycle can be induced with parthenogenic activators (NH(,4)Cl or procaine) although no spontaneous cleavage will occur. To determine the effects of these agents on other cell cycle events, eggs from four species of sea urchins were activated with NH(,4)Cl or procaine and then fertilized at different times of the chromosome cycle. These eggs show a decrease in the time to first cleavage which is apparently coupled to the parthenogenecally induced chromosome cycle. The advance is seen whether the activated eggs are fertilized during the first or second chromosome cycle. This chromosome cycle of distinct condensation and decondensation phases can be uncoupled by selective removal of the agent. These removal effects are time dependent; once a cell has reached a critical point the chromosomes are committed to decondense. However, earlier removal results in maximally condensed chromosomes which do not decondense. In either case, if these eggs are then fertilized, cleavage advance is not affected, demonstrating that it is independent of chromosome decondensation. To determine whethter cleavage advance is directly linked to other cycle related events, eggs were activated while in the presence of protein synthesis inhibitors cycloheximide and emetine. Results using these inhibitors indicates that chromosome condensation, but not decondensation, can occur in activated, inhibited eggs. There is no cleavage advance in these eggs. Activation of eggs by either NH(,4)Cl or Ionophore A23187 in Na and/or Ca free sea waters with protein synthesis inhibitors suggest that initiation of condensation is dependent upon cytoplasmic pH, but not upon external Na or Ca ions. Studies of the effects of NH(,4)Cl on surface morphology were also undertaken. NH(,4)Cl induces microvillar elongation approximately 60 minutes after addition. Longitudinally oriented microfilaments are observed in transmission electron micrographs of elongated microvilli. If NH(,4)Cl is removed, microvilli shorten in length and increase in diameter. No microfilaments are visible in short microvilli. When NH(,4)Cl is subsequently readded, microvilli elongate, becoming highly branched and/or knobbed in appearance. Microfilaments are observed in these elongate microvilli.
Belisle, Barbara Ellen wolfanger, "Mitotic Triggers in the Cell Cycle: Phase Dependency of Cleavage Advance on the Chromosome Cycle in Parthenogenetically Activated Sea Urchin Eggs." (1981). LSU Historical Dissertations and Theses. 3628.