Thymidine uptake by developing sea urchin embryos

Activation of Na+-dependent transport at fertilization in the sea urchin: requirements of both an early event associated with exocytosis and a later event involving increased energy metabolism

Transport of glycine, phosphate, and thymidine has been studied in parallel in eggs and embryos of Strongylocentrotus purpuratus. Uptake of each of these substrates is shown to be sodium dependent and expressed at fertilization in this species. The data indicate that activation of unfertilized eggs by ammonia does not result in significant expression of any of these transport systems compared to fertilized eggs. Activation of unfertilized eggs by sperm or by ionophore A-23187 in seawater results in complete development of these transport systems. However, if eggs are activated by ionophore in sodium-free seawater or fertilized by sperm in complete seawater and transferred to sodium-free seawater, Na+-dependent transport does not develop. Ammonia reverses the inhibitory effect of sodium-free seawater on development of these transport systems. This reversal is sensitive to 2,4-dinitrophenol. The data suggest that transport of glycine, phosphate, and thymidine share a common mechanism of activation. Moreover, this activation requires both an early event (less than 5 min postinsemination) and a later event involving increased energy metabolism.

Increased uptake of thymidine in the activation of sea urchin eggs. II. Cooperativity with phosphorylation, involvement of the cortex, and partial localization of the kinases

Uptake and phosphorylation of exogenously supplied thymidine are stimulated in Strongylocentrotus purpuratus eggs after fertilization. Before fertilization, the rate of uptake is low and less than 10% of the thymidine entering the egg is phosphorylated. After fertilization, the rate of uptake increases over 50-fold and greater than 90% of the thymidine is immediately phosphorylated. These results imply that there is close cooperativity between fertilization-induced uptake and phosphorylation of thymidine. To gain insight into the structural basis of this apparent cooperativity and to provide a partial localization of the kinases, uptake and phosphorylation were measured in centrifuged eggs, and in centrifuged nucleate and anucleate merogons. Electron micrographs show that in these cells, the inner cytoplasmic contents are stratified according to density and displaced within the egg, whereas the outer cortical region of the cytoplasm remains intact. Uptake and phosphorylation of thymidine are fully stimulated in these eggs and merogons after fertilization, suggesting that both processes are mediated by an intact egg cortex. In support of this suggestion, we report that controlled disruption of the egg cortex prior to fertilization by treatment with cytochalasin B (CB) significantly reduces the rates of uptake and phosphorylation after fertilization. The full stimulation of phosphorylation in nucleate and anucleate merogons eliminates any localization of the catalyzing enzymes (thymidine kinase and thymidylate kinase) in the maternal nucleus and other inner cytoplasmic contents differentially segregated by centrifugation.

Effect of 5-bromodeoxyuridine on differentiation. I. Probability distribution of BUdR-containing DNA-strands in subsequent divisions

The consequences on transcription of BUdR substitution of the normal thymidine in nuclear DNA are analyzed from a probabilistic point of view. The probability that at any stage subsequent to the administration of BUdR, a randomly chosen cell will be able to make good transcripts is calculated both under the assumption that incorporation of BUdR is complete and under the assumption that incorporation is partial. In the latter case both gradually increasing and decreasing incorporation rates are considered. The presented calculations may be of value for the investigation of any type of differentiating cells. Using the example of the sea urchin embryo it is shown how to calculate the probability that a particular group of cells, which in further development differentiate to a certain organ, will inherit as sufficient number of essential genes unaffected by BUdR in order to ensure a normal development. It is shown that the probability that the four macromeres (or micromeres) in the vegetal half of the 16-cell stage of the sea urchin embryo will contain at least one, two or four plus (or minus) strands of DNA uninfluenced by BUdR gradually decreases the earlier BUdR is administered. The theoretical calculations are in agreement with experimental observations on sea urchin embryos (described in part II).

Effect of 5-bromodeoxyuridine on differentiation. II. The effect of early BUdR treatment on gastrulation of sea urchin embryos

BUdR treatment of early cleavage stages is more effective on gastrulation the earlier the drug is administered before the 16-cell stage. The distribution of percentages of embryos gastrulating is in good agreement with the theoretical calculated values presented in part I and permit the assumption that BUdR may act by preventing normal transcription at, or shortly after, the 16-cell stage.