Ultrastructural Study of Asters Induced by Microinjection with Sperm Centriolar Fraction in Sea Urchin Eggs

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Abstract
Multiple asters can be artificially induced in sea urchin fertilized eggs by the microinjection of the centriolar fraction of sperm homogenate. Investigation was continued by the EM to determine whether the multi-aster formation was due to the centrioles or the contaminants in the injected sperm fraction. In 3 operated eggs, 33 asters were thoroughly examined, and the presence of centrioles was confirmed in the central region of 26 asters. The rest of them might have contained the centrioles in the sections lost during the preparation procedures. Fragmented axoneme, the plug of electron dense material and the centriolar fossa, which were usually accompanied with the isolated centrioles, disappeared from the centrioles in these multiple asters. Electron dense, amorphous materials were formed associating with the triplet blades and distributed around the centrioles. Many astral microtubules were terminated in these pericentriolar materials. Although the pericentriolar material is acting as the microtubule organizing center, all multiple asters, except those derived from fertilization (2 asters per egg), are most likely induced by the injected centrioles and not by the contaminants.