The follicular envelope of ascidian eggs: A site of messenger RNA and protein synthesis during early embryogenesis

Spawned ascidian eggs are surrounded by an extra‐embryonic envelope that consists of test cells and follicle cells and remains closely associated with the developing embryo until the time of larval hatching. The purpose of the present study was to determine whether this envelope is involved in macromolecular synthesis during early embryogenesis. Autoradiographic detection of radioactive uridine and leucine incorporation into the egg envelope of Ciona intestinalis and Styela plicata showed that this structure was active in RNA and protein synthesis between fertilization and the sixteen‐cell stage. In both ascidian species the components of the envelope active in macromolecular synthesis were the follicle cells. As shown by in situ hybridization with an [3H]‐poly (U) probe, the follicle cells but not the test cells contained substantial accumulations of poly(A)‐containing RNA during embryogenesis. Approximately 90% of the RNA synthesized in the follicle cells was poly(A)‐containing RNA. Both follicle cells and test cells, however, participated in macromolecular synthesis during oogenesis. The follicle cells of unfertilized Ciona eggs primarily synthesized a 42,000 molecular weight polypeptide which may be coded for by a major 16S putative mRNA species. It is concluded that the follicle cells are active in RNA and protein synthesis during early ascidian development. The production of a major polypeptide species suggests an active physiological role for these cells during embryogenesis.