Ovulation and embryo-parent relationships in Botrylloides leachi (Ascidiacea).

The ultrastructural events of ovulation and differentiation of the brood pouch are studied in the colonial ascidian Botrylloides leachi. Each zooid matures one or two yolked oocytes enveloped by test cells, vitelline coat, and inner and outer follicle cells. A small vesicular oviduct lies between the outer follicle cells and the brood pouch, a flask-shaped structure differentiated from the wall of the atrial chamber. The oviduct epithelium fuses with the outer follicle and brood pouch cells and breaks, creating a pathway which drives the oocyte to the brood pouch. Embryos develop in isolation in the latter whose epithelium undergoes differentiation: the cell surface increases enormously, forming deeper and deeper baso-lateral foldings, cell interdigitations and apical polymorphic protrusions. Extended tight junctions separate the different domains of the plasmalemma and create a barrier between the pouch lumen and the blood. Large quantities of glycogen, mainly in the form of rosettes, are formed and accumulate progressively in extensive apical regions. Glycogen, presumably released by apocrine secretion, fills the space between the brood pouch epithelium and the inner follicle cells surrounding the embryo. All features are indicative of a transfer of substances from the blood toward the embryo. The significance of the embryo-parent exchange in B. leachi and comparative aspects of ovoviviparity and viviparity in ascidians are discussed.