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Summary
Botrylloides violaceus, a viviparous colonial ascidian, produces small alecithal eggs which develop into large tadpole larvae after a long period of gestation (about 1 month) in the colonial tunic. Unlike other botryllids, embryo development occurs in a brood pouch which soon becomes detached from the atrial epithelium to reach the colonial tunic and is independent of parent life. This study analyzes the ultrastructural differentiation of the brood pouch—from ovulation to larva release—in order to verify its role in the production, secretion and transport of nutrients for the embryo. Results show that brood pouch cells undergo modifications during embryogenesis, including: (a) increase in baso-lateral foldings of membranes and formation of long basal membranal channels; (b) shifting of tight junctions towards the base of the cells and increase in apical cell region by means of polymorphic, cytoplasmic protrusions; (c) synthesis, storage and secretion towards the embryo of proteinaceous materials and glycogen. All these features indicate that the brood pouch epithelium is involved in nutritional exchanges between the blood flowing into the tunic and the embryo. Aspects of viviparity in ascidians and comparison between the brood pouches of B. violaceus and of the ovoviviparous Botrylloides leachi are discussed.
