Oogenesis and oocyte envelope differentiation in the viviparous ascidianBotrylloides violaceus

Abstract

Summary Oogenesis and oocyte envelope differentiation were examined in the viviparous Botrylloides violaceus, a compound ascidian producing small alecithal eggs about 80 μm in diameter. Oogenesis was divided into five stages, defined according to ultrastructural changes in envelopes and oocyte growth. In the gonadal blastema of young buds, each oocyte in the early meiotic prophase is partially enveloped by a few primary follicle cells (stage I) which adhere to the oolemma at several junctional points. In stages II and III, these cells give rise to an external continuous layer (presumptive outer follicle cells) and to underlying scattered elements which later differentiate into test cells and inner follicle cells (stage IV). The direct contribution of mesenchymatous elements was not observed in inner follicle cells and test cells. Observations on the time of appearance and development of organelles, microvilli and endocytotic activity in growing oocytes are reported. Oocytes progressively accumulate “clear...