Ultrastructural Study of Oogenesis in the Compound Ascidian Botryllus schlosseri(Tunicata)

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Abstract
On the basis of ultrastructural observations, Botryllus schlosserioogenesis is divided into five stages, of which stages 1–3 and 4–5 refer to previtellogenic and vitellogenic phases, respectively. Special attention was paid to the mechanisms of acquisition of nutrient, organelle differentiation, yolk formation and relationships between oocyte and its complex investment.In the gonadal blastema of young buds, small oocytes are recognized early, partially covered by a few primary follicle cells, the presumptive source of all the cellular envelopes of the fully grown oocyte. Some oocytes are seen in the zygotene–pachytene phase, characterized by synaptonemal complexes, but most are found at the end of the diplotene, when oocytes commonly stop meiosis, grow and migrate through blood vessels into blastozooids of successive generations. In laboratory animals, only 1–3 oocytes per blastozooid can reach complete maturity.Detailed observations on the time of appearance and development of organelles, microvilli and endocytotic activity in growing oocytes are also reported. Ultrastructural differentiation and the relationships between oocytes and their envelopes all indicate that both auto– and hetero–synthetic mechanisms for yolk formation occur in B. schlosseri.These mechanisms and the possible role of egg envelopes in the process are discussed, comparing the large yolky eggs of B. schlosseriwith those of solitary species.