Ovulation and placentation in Botryllus schlosseri (Ascidiacea): an ultrastructural study

Abstract

The mode of ovulation and placentation was studied by light and electron microscopy in the ovoviviparous ascidian Botryllus schlosseri using colonies from the laboratory. The full-grown oocyte is surrounded by the outer and inner follicle cell layers, the acellular vitelline coat (chorion), and the test cells, and it is furnished with its own vesicular oviduct which is interposed between the egg and the atrial epithelium. In contrast to most ascidians, the outer follicle is thick and has an ultrastructure consistent with intense protein synthesis. At ovulation the outer follicle shows signs of involution where it contacts the oviduct. When the oviducal wall breaks and the egg moves through the oviduct, the outer follicle cells are discharged in the mantle to form a sort of corpus luteum. The egg remains hanging in the atrial chamber by means of a cuplike "placenta." The placental tissues are all of maternal origin, being derived from both the atrial and oviducal epithelia together with some of the inner follicle cells. These latter anchor to the oviducal epithelium by means of junctional spots and a filamentous cementing secretion. Our results suggest that the main role of the "placenta" is to attach the embryo to the parent, thus exposing it to the flow of seawater.

Mechanism of neurogenesis during the embryonic development of a tunicate

“…Embryos are retained in the peribranchial chamber by means of a placental cup; their developmental phases follow the development of the colony, so that they mature and hatch just before adult regresses. After about a week of gestation (Zaniolo et al, 1987), embryos hatch as natatory larvae leaving the colony through the atrial aperture of the parent. The larva, after a short period of swimming life, undergo metamorphosis, giving rise to the sessile oozooid, which reproduces asexually by budding.…”

Sexual and asexual reproduction in the colonial ascidian Botryllus schlosseri

“…The timing of ovulation, sperm release and fertilization in a blastogenetic cycle of sexually mature colonies can be summarized in the following points (Manni et al ., ; Sabbadin, , 1960; Zaniolo et al ., ): oocytes ovulate when the primary buds approach the adult stage; adults discharge most of the sperm one to two days after ovulation (with sperm release continuing up to the end of zooid life‐span); fertilization occurs just after the ovulation; larvae are released just before the colonial take‐over. …”

Ascidian follicle cells: Multifunctional adjuncts to maturation and development

“…In the colonial stolidobranch Botryllus schlosseri , follicle cells secrete a glue‐like substance, which anchors the brooded embryo to its somatic attachment site (Zaniolo et al. 1986).…”

“…Electron microscopy of ascidian follicle cells discloses multiple signs of protein synthesis including a well developed rough endoplasmic reticulum (Cloney 1995;Burighel & Cloney 1997) and secretory granules (Cloney 1995). In the colonial stolidobranch Botryllus schlosseri the outer follicle cells are much larger than the inner follicle cells; these outer follicle cells secrete protein yolk during oogenesis (Zaniolo et al 1986;Martinucci et al 1988) which is incorporated into growing oocytes. Direct autoradiographic Jeffery (1980) demonstrated nuclear incorporation of radioactive uridine and cytoplasmic incorporation of radioactive leucine.…”