The ascidian egg envelope in fertilization: structural and molecular features

Abstract

In this report, unpublished and recent findings concerning the structure and function of the ascidian egg coat are compiled in context with fertilization. In the initial stage of ascidian fertilization, sperm interact with a complex egg investment that consists of a layer of follicle cells attached to an acellular vitelline coat. Increasing evidence exists that ascidian sperm are activated at their encounter with the follicle cells. The molecular basis of sperm-follicle cell interactions is discussed in context with sperm binding, membrane proteins and sperm bound glycosidase. The model that suggests a block to polyspermy established by glycosidase released from the follicle cells on fertilization is evaluated and compared with assured facts. Although a number of questions remain to be answered, our recent findings that a cloned β-hexosaminidase from P. mammillata binds exclusively to the follicle cells of unfertilized but not fertilized eggs, indicates that the follicle cells participate in the block to polyspermy. A dual function, mediating sperm activation and a block to polyspermy attributes to the ascidian follicle cells a key position in fertilization. KEY WORDS: fertilization, ascidian, egg coat, follicle cellThe ascidian egg envelope: a survey of the structural features Ascidian eggs are surrounded by an acellular envelope called chorion or vitelline coat (VC) to which follicle cells (FCs) adhere externally and test cells (TC) internally. Multitudinous functions have been attributed to the egg envelope as entity and to each of its structural components. In the present paper we focus on the role of the egg envelope in fertilization particularly the function of FCs in sperm egg-interaction.The pictures of Phallusia mammillata eggs ( Fig. 1 A,B) give a view of the egg and egg envelope representative for solitary and colonial ascidians with external fertilization. The egg of about 130 µm in diameter exhibits a relatively smooth plasma membrane with only few short microvilli. It is surrounded by an acellular VC with test cells attached to the inner and follicle cells to the outer surface. In most solitary species, the follicle cells are cubical or columnar or rarely tall cone-shaped as in Ciona. Test cells typically adhere to the inner matrix of the VC not to the egg surface. In some species, they form an almost closed sheet in others they are loosely arranged in groups. Although scanning electron microscopy (SEM) pictures reveal a good 3D impression, Int. J. Dev. Biol. 52: 527-533 (2008) the real dimension of the highly hydrated VC layers is never preserved and the highly vacuolated follicle cells are shriveled. This is also the case if conventional preparation is applied for transmission electron microscopy (TEM). Due to dehydration the VC appears as a relatively thin bi-or tripartite filamentous structure of about 400 to 900 nm (Honegger, 1986;Xie and Honegger, 1993;Cloney, 1995;Burighel and Cloney, 1997). Especially the filamentous structure of the inner VC layer mostly appears compressed and...

Ascidian follicle cells: Multifunctional adjuncts to maturation and development

“…In scanning electron micrographs the surface of Phallusia and Ascidia follicle cells is covered by microannuli (Fig. 2D,E) or membrane protrusions varying in size between 1.0 and 2.5 μm (Villa & Patricolo 1993; Honegger & Füglister 2001; Honegger & Koyanagi 2008) which seem to be absent in styelid follicle cells (Villa & Patricolo 2000). These surface ornaments preferentially bind wheat germ agglutinin (WGA) in Ascidia ceratodes (Fig.…”

Glycobiology of Reproductive Processes in Marine Animals: The State of the Art

“…The most common molecules on the surface of oocyte envelope are glycoproteins [71,72,73,74] that are provided by the liver [75] or by the ovary [76,77,78] or by both of them [79]. The vitelline envelope possesses many functions, such as prevention of polyspermy, protection of the growing oocyte and the developing embryo, uptake of nutrients and other molecules during oogenesis, and guidance of the spermatozoa to the oocyte.…”

“…Sperm-egg binding triggers eggs to release large quantities of glycosidase that rapidly bind the vitelline-coat surface as spermatozoa surface glycosidase blocking the binding of supernumerary spermatozoa [74,101,204]. …”

Drosophila sperm surface alpha-l-fucosidase interacts with the egg coats through its core fucose residues

“…Gamete binding in many species has been proposed to involve interactions between proteins on sperm and glycoconjugates on the egg surface (Mengerink and Vacquier, 2001;Evans, 2012;Clark, 2013Clark, , 2014. In particular, experimental evidence suggest that sperm surface glycosidases could act in a non-catalytic or lectinlike manner, recognizing complementary sugar moieties in the egg envelope (Miranda et al, 2000;Martinez et al, 2000;Perotti et al, 2001;Miller et al, 2002;Intra et al, 2006;Zitta et al, 2006;Hedrick, 2008;Honegger and Koyanagi, 2008;Intra et al, 2011). Examples of glycosidases on the sperm plasma membrane include mannosidases and fucosidases in mammalian sperm (Tulsiani et al, 1989(Tulsiani et al, , 1990Venditti et al, 2010;Phopin et al, 2012Phopin et al, , 2013, fucosidases and N-acetylglucosaminidases in tunicates (Hoshi et al, 1983(Hoshi et al, , 1985(Hoshi et al, , 1994Hoshi, 1986;Godknecht and Honegger, 1991;Matsumoto et al, 2002;Downey and Lambert, 1994), and mannosidases, fucosidases and N-acetylglucosaminidases in flies (Cattaneo et al, 1997(Cattaneo et al, , 2002Pasini et al, 1999;Perotti et al, 2001;Intra et al, 2006Intra et al, , 2009Intra et al, , 2011.…”