Organisation ofXenopus oocyte and egg cortices

Abstract

The division of the Xenopus oocyte cortex into structurally and functionally distinct “animal” and “vegetal” regions during oogenesis provides the basis of the organisation of the early embryo. The vegetal region of the cortex accummulates specific maternal mRNAs that specify the development of the endoderm and mesoderm, as well as functionally‐defined “determinants” of dorso‐anterior development, and recognisable “germ plasm” determinants that segregate into primary germ cells. These localised elements on the vegetal cortex underlie both the primary animal‐vegetal polarity of the egg and the organisation of the developing embryo. The animal cortex meanwhile becomes specialised for the events associated with fertilisation: sperm entry, calcium release into the cytoplasm, cortical granule exocytosis, and polarised cortical contraction. Cortical and subcortical reorganisations associated with meiotic maturation, fertilisation, cortical rotation, and the first mitotic cleavage divisions redistribute the vegetal cortical determinants, contributing to the specification of dorso‐anterior axis and segregation of the germ line. In this article we consider what is known about the changing organisation of the oocyte and egg cortex in relation to the mechanisms of determinant localisation, anchorage, and redistribution, and show novel ultrastructural views of cortices isolated at different stages and processed by the rapid‐freeze deep‐etch method. Cortical organisation involves interactions between the different cytoskeletal filament systems and internal membranes. Associated proteins and cytoplasmic signals probably modulate these interactions in stage‐specific ways, leaving much to be understood. Microsc. Res. Tech. 44:415–429, 1999. © 1999 Wiley‐Liss, Inc.

Apparent mitochondrial asymmetry in Xenopus eggs

“…In Xenopus eggs and oocytes, polarity along the animal/vegetal (AV) axis is evident by the asymmetric distribution of cytoplasmic components, including proteins, mRNAs, and organelles (reviewed in Danilchik and Gerhart, 1987;Gard, 1995;Chang et al, 1999;Mowry and Cote, 1999;Wylie, 1999). One of the first manifestations of polarity in the developing Xenopus oocyte is the presence of a mitochondrial cloud within the vegetal hemisphere of immature (stage I; Dumont, 1972) oocytes (Heasman et al, 1984).…”

Fluid dynamics in developmental biology: Moving fluids that shape ontogeny

“…Moreover, there is abundant experimental evidence that links the alignment of cortical microtubules-those just below the cell membrane-with the spontaneous cortical rotation that happens before the first cell division (Elinson and Rowning, 1988;Gerhart et al, 1989). This cortical rotation is known to correlate precisely with the later developing dorsoventral and left-right axes of the embryo (Elinson and Holowacz, 1995;Chang et al, 1999). Some experiments also show that forced cortical rotation-rotating the cortex using an external apparatusworks as well in dorsal-ventral axis determination (Phillips et al, 1996).…”

Établissement et expression des axes embryonnaires : comparaisons entre différents organismes modèles

“…La plus significative et souvent la première de ces différenciations cytoplasmiques axiales des ovocytes est la formation de domaines cytoplasmiques particuliers, appelés plasmes germinatifs, qui sont transmis par la mère via les cellules germinales [15,16] [20,21]. Lorsque le nuage mitochondrial atteint le cortex végétatif, il se disperse sous forme d'îlots de plasmes germinatifs associés à des mitochondries, les ARNm apparaissant alors plaqués contre le cortex végétatif (Poster, Fc-d) [22]. Entre les stades II et IV de l'ovogenèse, une autre classe d'ARNm parvient au cortex: elle comprend en particulier Vg1 (codant pour un facteur de croissance) et VegT (codant pour un facteur de transcription), ces facteurs jouant par la suite un rôle essentiel dans la formation des tissus endomésodermiques [23].…”

“…Ainsi, la protéine membranaire GLP-1 (homologue de Notch) et les protéines cytoplasmiques liant l'ARN (MEX-5/6) sont localisées dans la cellule antérieure AB et ses descendants, alors que la protéine PIE-1, déterminant des cellules germinales, est principalement localisée dans les noyaux et les granules P de la cellule posté-rieure P1 et de ses descendants (Poster, Bj). Chez l'ascidie et le xénope, les réorganisations du cytosquelette suivent l'évolution des facteurs contrôlant le cycle cellulaire [13,22,52]. La méiose s'achève dans la demi-heure qui suit la fécondation.…”