RESULTS

Cells are arranged into species-specific patterns during early embryogenesis. Such cell division patterns are important since they often reflect the distribution of localized cortical factors from eggs/fertilized eggs to specific cells as well as the emergence of organismal form. However, it has proven difficult to reveal the mechanisms that underlie the emergence of cell positioning patterns that underlie embryonic shape, likely because a systems-level approach is required that integrates cell biological, genetic, developmental, and mechanical parameters. The choice of organism to address such questions is also important. Because ascidians display the most extreme form of invariant cleavage pattern among the metazoans, we have been analyzing the cell biological mechanisms that underpin three aspects of cell division (unequal cell division (UCD), oriented cell division (OCD), and asynchronous cell cycles) which affect the overall shape of the blastula-stage ascidian embryo composed of 64 cells. In ascidians, UCD creates two small cells at the 16-cell stage that in turn undergo two further successive rounds of UCD. Starting at the 16-cell stage, the cell cycle becomes asynchronous, whereby the vegetal half divides before the animal half, thus creating 24-, 32-, 44-, and then 64-cell stages. Perturbing either UCD or the alternate cell division rhythm perturbs cell position. We propose that dynamic cell shape changes propagate throughout the embryo via cell-cell contacts to create the ascidian-specific invariant cleavage pattern

Embryos of the viviparous dermapteran, Arixenia esau develop sequentially in two compartments: terminal ovarian follicles and the uterus.

Three main reproductive strategies have been described among insects: most common oviparity, ovoviviparity and viviparity. In the latter strategy, the embryonic development takes place within the body of the mother which provides gas exchange and nutrients for embryos. Here we present the results of histological and EM analyses of the female reproductive system of the viviparous earwig, Arixenia esau, focusing on all the modifications related to the viviparity. We show that in the studied species the embryonic development consists of two "physiological phases" that take place in two clearly disparate compartments, i.e. the terminal ovarian follicle and the uterus. In both compartments the embryos are associated with synthetically active epithelial cells. We suggest that these cells are involved in the nourishment of the embryo. Our results indicate that viviparity in arixeniids is more complex than previously considered. We propose the new term "pseudoplacento-uterotrophic viviparity" for this unique two-phase reproductive strategy

Asymmetric Divisions in Oogenesis

International audienceIn the majority of animals, the oocyte/egg is structurally, molecularly, and functionally asymmetric. Such asymmetry is a prerequisite for a flawless fertilization and faithful segregation of maternal determinants during subsequent embryonic development. The oocyte asymmetry develops during oogenesis and must be maintained during consecutive and obligatorily asymmetric oogonial divisions, which depending on the species lead to the formation of either oocyte alone or oocyte and nurse cell complex. In the following chapter, we summarize current knowledge on the asymmetric oogonial divisions in invertebrate (insects) and vertebrate (Xenopus) species

Gametic synapses, nanotubes and sperm RNAs - Redefining the origin of maternal determinants

International audienceThe female germline cells, i.e., the oocytes/eggs, contain a subpopulation of unique organelles and molecules ( RNA and proteins) collectively called "the maternal determinants" that are indispensable for the determination of cell fate in the developing embryo. Although it has been known for some time that somatic cells deliver low-molecular-weight molecules to the oocyte/egg, the paradigm has been that the larger molecules and organelles are synthesized by the female germline cells without input from the surrounding somatic cells. However, recent discoveries of novel types of intercellular connections such as gametic synapses and tunneling nanotubes, allowing the transfer of large, externally derived molecules to the oocyte/egg, may dismantle the paradigm of the transcriptional/translational self-containment of the female gamete and add novel and unexpected aspects to the origin and identity of maternal determinants. In addition, the discovery that sperm delivers various RNAs to the egg suggests that sperm may not only epigenetically modify the egg genome but also influence or modify information contained in the maternal determinants

Secretory activity of the uterus epithelial cells.

<p>Transverse section through the infolding of the uterus wall (boxed fragment of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064087#pone-0064087-g001" target="_blank">Figure 1C</a>). Two closely opposed epithelial layers are visible. Note argyrophylic secretory vacuoles in the cytoplasm of epithelial cells (encircled). Basement lamina (empty arrows), tracheal branches (black arrowheads), argyrophylic material on the cell surface (white asterisks). Semi-thin section stained with AgNOR technique. LM, scale bar: 24 µm.</p

Morphology of the female reproductive system.

<p>(<b>A</b>) Three ovarioles (ov) attached to the uterus (u). White arrow indicates circumferential muscle fibers surrounding the posteriormost section of the uterus. Tracheal branches (black arrowheads). SEM, scale bar: 0.5 mm. (<b>B, C</b>) Transverse section through the uterus (fragment). Note folded epithelium (ep) lining the uterus (u). Basement lamina (empty arrows), muscle fibers (white arrowheads), tracheal branches (black arrowheads), grooves separating ridges of the epithelium (arrows), black asterisks in (C) indicate argyrophylic material on the cell surface. Boxed fragment in (C) is enlarged in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064087#pone-0064087-g002" target="_blank">Figure 2</a>. (B) Semi-thin section stained with methylene blue. (C) Semi-thin section stained with AgNOR technique. LM, scale bar: 24 µm. (<b>D</b>) Basement lamina (bl) supporting epithelial cells (ep) of the uterus. The lamina is penetrated by canals (c) containing tracheal branches (black arrowheads) immersed in filamentous material. TEM, scale bar: 1 µm. (<b>E, F</b>) Apical compartment of an epithelial cell lining the uterus. Note large secretory vacuoles (v), fibro-granular material covering the tips of the microvilli (white asterisk) and an adherens junction (aj) connecting membranes of neighboring cells. Mitochondria (m). TEM, scale bar: 1 µm. (<b>G</b>) Basal compartments of epithelial cells of the uterus. Mitochondria (m), elements of rough endoplasmic reticulum (rer). Note that plasma membranes are folded and closely adjoined. TEM, scale bar: 1 µm.</p