Polarity and reorganization of the endoplasmic reticulum during fertilization and ooplasmic segregation in the ascidian egg.

Abstract

Abstract. During the first cell cycle of the ascidian egg, two phases of ooplasmic segregation create distinct cytoplasmic domains that are crucial for later development. We recently defined a domain enriched in ER in the vegetal region of Phaltusia mammillata eggs. To explore the possible physiological and developmental function of this ER domain, we here investigate its organization and fate by labeling the ER network in vivo with DiICl6(3), and observing its distribution before and after fertilization in the living egg. In unfertilized eggs, the ER-rich vegetal cortex is overlaid by the ER-poor but mitochondria-rich subcortical myoplasm. Fertilization results in striking rearrangements of the ER network. First, ER accumulates at the vegetal-contraction pole as a thick layer between the plasma membrane and the myoplasm. This accompanies the relocation of the myoplasm toward that region during the first phase of ooplasmic segregation. In other parts of the cytoplasm, ER becomes progressively redistributed into ER-rich and ER-poor microdomains. As the sperm aster grows, ER accumulates in its centrosomal area and along its astral rays. During the second phase of ooplasmic segregation, which takes place once meiosis is completed, the concentrated ER domain at the vegetal-contraction pole moves with the sperm aster and the bulk of the myoplasm toward the future posterior side of the embryo. These results show that after fertilization, ER first accumulates in the vegetal area from which repetitive calcium waves are known to originate (Speksnijder, J. E. 1992. . This ER domain subsequently colocalizes with the myoplasm to the presumptive primary muscle cell region.T nE ER is a key cellular organelle involved in the synthesis of membrane lipids and proteins, secretion, and calcium homeostasis. Morphologically, it is organized as a continuous and extensive network of tubules and cisternae with typical three-way junctions. A number of recent studies have revealed that it is a highly dynamic structure which interacts with a variety of cellular constituents, including microtubules and actin filaments (Terasaki et al.,

Calcium wave pacemakers in eggs

“…In eggs that display repetitive Ca 2+ waves, the interconnected network of ER sheets and tubes is organized into ER-rich domains (also called ER clusters). The cytoplasm also has ER-poor domains, which contain high densities of mitochondria and/or other vesicular organelles (Speksnijder et al, 1993;Stricker et al, 1998;Kline et al, 1999;Dumollard and Sardet, 2001). The ER clusters are made of densely packed tubes and sheets of ER membrane (Speksnijder et al, 1993;Fissore et al, 1999;Terasaki et al, 2001;Dumollard and Sardet, 2001).…”

“…The cytoplasm also has ER-poor domains, which contain high densities of mitochondria and/or other vesicular organelles (Speksnijder et al, 1993;Stricker et al, 1998;Kline et al, 1999;Dumollard and Sardet, 2001). The ER clusters are made of densely packed tubes and sheets of ER membrane (Speksnijder et al, 1993;Fissore et al, 1999;Terasaki et al, 2001;Dumollard and Sardet, 2001). In oscillating eggs [nemertean (Stricker et al, 1998); mouse (Kline et al, 1999); ascidian (Dumollard and Sardet, 2001;Sardet et al, 2002)] as well as in Xenopus eggs (Terasaki et al, 2001), ER-rich domains are concentrated in the 2-6 µm thick layer beneath the plasma membrane and are more dispersed in the deeper cytoplasm (deeper than 5 µm).…”

“…The egg cortex and cytoplasm are filled with an extensive and continuous ER network (Speksnijder et al, 1993;Jaffe and Terasaki, 1993;Terasaki et al, 1996;Terasaki et al, 2001;Stricker et al, 1998;Kline et al, 1999). The ER network contains the intracellular Ca 2+ channels -the Ins(1,4,5)P3 receptors (IP3Rs) and ryanodine receptors (RyRs) -as well as the sarco-endoplasmic reticulum Ca 2+ -ATPases (SERCAs) that pump calcium back into the ER.…”

Fertilisation calcium signals in the ascidian egg

“…The ascidian egg possesses a complex Ca 2+ signalling machinery hosted by the plasma membrane, the endoplasmic reticulum (ER) and mitochondria. The interior of the mature, metaphase I-arrested egg is organised into cortical and cytoplasmic domains made of different concentrations of intracellular organelles (the three main ones being ER, mitochondria and yolk platelets, Speksnijder et al, 1993;Roegiers et al, 1999;Dumollard and Sardet 2001). The Ca 2+ waves are initiated in the cortex and they spread through the subcortex and the deeper cytoplasm of the egg.…”

“…The fertilisation Ca 2+ wave triggers an actomyosin-driven contraction that reorganises the cortex and subcortex of the egg to form a vegetal constriction called the contraction pole, a specialised domain dedicated to the initiation of meiotic Ca 2+ waves (Speksnijder et al, 1993;McDougall and Sardet, 1995;Roegiers et al, 1995;Roegiers et al, 1999;Dumollard et al, 2002). A first Ca 2+ wave pacemaker functions during this cortical contraction and it is translocated towards the vegetal pole of the egg.…”