Strongylocentrotus drobachiensis oocytes maintain a microtubule organizing center throughout oogenesis: Implications for the establishment of egg polarity in sea urchins

Abstract

Although it has been known for over a century that sea urchin eggs are polarized cells, very little is known about the mechanism responsible for establishing and maintaining polarity. Our previous studies of microtubule organization during sea urchin oogenesis described a cortical microtubule-organizing center (MTOC) present during germinal vesicle (GV) migration in large oocytes. This MTOC was localized within the future animal pole of the mature egg. In this study we have used electron microscopy and immunocytochemistry to characterize the structure of this MTOC and have established that this organelle appears prior to GV migration. We show that the cortical MTOC contains all the components of a centrosome, including a pair of centrioles. Although a centrosome proper was not found in small oocytes, the centriole pair in these cells was always found in association with a striated rootlet, a structural remnant of the flagellar apparatus present in precursor germinal cells (PGCs). The centrioles/striated rootlet complex was asymmetrically localized to the side of the oocyte closest to the gonadal wall. These data are consistent with the previously proposed hypothesis that in echinoderms the polarity of the PGCs in the germinal epithelium influences the final polarity of the mature egg.

A mechanism for the elimination of the female gamete centrosome in Drosophila melanogaster

“…Echinoderms are an exception to centriole elimination in early prophase I arrest, because most centrioles are eliminated through extrusion within the polar bodies during both of the meiotic divisions, with a last centriole being eliminated after meiosis exit (8). Remarkably, echinoderm centrioles are surrounded by PCM (g-tubulin and pericentrin) in all stages of oogenesis (43), which may protect them, further supporting the generality of a PCMdependent centriole maintenance mechanism. It is also known that Polo substrate phosphorylation is important, both for their localization and to reinforce Polo's own localization at the centrosome, in a positive feedback loop (26).…”

Differential Regulation of Disheveled in a Novel Vegetal Cortical Domain in Sea Urchin Eggs and Embryos: Implications for the Localized Activation of Canonical Wnt Signaling

“…To determine when the Dsh protein begins to accumulate in the VCD during oogenesis we carried out immunolocalization of Dsh in oocytes at different oogenic stages obtained from excised ovaries. We noted that the smallest oocytes showed low levels of Dsh expression throughout the cytoplasm and these oocytes did not have the microtubule organizing center (MTOC) that forms at the animal pole prior to meiosis and polar body formation [ 45 ] ( Figure 4A-E ). We observed that the Dsh protein begins to accumulate asymmetrically in the cortex of small oocytes and this pole was identified as the vegetal pole by the presence of an MTOC at the opposite pole of the cell ( Figure 4F-J ).…”

“…Similarly, asteroid (sea star) oocytes express an AV polarity that can be identified by apical centrioles [ 65 , 66 ], an apically displaced nucleus, an absence of large vacuoles, and actin-filled spikes at the animal pole [ 66 ]. In several species of echinoids (sea urchins and sand dollars), the animal pole of oocytes can be traced by the location of the jelly canal, the site of polar body extrusion [ 44 , 56 ], and a cortical microtubule-organizing center (MTOC) [ 45 ]. Hence, the AV polarity becomes morphologically evident during oogenesis, but exactly when during this process the two poles first become polarized at the molecular level has heretofore been unclear [ 67 ].…”

Localization of germ plasm‐related structures during sea urchin oogenesis

“…Additionally, as in many animals with a determinative mode, maternally accumulated germ line determinants in sea urchin eggs are associated with the cortical cytoskeleton. Although sea urchin eggs are committed to an early establishment of the animal–vegetal axis through the activation of Wnt/ β‐catenin signaling (Logan et al, ; Ettensohn, ; Egaña et al, ; Peng and Wikramanayake, ), these eggs do not establish polarity by germ plasm localization. Interestingly, Vasa can solely regulate a cell cycle and interact with the mitotic spindle during the initial cleavages of embryo (Yajima and Wessel, ).…”