Specification of Larval Axes of Partial Embryos in the Temnopleurid Temnopleurus toreumaticus and the Strongylocentroid Hemicentrotus pulcherrimus

Abstract

Many sea urchins, including the strongylocentroid Hemicentrotus pulcherrimus, produce an amniotic cavity on the left for adult rudiment formation at the late larval stage. In contrast, temnopleurids form a cell mass at the early larval stage instead of an amniotic cavity. Although the mechanisms establishing left-right polarity of the amniotic cavity involve cell-cell interactions and signaling pathways, corresponding pathways for the cell mass are unknown. We analyzed the effects of blastomere isolation on the specification of larval axes in the temnopleurid Temnopleurus toreumaticus and compared them to those in H. pulcherrimus. Blastomere isolation at the two- or four-cell stages in T. toreumaticus disturbed the location of the cell mass and adult rudiment in approximately 10-20% of specimens. In contrast, isolation at the two-cell stage in H. pulcherrimus caused the left-right polarity to become random. When blastomeres isolated at the two-cell stage were cultured as pairs, approximately 20% of pairs had atypical polarity in both species. Following isolation at the four-cell stage, 71.4% of quartets produced larvae with atypical polarity in T. toreumaticus. Thus, cell-cell interaction between two daughter blastomeres after the second cleavage may be involved in the mechanism determining left-right polarity. Dye injection into a blastomere and subsequent observations indicated that the location of the boundary of the first cleavage showed similar patterns in both species. These observations suggest that species-specific mechanisms establish the larval axes and blastomeres at the two- and four-cell stages redistribute their cytoplasm, forming gradients that establish left-right polarity.

Revealing the cells fated to form the cell mass in embryos of temnopleurid sea urchins

“…Next, we analyzed the number of Mesos necessary for CM formation, by isolating the Mesos at the 16‐cell stage. It was reported that larvae of T. toreumaticus derived from each blastomere isolated at the 2‐ or 4‐cell stage exhibited the potential to form a CM (Kitazawa et al, 2017), and this result indicated that CM formation requires at least 2 Mesos. In the present study, CM formation was confirmed at approximately 4 days after fertilization.…”

“…The size of the CM in specimens derived from a single blastomere at the 2-cell stage increased from about 15 µm 4 days after fertilization to 25 µm 5 days after fertilization (Kitazawa et al, 2017). This suggests that at least 4 Mesos are needed for an embryo to form a CM with a normal size.…”

“…Therefore, if similar mechanisms controlling differentiation of the oral ectoderm are also conserved in temnopleurid sea urchins, an2 progenitor cells obtain signals from the vegetal region until the mesenchyme blastula stage and then obtain the potential to form a CM (Figure 13). In temnopleurid sea urchins, there may exist some cytoplasmic polarity in blastomeres by at least the 4-cell stage (Kitazawa et al, 2017). Consideration of the fate of the vegetal hemisphere suggests signals may be released from the vegetal hemisphere to the an2 region.…”

Effects of Nodal inhibition on development of temnopleurid sea urchins

“…It is believed that among species of the infraorder Temnopleuridea (Camarodonta), patterning inputs for adult rudiment formation changed from the more external amniotic cavity to the internal CM, thereby providing greater protection for the rudiment (Kitazawa et al 2012(Kitazawa et al , 2014. Also, blastomere isolation during the two-and four-cell stages for T. toreumaticus caused atypical left-right polarity (Kitazawa, Nakano, Yamaguchi, Miyahara, & Yamanaka, 2017). However, the mechanisms of adult rudiment formation from the CM are still unclear.…”

“…In experiments with T. toreumaticus, it was reported that position of the CM was disturbed by treatments involving lower specific gravity and inhibition of H + /K + -ATPase activity (Kitazawa, Nakano, Kobayashi, & Yamanaka, 2013), with slightly different treatment effects than found with other species. Also, blastomere isolation during the two-and four-cell stages for T. toreumaticus caused atypical left-right polarity (Kitazawa, Nakano, Yamaguchi, Miyahara, & Yamanaka, 2017).…”