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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.11.035 Byline: Atsuko Yamazaki (a), Rika Kawabata (a), Kosuke Shiomi (a), Jun Tsuchimoto (a), Masato Kiyomoto (b), Shonan Amemiya (c), Masaaki Yamaguchi (a) Keywords: Kruppel-like; Mesoderm specification; Delta/Notch signaling; Effector; Sea urchin embryo Abstract: The canonical Wnt pathway plays a central role in specifying vegetal cell fate in sea urchin embryos. SpKrl has been cloned as a direct target of nuclear [beta]-catenin. Using Hemicentrotus pulcherrimus embryos, here we show that HpKrl controls the specification of secondary mesenchyme cells (SMCs) through both cell-autonomous and non-autonomous means. Like SpKrl, HpKrl was activated in both micromere and macromere progenies. To examine the functions of HpKrl in each blastomere, we constructed chimeric embryos composed of blastomeres from control and morpholino-mediated HpKrl-knockdown embryos and analyzed the phenotypes of the chimeras. Micromere-swapping experiments showed that HpKrl is not involved in micromere specification, while micromere-deprivation assays indicated that macromeres require HpKrl for cell-autonomous specification. Transplantation of normal micromeres into a micromere-less host with morpholino revealed that macromeres are able to receive at least some micromere signals regardless of HpKrl function. From these observations, we propose that two distinct pathways of endomesoderm formation exist in macromeres, a Krl-dependent pathway and a Krl-independent pathway. The Krl-independent pathway may correspond to the Delta/Notch signaling pathway via GataE and Gcm. We suggest that Krl may be a downstream component of nuclear [beta]-catenin required by macromeres for formation of more vegetal tissues, not as a member of the Delta/Notch pathway, but as a parallel effector of the signaling (Krl-dependent pathway). Author Affiliation: (a) Division of Life Science, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan (b) Tateyama Marine Laboratory, Marine and Coastal Research Center, Ochanomizu University, Kou-yatsu, Tateyama 294-0301, Japan (c) Misaki Marine Biological Station, Graduate School of Science, University of Tokyo, Misaki, Miura 238-0225, Japan Article History: Received 22 June 2007; Revised 6 November 2007; Accepted 20 November 2007

The larval skeleton of sea urchin embryos is formed by primary mesenchyme cells (PMCs). Thereafter, the larvae start feeding and additional arms develop. An adult rudiment that contains spines, tube feet, tests, and other parts of the adult body is formed in the eight-armed larva. The cellular mechanism of the later skeletogenesis and the lineage of the adult skeletogenic cells are not known. In this study, the morphogenesis of larval and adult skeletons during larval development of the sea urchin Hemicentrotus pulcherrimus was investigated by immunostaining cells with PMC-specific monoclonal antibodies, which are useful markers of skeletogenic cells. All spicules and the associated cells in the later larvae were stained with the antibodies. We could observe the initiation of skeletal morphogenesis at each developmental stage and visualize the cellular basis of skeleton formation in whole-mount embryos that possessed an intact morphology. There were some similarities between PMCs and the later skeletogenic cells. Both had a rounded shape with some filopodia, and the antigen expression started just before overt spicule formation. In the later-stage embryos, cells with filopodia and faint antigen expression were observed migrating in the blastocoel or aggregating in the presumptive location of new skeletogenesis., Introduction The adult rudiment, which is an adult organ formed in a part of the larval body in echinoderms, has been attracting interest from embryologists and cell biologists for a [...]

Coelomogenesis in the isocrinid sea lily, Metacrinus rotundus, is described through the swimming larval stages. After the late gastrula stage, the archenteron separates from the ectoderm to form an archenteral sac, which develops into a dumbbell shape consisting of anterior and posterior lobes, and a middle part connecting both lobes. The anterior and posterior lobes, and the middle part, become separated into an axo‐hydrocoel, the left and right somatocoels and an enteric sac, respectively. The hydrocoel forms from the left lower edge of the axo‐hydrocoel and becomes separated from the axocoel by the late dipleurula stage, when chambered organs and coelom X bud off from the anterior tip of the right and left somatocoels, respectively. Coelom X does not occur in comatulid crinoids (feather stars), and its fate is unclear. The pore canal extends from the axocoel. The hydrocoel differentiates into a crescent shape at the overtime semidoliolaria stage, a few days after the semidoliolaria becomes competent to settle. Coelomogenesis in M. rotundus is much simpler than in the comatulids and probably represents the ancestral mode of the crinoids. As each portion of the dumbbell sac differentiates almost in situ into each coelom, presumptive fates in the sac are easily followed in M. rotundus. [ABSTRACT FROM AUTHOR]