Mechanisms of cnidocyte development in the moon jellyfish Aurelia.

Stinging cells called cnidocytes are a defining trait of the cnidarians (sea anemones, corals, jellyfish, and their relatives). In hydrozoan cnidarians such as Hydra, cnidocytes develop from interstitial stem cells set aside in the ectoderm. It is less clear how cnidocytes develop outside the Hydrozoa, as other cnidarians appear to lack interstitial stem cells. We addressed this question by studying cnidogenesis in the moon jellyfish (Aurelia) through the visualization of minicollagen-a protein associated with cnidocyte development-as well as transmission electron microscopy. We discovered that developing cnidoblasts are rare or absent in feeding structures rich in mature cnidocytes, such as tentacles and lappets. Using transmission electron microscopy, we determined that the progenitors of cnidocytes have traits consistent with epitheliomuscular cells. Our data suggests a dynamic where cnidocytes develop at high concentrations in the epithelium of more proximal regions, and subsequently migrate to more distal regions where they exhibit high usage and turnover. Similar to some anthozoans, cnidocytes in Aurelia do not appear to be generated by interstitial stem cells; instead, epitheliomuscular cells appear to be the progenitor cell type. This observation polarizes the evolution of cnidogenesis, and raises the question of how interstitial stem cells came to regulate cnidogenesis in hydrozoans.

Cell tracking supports secondary gastrulation in the moon jellyfish Aurelia.

The moon jellyfish Aurelia exhibits a dramatic reorganization of tissue during its metamorphosis from planula larva to polyp. There are currently two competing hypotheses regarding the fate of embryonic germ layers during this metamorphosis. In one scenario, the original endoderm undergoes apoptosis and is replaced by a secondary endoderm derived from ectodermal cells. In the second scenario, both ectoderm and endoderm remain intact through development. In this study, we performed a pulse-chase experiment to trace the fate of larval ectodermal cells. We observed that prior to metamorphosis, ectodermal cells that proliferated early in larval development concentrate at the future oral end of the polyp. During metamorphosis, these cells migrate into the endoderm, extending all the way to the aboral portion of the gut. We therefore reject the hypothesis that larval endoderm remains intact during metamorphosis and provide additional support for the "secondary gastrulation" hypothesis. Aurelia appears to offer the first and only described case where a cnidarian derives its endoderm twice during normal development, adding to a growing body of evidence that germ layers can be dramatically reorganized in cnidarian life cycles.

Structural and Developmental Disparity in the Tentacles of the Moon Jellyfish Aurelia sp.1.

Tentacles armed with stinging cells (cnidocytes) are a defining trait of the cnidarians, a phylum that includes sea anemones, corals, jellyfish, and hydras. While cnidarian tentacles are generally characterized as structures evolved for feeding and defense, significant variation exists between the tentacles of different species, and within the same species across different life stages and/or body regions. Such diversity suggests cryptic distinctions exist in tentacle function. In this paper, we use confocal and transmission electron microscopy to contrast the structure and development of tentacles in the moon jellyfish, Aurelia species 1. We show that polyp oral tentacles and medusa marginal tentacles display markedly different cellular and muscular architecture, as well as distinct patterns of cellular proliferation during growth. Many structural differences between these tentacle types may reflect biomechanical solutions to different feeding strategies, although further work would be required for a precise mechanistic understanding. However, differences in cell proliferation dynamics suggests that the two tentacle forms lack a conserved mechanism of development, challenging the textbook-notion that cnidarian tentacles can be homologized into a conserved bauplan.

Evolutionary origin of rhopalia: insights from cellular-level analyses of Otx and POU expression patterns in the developing rhopalial nervous system.

In Cnidaria, the medusae of Scyphozoa and its sister-group Cubozoa uniquely possess rhopalia at their bell margin. These sensory centers coordinate behavior and development. We used fluorescent in situ hybridization and confocal microscopy to examine mRNA expression patterns in Aurelia sp.1 (Cnidaria, Scyphozoa) during early medusa formation, while simultaneously visualizing the developing nervous system by immunofluorescence. The genes investigated include AurOtx1, and the POU genes, AurPit1, and AurBrn3, homologs of genes known to function in cephalar neural organization and sensory cell differentiation across Bilateria. Our results show that AurOtx1 expression defines the major part of the oral neuroectodermal domain of the rhopalium, within which distinct populations of AurBrn3- and AurPit1-expressing sensory cells develop. Thus, despite the unique attributes of rhopalial evolution, we suggest that the rhopalial nervous system of scyphozoan medusae involves similar patterns of differential expression of genes that function in bilaterian cephalic structure and neuroendocrine system development. We propose that rhopalia evolved from preexisting sensory structures that developed distinct populations of sensory cells differentially expressing POU genes within Otx oral-neuroectodermal domains. This implies some commonality of developmental genetic functions involving these genes in the still poorly constrained common ancestor of bilaterians and cnidarians.

Development of the rhopalial nervous system in Aurelia sp.1 (Cnidaria, Scyphozoa).

We examined the development of the nervous system in the rhopalium, a medusa-specific sensory structure, in Aurelia sp.1 (Cnidaria, Scyphozoa) using confocal microscopy. The rhopalial nervous system appears primarily ectodermal and contains neurons immunoreactive to antibodies against tyrosinated tubulin, taurine, GLWamide, and FMRFamide. The rhopalial nervous system develops in an ordered manner: the presumptive gravity-sensing organ, consisting of the lithocyst and the touch plate, differentiates first; the "marginal center," which controls swimming activity, second; and finally, the ocelli, the presumptive photoreceptors. At least seven bilaterally arranged neuronal clusters consisting of sensory and ganglion cells and their neuronal processes became evident in the rhopalium during metamorphosis to the medusa stage. Our analysis provides an anatomical framework for future gene expression and experimental studies of development and functions of scyphozoan rhopalia.

Early development, pattern, and reorganization of the planula nervous system in Aurelia (Cnidaria, Scyphozoa).

We examined the development of the nervous system in Aurelia (Cnidaria, Scyphozoa) from the early planula to the polyp stage using confocal and transmission electron microscopy. Fluorescently labeled anti-FMRFamide, antitaurine, and antityrosinated tubulin antibodies were used to visualize the nervous system. The first detectable FMRFamide-like immunoreactivity occurs in a narrow circumferential belt toward the anterior/aboral end of the ectoderm in the early planula. As the planula matures, the FMRFamide-immunoreactive cells send horizontal processes (i.e., neurites) basally along the longitudinal axis. Neurites extend both anteriorly/aborally and posteriorly/orally, but the preference is for anterior neurite extension, and neurites converge to form a plexus at the aboral/anterior end at the base of the ectoderm. In the mature planula, a subset of cells in the apical organ at the anterior/aboral pole begins to show FMRFamide-like and taurine-like immunoreactivity, suggesting a sensory function of the apical organ. During metamorphosis, FMRFamide-like immunoreactivity diminishes in the ectoderm but begins to occur in the degenerating primary endoderm, indicating that degenerating FMRFamide-immunoreactive neurons are taken up by the primary endoderm. FMRFamide-like expression reappears in the ectoderm of the oral disc and the tentacle anlagen of the growing polyp, indicating metamorphosis-associated restructuring of the nervous system. These observations are discussed in the context of metazoan nervous system evolution.

Embryonic development and metamorphosis of the scyphozoan Aurelia.

We investigated the development of Aurelia (Cnidaria, Scyphozoa) during embryogenesis and metamorphosis into a polyp, using antibody markers combined with confocal and transmission electron microscopy. Early embryos form actively proliferating coeloblastulae. Invagination is observed during gastrulation. In the planula, (1) the ectoderm is pseudostratified with densely packed nuclei arranged in a superficial and a deep stratum, (2) the aboral pole consists of elongated ectodermal cells with basally located nuclei forming an apical organ, which is previously only known from anthozoan planulae, (3) endodermal cells are large and highly vacuolated, and (4) FMRFamide-immunoreactive nerve cells are found exclusively in the ectoderm of the aboral region. During metamorphosis into a polyp, cells in the planula endoderm, but not in the ectoderm, become strongly caspase 3 immunoreactive, suggesting that the planula endoderm, in part or in its entirety, undergoes apoptosis during metamorphosis. The polyp endoderm seems to be derived from the planula ectoderm in Aurelia, implicating the occurrence of "secondary" gastrulation during early metamorphosis.