Study of larval and adult skeletogenic cells in developing sea urchin larvae

Localization and origins of juvenile skeletogenic cells in the sea urchin Lytechinuspictus

The development of the sea urchin larval body plan is well understood from extensive studies of embryonic patterning. However, fewer studies have investigated the late larval stages during which the unique pentaradial adult body plan develops. Previous work on late larval development highlights major tissue changes leading up to metamorphosis, but the location of specific cell types during juvenile development is less understood. Here, we improve on technical limitations by applying highly sensitive hybridization chain reaction fluorescent in situ hybridization (HCR-FISH) to the fast-developing and transparent sea urchin Lytechinus pictus, with a focus on skeletogenic cells. First, we show that HCR-FISH can be used in L. pictus to precisely localize skeletogenic cells in the rudiment. In doing so, we provide a detailed staging scheme for the appearance of skeletogenic cells around the rudiment prior to and during biomineralization and show that many skeletogenic cells unassociated with larval rods localize outside of the rudiment prior to localizing inside. Second, we show that downstream biomineralization genes have similar expression patterns during larval and juvenile skeletogenesis, suggesting some conservation of skeletogenic mechanisms during development between stages. Third, we find co-expression of blastocoelar and skeletogenic cell markers around juvenile skeleton located outside of the rudiment, which is consistent with data showing that cells from the non-skeletogenic mesoderm embryonic lineage contribute to the juvenile skeletogenic cell lineage. This work sets the foundation for subsequent studies of other cell types in the late larva of L. pictus to better understand juvenile body plan development, patterning, and evolution.

Thyroid hormone-induced cell death in sea urchin metamorphic development

Thyroid hormones (THs) are important regulators of development, metabolism and homeostasis in metazoans. Specifically, they have been shown to regulate the metamorphic transitions of vertebrates and invertebrates alike. Indirectly developing sea urchin larvae accelerate the formation of juvenile structures in response to thyroxine (T4) treatment, while reducing their larval arm length. The mechanisms underlying larval arm reduction are unknown and we hypothesized that programmed cell death (PCD) is linked to this process. To test this hypothesis, we measured larval arm retraction in response to different THs (T4, T3, rT3, Tetrac) and assessed cell death in larvae using three different methods (TUNEL, YO-PRO-1 and caspase-3 activity) in the sea urchin Strongylocentrotus purpuratus. We also compared the extent of PCD in response to TH treatment before and after the invagination of the larval ectoderm, which marks the initiation of juvenile development in larval sea urchin species. We found that T4 treatment results in the strongest reduction of larval arms but detected a significant increase of PCD in response to T4, T3 and Tetrac in post-ingression but not pre-ingression larvae. As post-ingression larvae have initiated metamorphic development and therefore allocate resources to both larval and the juvenile structures, these results provide evidence that THs regulate larval development differentially via PCD. PCD in combination with cell proliferation likely has a key function in sea urchin development.

Sperm lacking Bindin are infertile but are otherwise indistinguishable from wildtype sperm

Cell-cell fusion is limited to only a few cell types in the body of most organisms and sperm and eggs are paradigmatic in this process. The specialized cellular mechanism of fertilization includes the timely exposure of gamete-specific interaction proteins by the sperm as it approaches the egg. Bindin in sea urchin sperm is one such gamete interaction protein and it enables species-specific interaction with a homotypic egg. We recently showed that Bindin is essential for fertilization by use of Cas9 targeted gene inactivation in the sea urchin, Hemicentrotus pulcherrimus. Here we show phenotypic details of Bindin-minus sperm. Sperm lacking Bindin do not bind to nor fertilize eggs at even high concentrations, yet they otherwise have wildtype morphology and function. These features include head shape, tail length and beating frequency, an acrosomal vesicle, a nuclear fossa, and they undergo an acrosomal reaction. The only phenotypic differences between wildtype and Bindin-minus sperm identified is that Bindin-minus sperm have a slightly shorter head, likely as a result of an acrosome lacking Bindin. These data, and the observation that Bindin-minus embryos develop normally and metamorphose into normal functioning adults, support the contention that Bindin functions are limited to species-specific sperm-egg interactions. We conclude that the evolutionary divergence of Bindin is not constrained by any other biological roles.

Genetic manipulation of the pigment pathway in a sea urchin reveals distinct lineage commitment prior to metamorphosis in the bilateral to radial body plan transition

Echinoderms display a vast array of pigmentation and patterning in larval and adult life stages. This coloration is thought to be important for immune defense and camouflage. However, neither the cellular nor molecular mechanism that regulates this complex coloration in the adult is known. Here we knocked out three different genes thought to be involved in the pigmentation pathway(s) of larvae and grew the embryos to adulthood. The genes tested were polyketide synthase (PKS), Flavin-dependent monooxygenase family 3 (FMO3) and glial cells missing (GCM). We found that disabling of the PKS gene at fertilization resulted in albinism throughout all life stages and throughout all cells and tissues of this animal, including the immune cells of the coelomocytes. We also learned that FMO3 is an essential modifier of the polyketide. FMO3 activity is essential for larval pigmentation, but in juveniles and adults, loss of FMO3 activity resulted in the animal becoming pastel purple. Linking the LC-MS analysis of this modified pigment to a naturally purple animal suggested a conserved echinochrome profile yielding a pastel purple. We interpret this result as FMO3 modifies the parent polyketide to contribute to the normal brown/green color of the animal, and that in its absence, other biochemical modifications are revealed, perhaps by other members of the large FMO family in this animal. The FMO modularity revealed here may be important in the evolutionary changes between species and for different immune challenges. We also learned that glial cells missing (GCM), a key transcription factor of the endomesoderm gene regulatory network of embryos in the sea urchin, is required for pigmentation throughout the life stages of this sea urchin, but surprisingly, is not essential for larval development, metamorphosis, or maintenance of adulthood. Mosaic knockout of either PKS or GCM revealed spatial lineage commitment in the transition from bilaterality of the larva to a pentaradial body plan of the adult. The cellular lineages identified by pigment presence or absence (wild-type or knock-out lineages, respectively) followed a strict oral/aboral profile. No circumferential segments were seen and instead we observed 10-fold symmetry in the segments of pigment expression. This suggests that the adult lineage commitments in the five outgrowths of the hydropore in the larva are early, complete, fixed, and each bilaterally symmetric. Overall, these results suggest that pigmentation of this animal is genetically determined and dependent on a population of pigment stem cells that are set-aside in a sub-region of each outgrowth of the pentaradial adult rudiment prior to metamorphosis. This study reveals the complex chemistry of pigment applicable to many organisms, and further, provides an insight into the key transitions from bilateral to pentaradial body plans unique to echinoderms.

DNA damages and offspring quality in sea urchin Paracentrotus lividus sperms exposed to ZnO nanoparticles

The recent advances in nanotechnology lead to a potential increase of the release of nanoparticles (NPs) into marine environment through different routes, with possible toxic effects upon the living part of this ecosystem. One of the ways of NPs marine contamination gaining today increasing concern stems from the widespread use cosmetics containing ZnO NPs as UV-filter. Although the possible adverse effects on marine organisms have been already ascertained, the information about the possible genotoxicity of ZnO NPs is still scant. In this work the spermiotoxicity of ZnO particles of different sizes (ZnO Bulk > 200 nm, ZnO NPs 100 nm and ZnO NPs 14 nm) was assessed, using Paracentrotus lividus spermatozoa, by evaluating the DNA damage of the exposed sperm, fertilization capability and DNA damage transmission to progeny. Our results showed that ZnO NPs induced DNA damages in spermatozoa after 30 min of exposure. While the sperm fertilization capability was not affected, morphological alterations (skeletal alterations) in offspring were observed and a positive correlation between sperm DNA damage and offspring quality was reported. This study underlines that a possible spermiotoxic action of ZnO NPs at concentration close to those reported in marine coastal water could occur.

Thyroid Hormones Accelerate Initiation of Skeletogenesis via MAPK (ERK1/2) in Larval Sea Urchins (Strongylocentrotus purpuratus)

Thyroid hormones are important regulators of development and metabolism in animals. Their function via genomic and non-genomic actions is well-established in vertebrate species but remains largely elusive among invertebrates. Previous work suggests that thyroid hormones, principally 3,5,3',5'-Tetraiodo-L-thyronine (T4), regulate development to metamorphosis in sea urchins. Here we show that thyroid hormones, including T4, 3,5,3'-triiodo-l-thyronine (T3), and 3,5-Diiodothyronine (T2) accelerate initiation of skeletogenesis in sea urchin gastrulae and pluteus larvae of the sea urchin Strongylocentrotus purpuratus, as measured by skeletal spicule formation. Fluorescently conjugated hormones show T4 binding to primary mesenchyme cells in sea urchin gastrulae. Furthermore, our investigation of TH mediated skeletogenesis shows that Ets1, a transcription factor controlling initiation of skeletogenesis, is a target of activated (phosphorylated) mitogen-activated protein kinase [MAPK; extracellular signal-regulated kinase 1/2 (ERK1/2)]. As well, we show that PD98059, an inhibitor of ERK1/2 MAPK signaling, prevents the T4 mediated acceleration of skeletogenesis and upregulation of Ets1. In contrast, SB203580, an inhibitor of p38 MAPK signaling, did not inhibit the effect of T4. Immunohistochemistry revealed that T4 causes phosphorylation of ERK1/2 in presumptive primary mesenchyme cells and the basal membrane of epithelial cells in the gastrula. Pre-incubation of sea urchin gastrulae with RGD peptide, a competitive inhibitor of TH binding to integrins, inhibited the effect of T4 on skeletogenesis. Together, these experiments provide evidence that T4 acts via a MAPK- (ERK1/2) mediated integrin membrane receptor to accelerate skeletogenesis in sea urchin mesenchyme cells. These findings shed light, for the first time, on a putative non-genomic pathway of TH action in a non-chordate deuterostome and help elucidate the evolutionary history of TH signaling in animals.

Skeletal regeneration in the brittle star Amphiura filiformis

Background

Brittle stars regenerate their whole arms post-amputation. Amphiura filiformis can now be used for molecular characterization of arm regeneration due to the availability of transcriptomic data. Previous work showed that specific developmental transcription factors known to take part in echinoderm skeletogenesis are expressed during adult arm regeneration in A. filiformis; however, the process of skeleton formation remained poorly understood. Here, we present the results of an in-depth microscopic analysis of skeletal morphogenesis during regeneration, using calcein staining, EdU labeling and in situ hybridization.

Results

To better compare different samples, we propose a staging system for the early A. filiformis arm regeneration stages based on morphological landmarks identifiable in living animals and supported by histological analysis. We show that the calcified spicules forming the endoskeleton first appear very early during regeneration in the dermal layer of regenerates. These spicules then mature into complex skeletal elements of the differentiated arm during late regeneration. The mesenchymal cells in the dermal area express the skeletal marker genes Afi-c-lectin, Afi-p58b and Afi-p19; however, EdU labeling shows that these dermal cells do not proliferate.

Conclusions

A. filiformis arms regenerate through a consistent set of developmental stages using a distalization-intercalation mode, despite variability in regeneration rate. Skeletal elements form in a mesenchymal cell layer that does not proliferate and thus must be supplied from a different source. Our work provides the basis for future cellular and molecular studies of skeleton regeneration in brittle stars.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0149-x) contains supplementary material, which is available to authorized users.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

Exposure of Paracentrotus lividus male gametes to engineered nanoparticles affects skeletal bio-mineralization processes and larval plasticity

The aim of this study is to contribute to the understanding of the mechanisms underlying nanoparticle (NP)-induced embryotoxicity in aquatic organisms. We previously demonstrated that exposure of male gametes to NPs causes non-dose-dependent skeletal damage in sea urchin (Paracentrotus lividus) larvae. In the present study, the molecular mechanisms responsible for these anomalies in sea urchin development from male gametes exposed to cobalt (Co), titanium dioxide (TiO2) and silver (Ag) NPs were investigated by histochemical, immunohistochemical and Western blot analyses. P. lividus sperm were exposed to different NP concentrations (from 0.0001 to 1 mg/L). The distribution of molecules related to skeletogenic cell identification, including ID5 immunoreactivity (IR), wheat germ agglutinin (WGA) affinity and fibronectin (FN) IR, were investigated by confocal laser scanning microscopy at the gastrula (24 h) and pluteus (72 h) stages. Our results identified a spatial correspondence among PMCs, ID5 IR and WGA affinity sites. The altered FN pattern suggests that it is responsible for the altered skeletogenic cell migration, while the Golgi apparatus of the skeletogenic cells, denoted by their WGA affinity, shows different aspects according to the degree of anomalies caused by NP concentrations. The ID5 IR, a specific marker of skeletogenic cells in sea urchin embryos (in particular of the msp130 protein responsible for Ca(2+) and Mg(2+) mineralization), localized in the cellular strands prefiguring the skeletal rods in the gastrula stage and, in the pluteus stage, was visible according to the degree of mineralization of the skeleton. In conclusion, the present study suggests that the investigated NPs suspended in seawater interfere with the bio-mineralization processes in marine organisms, and the results of this study offer a new series of specific endpoints for the mechanistic understanding of NP toxicity.

A detailed staging scheme for late larval development in Strongylocentrotus purpuratus focused on readily-visible juvenile structures within the rudiment

BACKGROUND

The purple sea urchin, Strongylocentrotus purpuratus, has long been the focus of developmental and ecological studies, and its recently-sequenced genome has spawned a diversity of functional genomics approaches. S. purpuratus has an indirect developmental mode with a pluteus larva that transforms after 1-3 months in the plankton into a juvenile urchin. Compared to insects and frogs, mechanisms underlying the correspondingly dramatic metamorphosis in sea urchins remain poorly understood. In order to take advantage of modern techniques to further our understanding of juvenile morphogenesis, organ formation, metamorphosis and the evolution of the pentameral sea urchin body plan, it is critical to assess developmental progression and rate during the late larval phase. This requires a staging scheme that describes developmental landmarks that can quickly and consistently be used to identify the stage of individual living larvae, and can be tracked during the final two weeks of larval development, as the juvenile is forming.

RESULTS

Notable structures that are easily observable in developing urchin larvae are the developing spines, test and tube feet within the juvenile rudiment that constitute much of the oral portion of the adult body plan. Here we present a detailed staging scheme of rudiment development in the purple urchin using soft structures of the rudiment and the primordia of these juvenile skeletal elements. We provide evidence that this scheme is robust and applicable across a range of temperature and feeding regimes.

CONCLUSIONS

Our proposed staging scheme provides both a useful method to study late larval development in the purple urchin, and a framework for developing similar staging schemes across echinoderms. Such efforts will have a high impact on evolutionary developmental studies and larval ecology, and facilitate research on this important deuterostome group.

Growth factors and early mesoderm morphogenesis: insights from the sea urchin embryo

The early morphogenesis of the mesoderm is critically important in establishing the body plan of the embryo. Recent research has led to a better understanding of the mechanisms that underlie this process, and growth factor signaling pathways have emerged as key regulators of the directional movements of mesoderm cells during gastrulation. In this review, we undertake a comparative analysis of the various essential functions of growth factor signaling pathways in regulating early mesoderm morphogenesis, with an emphasis on recent advances in the sea urchin embryo. We focus on the roles of the vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) pathways in the migration of primary mesenchyme cells and the formation of the embryonic endoskeleton. We compare the functions of VEGF and FGF in sea urchins with the roles that these and other growth factors play in regulating mesoderm migration during gastrulation in Drosophila and vertebrates.

Piwi regulates Vasa accumulation during embryogenesis in the sea urchin

BACKGROUND

Piwi proteins are essential for germ line development, stem cell maintenance, and more recently found to function in epigenetic and somatic gene regulation. In the sea urchin Strongylocentrotus purpuratus, two Piwi proteins, Seawi and Piwi-like1, have been identified, yet their functional contributions have not been reported.

RESULTS

Here we found that Seawi protein was localized uniformly in the early embryo and then became enriched in the primordial germ cells (PGCs) (the small micromere lineage) from blastula stage and thereafter. Morpholino knockdown of Sp-seawi diminished PGC-specific localization of Seawi proteins, and altered expression of other germ line markers such as Vasa and Gustavus, but had no effect on Nanos. Furthermore, Seawi knockdown transiently resulted in Vasa positive cell proliferation in the right coelomic pouch that appear to be derived from the small micromere lineage, yet they quickly disappeared with an indication of apoptosis by larval stage. Severe Seawi knockdown resulted in an increased number of apoptotic cells in the entire gut area.

CONCLUSION

Piwi proteins appear to regulate PGC proliferation perhaps through control of Vasa accumulation. In this organism, Piwi is likely regulating mRNAs, not just transposons, and is potentially functioning both inside and outside of the germ line during embryogenesis.

Developmental abnormalities and changes in cholinesterase activity in sea urchin embryos and larvae from sperm exposed to engineered nanoparticles

The objective of this study is to examine the toxicity of engineered nanoparticles (NPs) that are dispersed in sea water by using an in vivo model. Because many products of nanotechnology contain NPs and are commonly used and well-established in the market, the accidental release of NPs into the air and water is quite possible. Indeed, at the end of their life cycle, some NPs are inevitably released into waste water and can reach marine ecosystem and affect the organisms there. Although there are few data on the presence of NPs in the marine environment, our awareness of their potential impact on environmental and organismal health is growing. Shallow-water benthonic organisms such as sea urchins provide planktonic larvae as a trophic base for finfish juveniles and are exposed to water from estuaries and precipitation. Such organisms can therefore be directly affected by NPs that are dispersed into those media. We evaluated the effects of exposure to different concentrations of nanosilver, titanium oxide and cobalt NPs on the sperm of the sea urchin Paracentrotus lividus by analyzing the functionality and the morphology and biochemistry of the first developmental stages of the sea urchin. Sperm were exposed to sea water containing suspensions of NPs ranging from 0.0001 mg/L to 1 mg/L. Fertilization ability was not affected, but developmental anomalies were identified in embryos from the gastrula to pluteus stages, including morphological alterations of the skeletal rods. In addition, the enzymatic activity (cholinesterase, ChE) of the larvae was measured. Acetylcholinesterase (AChE) and propionylcholinesterase activity (PrChE) was affected in all of the exposed samples. The results did not vary consistently with the concentration of NP, but controls were significantly different from exposed samples. Exposure of sea urchin to these NPs may cause neurotoxic damage, and the altered ChE activity may be involved in skeletogenic aberrations. In conclusion, the sea urchin represents a suitable and sensitive model for testing the toxicity and effects of engineered NPs that are dispersed in sea water.

Meiotic gene expression initiates during larval development in the sea urchin

BACKGROUND

Meiosis is a unique mechanism in gamete production and a fundamental process shared by all sexually reproducing eukaryotes. Meiosis requires several specialized and highly conserved genes whose expression can also identify the germ cells undergoing gametogenic differentiation. Sea urchins are echinoderms, which form a phylogenetic sister group of chordates. Sea urchin embryos undergo a feeding, planktonic larval phase in which they construct an adult rudiment prior to metamorphosis. Although a series of conserved meiosis genes (e.g., dmc1, msh5, rad21, rad51, and sycp1) is expressed in sea urchin oocytes, we sought to determine when in development meiosis would first be initiated.

RESULTS

We surveyed the expression of several meiotic genes and their corresponding proteins in the sea urchin Strongylocentrotus purpuratus. Surprisingly, meiotic genes are highly expressed not only in ovaries but beginning in larvae. Both RNA and protein localizations strongly suggest that meiotic gene expression initiates in tissues that will eventually give rise to the adult rudiment of the late larva.

CONCLUSIONS

These results demonstrate that broad expression of the molecules associated with meiotic differentiation initiates prior to metamorphosis and may have additional functions in these cells, or mechanisms repressing their function, until later in development when gametogenesis begins.

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

The process of germ line determination involves many conserved genes, yet is highly variable. Echinoderms are positioned at the base of Deuterostomia and are crucial to understanding these evolutionary transitions, yet the mechanism of germ line specification is not known in any member of the phyla. Here we demonstrate that small micromeres (SMics), which are formed at the fifth cell division of the sea urchin embryo, illustrate many typical features of primordial germ cell (PGC) specification. SMics autonomously express germ line genes in isolated culture, including selective Vasa protein accumulation and transcriptional activation of nanos; their descendants are passively displaced towards the animal pole by secondary mesenchyme cells and the elongating archenteron during gastrulation; Cadherin (G form) has an important role in their development and clustering phenotype; and a left/right integration into the future adult anlagen appears to be controlled by a late developmental mechanism. These results suggest that sea urchin SMics share many more characteristics typical of PGCs than previously thought, and imply a more widely conserved system of germ line development among metazoans.

CO2 induced seawater acidification impacts sea urchin larval development I: elevated metabolic rates decrease scope for growth and induce developmental delay

Anthropogenic CO(2) emissions are acidifying the world's oceans. A growing body of evidence is showing that ocean acidification impacts growth and developmental rates of marine invertebrates. Here we test the impact of elevated seawater pCO(2) (129 Pa, 1271 μatm) on early development, larval metabolic and feeding rates in a marine model organism, the sea urchin Strongylocentrotus purpuratus. Growth and development was assessed by measuring total body length, body rod length, postoral rod length and posterolateral rod length. Comparing these parameters between treatments suggests that larvae suffer from a developmental delay (by ca. 8%) rather than from the previously postulated reductions in size at comparable developmental stages. Further, we found maximum increases in respiration rates of +100% under elevated pCO(2), while body length corrected feeding rates did not differ between larvae from both treatments. Calculating scope for growth illustrates that larvae raised under high pCO(2) spent an average of 39 to 45% of the available energy for somatic growth, while control larvae could allocate between 78 and 80% of the available energy into growth processes. Our results highlight the importance of defining a standard frame of reference when comparing a given parameter between treatments, as observed differences can be easily due to comparison of different larval ages with their specific set of biological characters.

Molecular aspects of biomineralization of the echinoderm endoskeleton

Echinoderms possess a rigid endoskeleton composed of calcite and small amounts of occluded organic matrix proteins. The test (i.e., the shell-like structure of adults), spines, pedicellariae, tube feet, and teeth of adults, as well as delicate endoskeletal spicules found in larvae of some classes, are the main skeletal structures. They have been intensively studied for insight into the mechanisms of biomineralization. Recent work on characterization of the mineral phase and occluded proteins in embryonic skeletal spicules shows that these simple-looking structures contain scores of different proteins, and that the mineral phase is composed of amorphous calcium carbonate (ACC), which then transforms to an anhydrous ACC and eventually to calcite. Likewise, the adult tooth shows a similar transition from hydrated ACC to anhydrous ACC to calcite during its formation, and a similar transition is likely occurring during adult spine regeneration. We speculate that: (1) the ACC precursor is a general strategy employed in biomineralization in echinoderms, (2) the numerous occluded proteins play a role in post-secretion formation of the mature biomineralized structure, and (3) proteins with "multi-valent" intrinsically disordered domains are important for formation of occluded matrix structures, and regulation of crucial matrix-mineral interactions, such as ACC to calcite transitions and polymorph selection.

Evolutionary modification of mesenchyme cells in sand dollars in the transition from indirect to direct development

Peronella japonica, an intermediate type of direct-developing sand dollar, forms an abbreviated pluteus, followed by metamorphosis within 3 days without feeding. In this species, ingression of mesenchyme cells starts before hatching and continues until gastrulation, but no typical secondary mesenchyme cells (SMCs) migrate from the tip of the archenteron. Here, I investigated the cell lineage of mesenchyme cells through metamorphosis in P. japonica and found that mesenchyme cells migrating before hatching (early mesenchyme cells [EMCs]) were exclusively derived from micromeres and became larval skeletogenic cells, whereas cells migrating after hatching (late mesenchyme cells [LMCs]) appeared to contain several nonskeletogenic cells. Thus, it is likely that EMCs are homologous to primary mesenchyme cells (PMCs) and LMCs are similar to the SMCs of typical indirect developers, suggesting that heterochrony in the timing of mesenchyme cell ingression may have occurred in this species. EMCs disappeared after metamorphosis and LMCs were involved in adult skeletogenesis. Embryos from which micromeres were removed at the 16-cell stage formed armless plutei that went on to form adult skeletons and resulted in juveniles with normal morphology. These results suggest that in P. japonica, LMCs form adult skeletal elements, whereas EMCs are specialized for larval spicule formation. The occurrence of evolutionary modifications in mesenchyme cells in the transition from indirect to direct development of sand dollars is discussed.

A switch in the cellular basis of skeletogenesis in late-stage sea urchin larvae

Primary mesenchyme cells (PMCs) are solely responsible for the skeletogenesis during early larval development of the sea urchin, but the cells responsible for late larval and adult skeletal formation are not clear. To investigate the origin of larval and adult skeletogenic cells, I first performed transplantation experiments in Pseudocentrotus depressus and Hemicentrotus pulcherrimus, which have different skeletal phenotypes. When P. depressus PMCs were transplanted into H. pulcherrimus embryos, the donor phenotype was observed only in the early larval stage, whereas when secondary mesenchyme cells (SMCs) were transplanted, the donor phenotype was observed in late and metamorphic larvae. Second, a reporter construct driven by the spicule matrix protein 50 (SM50) promoter was introduced into fertilized eggs and their PMCs/SMCs were transplanted. In the resultant 6-armed pluteus, green fluorescent protein (GFP) expression was observed in both PMC and SMC transplantations, suggesting SMC participation in late skeletogenesis. Third, transplanted PMCs or SMCs tagged with GFP were analyzed by PCR in the transgenic chimeras. As a result, SMCs were detected in both larval and adult stages, but GFP from PMCs was undetectable after metamorphosis. Thus, it appears that SMCs participate in skeletogenesis in late development and that PMCs disappear in the adult sea urchin, suggesting that the skeletogenesis may pass from PMCs to SMCs during the late larval stage.