The synthesis and secretion of collagen by cultured sea urchin micromeres

The Evolution of Biomineralization through the Co-Option of Organic Scaffold Forming Networks

Biomineralization is the process in which organisms use minerals to generate hard structures like teeth, skeletons and shells. Biomineralization is proposed to have evolved independently in different phyla through the co-option of pre-existing developmental programs. Comparing the gene regulatory networks (GRNs) that drive biomineralization in different species could illuminate the molecular evolution of biomineralization. Skeletogenesis in the sea urchin embryo was extensively studied and the underlying GRN shows high conservation within echinoderms, larval and adult skeletogenesis. The organic scaffold in which the calcite skeletal elements form in echinoderms is a tubular compartment generated by the syncytial skeletogenic cells. This is strictly different than the organic cartilaginous scaffold that vertebrates mineralize with hydroxyapatite to make their bones. Here I compare the GRNs that drive biomineralization and tubulogenesis in echinoderms and in vertebrates. The GRN that drives skeletogenesis in the sea urchin embryo shows little similarity to the GRN that drives bone formation and high resemblance to the GRN that drives vertebrates’ vascular tubulogenesis. On the other hand, vertebrates’ bone-GRNs show high similarity to the GRNs that operate in the cells that generate the cartilage-like tissues of basal chordate and invertebrates that do not produce mineralized tissue. These comparisons suggest that biomineralization in deuterostomes evolved through the phylum specific co-option of GRNs that control distinct organic scaffolds to mineralization.

Culture of and experiments with sea urchin embryo primary mesenchyme cells

Skeletogenesis in the sea urchin embryo gives rise to a pair of intricate endoskeletal spicules. Deposition of these skeletal elements in the early larva is the outcome of a morphogenetic program that begins with maternal inputs in the early zygote and results in the specification of the large micromere-primary mesenchyme cell (PMC) lineage. PMCs are of considerable interest as a model system, not only to dissect the mechanism of specific developmental processes, but also to investigate their evolution and the unrivaled level of control over the formation of a graded, mechanically robust, yet single crystalline biomineral. The ability to study gene regulatory circuits, cellular behavior, signaling pathways, and molecular players involved in biomineralization is significantly boosted by the high level of autonomy of PMCs. In fact, in the presence of horse serum, micromeres differentiate into PMCs and produce spicules in vitro, separated from the embryonic milieu. PMC culture eliminates indirect effects that can complicate the interpretation of experiments in vivo, offers superior spatiotemporal control, enables PMC-specific readouts, and is compatible with most imaging and characterization techniques. In this chapter, we provide an updated protocol, based on the pioneering work by Okazaki and Wilt, for the isolation of micromeres and subsequent culture of PMCs, as well as protocols for fixation and staining for fluorescent microscopy, preparation of cell cultures for electron microscopy, and the isolation of RNA.

Transcriptomic responses to ocean acidification in larval sea urchins from a naturally variable pH environment

Some marine ecosystems already experience natural declines in pH approximating those predicted with future anthropogenic ocean acidification (OA), the decline in seawater pH caused by the absorption of atmospheric CO2 . The molecular mechanisms that allow organisms to inhabit these low pH environments, particularly those building calcium carbonate skeletons, are unknown. Also uncertain is whether an enhanced capacity to cope with present day pH variation will confer resistance to future OA. To address these issues, we monitored natural pH dynamics within an intertidal habitat in the Northeast Pacific, demonstrating that upwelling exposes resident species to pH regimes not predicted to occur elsewhere until 2100. Next, we cultured the progeny of adult purple sea urchins (Strongylocentrotus purpuratus) collected from this region in CO2 -acidified seawater representing present day and near future ocean scenarios and monitored gene expression using transcriptomics. We hypothesized that persistent exposure to upwelling during evolutionary history will have selected for increased pH tolerance in this population and that their transcriptomic response to low pH seawater would provide insight into mechanisms underlying pH tolerance in a calcifying species. Resulting expression patterns revealed two important trends. Firstly, S. purpuratus larvae may alter the bioavailability of calcium and adjust skeletogenic pathways to sustain calcification in a low pH ocean. Secondly, larvae use different strategies for coping with different magnitudes of pH stress: initiating a robust transcriptional response to present day pH regimes but a muted response to near future conditions. Thus, an enhanced capacity to cope with present day pH variation may not translate into success in future oceans.

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.

Parallel evolution of nacre building gene sets in molluscs

The capacity to biomineralize is closely linked to the rapid expansion of animal life during the early Cambrian, with many skeletonized phyla first appearing in the fossil record at this time. The appearance of disparate molluscan forms during this period leaves open the possibility that shells evolved independently and in parallel in at least some groups. To test this proposition and gain insight into the evolution of structural genes that contribute to shell fabrication, we compared genes expressed in nacre (mother-of-pearl) forming cells in the mantle of the bivalve Pinctada maxima and the gastropod Haliotis asinina. Despite both species having highly lustrous nacre, we find extensive differences in these expressed gene sets. Following the removal of housekeeping genes, less than 10% of all gene clusters are shared between these molluscs, with some being conserved biomineralization genes that are also found in deuterostomes. These differences extend to secreted proteins that may localize to the organic shell matrix, with less than 15% of this secretome being shared. Despite these differences, H. asinina and P. maxima both secrete proteins with repetitive low-complexity domains (RLCDs). Pinctada maxima RLCD proteins-for example, the shematrins-are predominated by silk/fibroin-like domains, which are absent from the H. asinina data set. Comparisons of shematrin genes across three species of Pinctada indicate that this gene family has undergone extensive divergent evolution within pearl oysters. We also detect fundamental bivalve-gastropod differences in extracellular matrix proteins involved in mollusc-shell formation. Pinctada maxima expresses a chitin synthase at high levels and several chitin deacetylation genes, whereas only one protein involved in chitin interactions is present in the H. asinina data set, suggesting that the organic matrix on which calcification proceeds differs fundamentally between these species. Large-scale differences in genes expressed in nacre-forming cells of Pinctada and Haliotis are compatible with the hypothesis that gastropod and bivalve nacre is the result of convergent evolution. The expression of novel biomineralizing RLCD proteins in each of these two molluscs and, interestingly, sea urchins suggests that the evolution of such structural proteins has occurred independently multiple times in the Metazoa.

Transcriptomic response of sea urchin larvae Strongylocentrotus purpuratus to CO2-driven seawater acidification

Ocean acidification from the uptake of anthropogenic CO2 is expected to have deleterious consequences for many calcifying marine animals. Forecasting the vulnerability of these marine organisms to climate change is linked to an understanding of whether species possess the physiological capacity to compensate for the potentially adverse effects of ocean acidification. We carried out a microarray-based transcriptomic analysis of the physiological response of larvae of a calcifying marine invertebrate, the purple sea urchin, Strongylocentrotus purpuratus, to CO2-driven seawater acidification. In lab-based cultures, larvae were raised under conditions approximating current ocean pH conditions (pH 8.01) and at projected, more acidic pH conditions (pH 7.96 and 7.88) in seawater aerated with CO2 gas. Targeting expression of ∼1000 genes involved in several biological processes, this study captured changes in gene expression patterns that characterize the transcriptomic response to CO2-driven seawater acidification of developing sea urchin larvae. In response to both elevated CO2 scenarios, larvae underwent broad scale decreases in gene expression in four major cellular processes:biomineralization, cellular stress response, metabolism and apoptosis. This study underscores that physiological processes beyond calcification are impacted greatly, suggesting that overall physiological capacity and not just a singular focus on biomineralization processes is essential for forecasting the impact of future CO2 conditions on marine organisms. Conducted on targeted and vulnerable species, genomics-based studies, such as the one highlighted here, have the potential to identify potential `weak links' in physiological function that may ultimately determine an organism's capacity to tolerate future ocean conditions.

The dynamics of secretion during sea urchin embryonic skeleton formation

Skeleton formation involves secretion of massive amounts of mineral precursor, usually a calcium salt, and matrix proteins, many of which are deposited on, or even occluded within, the mineral. The cell biological underpinnings of this secretion and subsequent assembly of the biomineralized skeletal element is not well understood. We ask here what is the relationship of the trafficking and secretion of the mineral and matrix within the primary mesenchyme cells of the sea urchin embryo, cells that deposit the endoskeletal spicule. Fluorescent labeling of intracellular calcium deposits show mineral precursors are present in granules visible by light microscopy, from whence they are deposited in the endoskeletal spicule, especially at its tip. In contrast, two different matrix proteins tagged with GFP are present in smaller post-Golgi vesicles only seen by electron microscopy, and the secreted protein are only incorporated into the spicule in the vicinity of the cell of origin. The matrix protein, SpSM30B, is post-translationally modified during secretion, and this processing continues after its incorporation into the spicule. Our findings also indicate that the mineral precursor and two well characterized matrix proteins are trafficked by different cellular routes.

A genome-wide analysis of biomineralization-related proteins in the sea urchin Strongylocentrotus purpuratus

Biomineralization, the biologically controlled formation of mineral deposits, is of widespread importance in biology, medicine, and engineering. Mineralized structures are found in most metazoan phyla and often have supportive, protective, or feeding functions. Among deuterostomes, only echinoderms and vertebrates produce extensive biomineralized structures. Although skeletons appeared independently in these two groups, ancestors of the vertebrates and echinoderms may have utilized similar components of a shared genetic "toolkit" to carry out biomineralization. The present study had two goals. First, we sought to expand our understanding of the proteins involved in biomineralization in the sea urchin, a powerful model system for analyzing the basic cellular and molecular mechanisms that underlie this process. Second, we sought to shed light on the possible evolutionary relationships between biomineralization in echinoderms and vertebrates. We used several computational methods to survey the genome of the purple sea urchin Strongylocentrotus purpuratus for gene products involved in biomineralization. Our analysis has greatly expanded the collection of biomineralization-related proteins. We have found that these proteins are often members of small families encoded by genes that are clustered in the genome. Most of the proteins are sea urchin-specific; that is, they have no apparent homologues in other invertebrate deuterostomes or vertebrates. Similarly, many of the vertebrate proteins that mediate mineral deposition do not have counterparts in the S. purpuratus genome. Our findings therefore reveal substantial differences in the primary sequences of proteins that mediate biomineral formation in echinoderms and vertebrates, possibly reflecting loose constraints on the primary structures of the proteins involved. On the other hand, certain cellular and molecular processes associated with earlier events in skeletogenesis appear similar in echinoderms and vertebrates, leaving open the possibility of deeper evolutionary relationships.

Cell adhesion and communication: a lesson from echinoderm embryos for the exploitation of new therapeutic tools

In this chapter, we summarise fundamental findings concerning echinoderms as well as research interests on this phylum for biomedical and evolutionary studies. We discuss how current knowledge of echinoderm biology, in particular of the sea urchin system, can shed light on the understanding of important biological phenomena and in dissecting them at the molecular level. The general principles of sea urchin embryo development are summarised, mainly focusing on cell communication and interactions, with particular attention to the cell-extracellular matrix and cell-cell adhesion molecules and related proteins. Our purpose is not to review all the work done over the years in the field of cellular interaction in echinoderms. On the contrary, we will rather focus on a few arguments in an effort to re-examine some ideas and concepts, with the aim of promoting discussion in this rapidly growing field and opening new routes for research on innovative therapeutic tools.

Phosphorylation-dependent regulation of skeletogenesis in sea urchin micromere-derived cells and embryos

Sea urchin embryo micromeres when isolated and cultured in vitro differentiate to produce spicules. Although several authors have used this model, almost nothing is known about the signaling pathways responsible for initiating skeletogenesis. In order to investigate the potential involvement of phosphorylation events in spiculogenesis, the effect of inhibitors of protein kinases and phosphatases on skeleton formation was studied. Results obtained using both cultured micromeres and embryos revealed that protein tyrosine kinase and phosphatase inhibitors blocked skeleton formation, but not serine/threonine phosphatase inhibitors. The inhibitors showed a dose-dependent effect and when removed from micromere or embryo culture, spicule formation resumed. Inhibition of tyrosine phosphatases resulted in an increase in the tyrosine phosphorylation level of two major proteins and a modest decrease in the expression of the mRNA coding for type I fibrillar collagen. These findings strongly suggest that tyrosine phosphorylation and dephosphorylation is required for micromere differentiation and for normal skeletogenesis during sea urchin embryo development.

Developmental characterization of the gene for laminin alpha-chain in sea urchin embryos

We describe the isolation and characterization of a cDNA clone encoding a region of the carboxy terminal globular domain (G domain) of the alpha-1 chain of laminin from the sea urchin, Strongylocentrotus purpuratus. Sequence analysis indicates that the 1.3 kb cDNA (spLAM-alpha) encodes the complete G2 and G3 subdomains of sea urchin a-laminin. The 11 kb spLAM-alpha mRNA is present in the egg and declines slightly in abundance during development to the pluteus larva. The spLAM-alpha gene is also expressed in a variety of adult tissues. Whole mount in situ hybridization of gastrula stage embryos indicates that ectodermal and endodermal epithelia and mesenchyme cells contain the spLAM-alpha mRNA. Immunoprecipitation experiments using an antibody made to a recombinant fusion protein indicates spLAM-alpha protein is synthesized continuously from fertilization as a 420 kDa protein which accumulates from low levels in the egg to elevated levels in the pluteus larva. Light and electron microscopy identify spLAM-alpha as a component of the basal lamina. Blastocoelic microinjection of an antibody to recombinant spLAM-alpha perturbs gastrulation and skeleton formation by primary mesenchyme cells suggesting an important role for laminin in endodermal and mesodermal morphogenesis.

A protein of the basal lamina of the sea urchin embryo

The purification, biochemical characterization and functional features of a novel extracellular matrix protein are described. This protein is a component of the basal lamina found in embryos from the sea urchin species Paracentrotus lividus and Hemicentrotus pulcherrimus. The protein has been named Pl-200K or Hp-200K, respectively, because of the species from which it was isolated and its apparent molecular weight in SDS-PAGE under reducing conditions. It has been purified from unfertilized eggs where it is found packed within cytoplasmic granules, and has different binding affinities to type I collagen and heparin, as assessed by affinity chromatography columns. By indirect immunofluorescence experiments it was shown that, upon fertilization, the protein becomes extracellular, polarized at the basal surface of ectoderm cells, and on the surface of primary mesenchyme cells at the blastula and gastrula stages. The protein serves as an adhesive substrate, as shown by an in vitro binding assay where cells dissociated from blastula embryos were settled on 200K protein-coated substrates. To examine the involvement of the protein in morphogenesis of sea urchin embryo, early blastula embryos were microinjected with anti-200K Fab fragments and further development was followed. When control embryos reached the pluteus stage, microinjected embryos showed severe abnormalities in arms and skeleton elongation and patterning. On the basis of current results, it was proposed that 200K protein is involved in the regulation of sea urchin embryo skeletogenesis.

Comparative analysis of fibrillar and basement membrane collagen expression in embryos of the sea urchin, Strongylocentrotus purpuratus

The time of appearance and location of three distinct collagen gene transcripts termed 1 alpha, 2 alpha, and 3 alpha, were monitored in the developing S. purpuratus embryo by in situ hybridization. The 1 alpha and 2 alpha transcripts of fibrillar collagens were detected simultaneously in the primary (PMC) and secondary (SMC) mesenchyme cells of the late gastrula stage and subsequently expressed in the spicules and gut associated cells of the pluteus stage. The 3 alpha transcripts of the basement membrane collagen appeared earlier than 1 alpha and 2 alpha, and were first detected in the presumptive PMC at the vegetal plate of the late blastula stage. The PMC exhibited high expression of 3 alpha at the mesenchyme blastula stage, but during gastrulation the level of expression was reduced differentially among the PMC. In the late gastrula and pluteus stages, both PMC and SMC expressed 3 alpha mRNA, and thus at these stages all three collagen genes displayed an identical expression pattern by coincidence. This study thus provides the first survey of onset and localization of multiple collagen transcripts in a single sea urchin species.

Ultrastructure and synthesis of the extracellular matrix of Pisaster ochraceus embryos preserved by freeze substitution

When asteroid embryos cryoprotected with propylene glycol are rapidly frozen in liquid propane and freeze substituted with ethanol, preservation of the cells and extracellular matrix (ECM) is excellent. The basal lamina, although thicker and less well defined than in conventionally fixed embryos, demonstrates a region of decreased density just below the cells that corresponds to the lamina lucida and a lamina densa. The former region is often occupied by fibrous material. In addition, as was previously described in conventionally fixed issues, the basal lamina of the ectoderm is generally thicker and more substantial than that of the endoderm, reinforcing an earlier suggestion that the structure of the basal is different in different regions of the embryo. The ECM of the blastocoel consists of thin "twig-like" elements that form a loose meshwork evenly distributed throughout the blastocoel. Bundles of 20 nm fibers, located within the meshwork, are oriented parallel to the base of the cells of the stomodeum. In the long axis of the embryo, similar fibers are present in the dorsal aspect of the animal between the stomach and the ectoderm and radiate out from the esophagus crossing the region between it and the ectoderm. Immunocytochemical work with three different monoclonal antibodies shows that glycoprotein molecules, synthesized in the Golgi apparatus, are also secreted here and form part of the matrix structure. The results suggest that the blastocoel is filled with a gel-like material reinforced with bundles of 20-nm fibers. The manner in which the observed arrangement could contribute to the development and maintainence of the shape of the embryo is discussed.

Characterization of the proteins comprising the integral matrix of Strongylocentrotus purpuratus embryonic spicules

In the present study, we enumerate and characterize the proteins that comprise the integral spicule matrix of the Strongylocentrotus purpuratus embryo. Two-dimensional gel electrophoresis of [35S]methionine radiolabeled spicule matrix proteins reveals that there are 12 strongly radiolabeled spicule matrix proteins and approximately three dozen less strongly radiolabeled spicule matrix proteins. The majority of the proteins have acidic isoelectric points; however, there are several spicule matrix proteins that have more alkaline isoelectric points. Western blotting analysis indicates that SM50 is the spicule matrix protein with the most alkaline isoelectric point. In addition, two distinct SM30 proteins are identified in embryonic spicules, and they have apparent molecular masses of approximately 43 and 46 kDa. Comparisons between embryonic spicule matrix proteins and adult spine integral matrix proteins suggest that the embryonic 43-kDa SM30 protein is an embryonic isoform of SM30. An adult 49-kDa spine matrix protein is also identified as a possible adult isoform of SM30. Analysis of the SM30 amino acid sequences indicates that a portion of SM30 proteins is very similar to the carbohydrate recognition domain of C-type lectin proteins.

Developmentally regulated protease expression during sea urchin embryogenesis

A temporal study of protease expression employing the technique of SDS-PAGE gelatin substrate zymography revealed a definitive appearance of proteases during early development in the sea urchins, Lytechinus pictus and Strongylocentrotus purpuratus. The levels of these proteases increase substantially during gastrulation in each species. The two major proteases with relative molecular masses of 57 and 50 kDa were found to be inhibited by the zinc chelator, 1,10-phenanthroline, the more nonspecific metal chelator, EDTA, and the reducing agent, dithiothreitol. The serine protease inhibitor, benzamidine, exerted no effects on the activities of these proteases, and both enzymes exhibited activity in the neutral to slightly basic pH range. Treatment of embryos with actinomycin D, an inhibitor of transcription, beginning up to 9 hr after fertilization, inhibited the subsequent appearances of the two proteases 48 hr after fertilization, as well as any morphological changes associated with gastrulation. Treatments beginning 15 and 21 hr after fertilization resulted in increased levels of proteases that correlate with arrests at successively more advanced stages of gastrulation. SDS-PAGE zymographic analyses of five different embryo fractions indicated that the 57- and 50-kDa proteases are localized in the blastocoel, and blastocoelic protease activity was further confirmed microscopically by in situ zymography. Hence, the 57- and 50-kDa proteases are characterized as metalloproteases. Their expression is dependent on transcription of the embryonic genome, and their spatiotemporal appearance suggests an involvement in blastocoelic matrix remodeling during gastrulation.

Matrix metalloproteases of the developing sea urchin embryo

A distinct group of metalloproteases has been identified in the developing sea urchin embryo by gelatin substrate gel zymography, a highly sensitive protease detection assay. The developing Arbacia embryo exhibited four prominent bands of gelatinase activity with apparent molecular masses of 55, 50, 42 and 38 kDa. The activity of the 55, 42 and 38 kDa tissue gelatinases increased and that of the 50 kDa tissue gelatinase decreased during embryonic development. All four enzymes were EDTA- and 1,10-phenanthroline sensitive and phenyl methyl sulphonyl fluoride (PMSF) insensitive. None of the enzymes had detectable caseinolytic activity in casein substrate gels. Although the Arbacia enzymes possessed a number of properties that are characteristic of the mammalian matrix metalloprotease family, they did not appear to be converted to lower molecular weight forms by organomercurial treatment and are distinct in this aspect. The Arbacia metalloproteases are candidate enzymes for the tissue and matrix remodeling that occurs during sea urchin embryo development.

Analysis of competence in cultured sea urchin micromeres

Sea urchin embryo micromeres form the primary mesenchyme, the skeleton-producing cells of the embryo. Almost nothing is known about nature and timing of the embryonic cues which induce or initiate spicule formation by these cells. A related question concerns the competence of the micromeres to respond to the cues. To examine competence in this system we have exposed cultured sea urchin micromeres to an inducing medium containing horse serum for various periods of time and have identified a period when micromeres are competent to respond to serum and form spicules. This window, between 30 and 50 h after fertilization, corresponds to the time when mesenchyme cells in vivo are aggregating and beginning to form the syncytium in which the spicule will be deposited. The loss of competence after 50 h is not due to impaired cell health since protein synthesis at this time is not significantly different from controls. Likewise the accumulation of a spicule matrix mRNA (SM 50) and a cell surface glycoprotein (msp 130), both indices of micromere/mesenchyme differentiation, still occurs in cells that have lost competence to respond to serum by forming spicules. These experiments demonstrate that the acquisition and loss of competence in these cells are regulated developmental events and establish an in vitro system for the identification of the molecular basis for inductive signal recognition and signal transduction.

Primary mesenchyme cells of the sea urchin embryo require an autonomously produced, nonfibrillar collagen for spiculogenesis

A collagen molecule in the sea urchin embryo was characterized by analysis of a 2.7-kb cDNA clone. This clone, Spcoll, was obtained by screening a gastrula stage Strongylocentrotus purpuratus cDNA library with a 237-bp genomic clone encoding a collagen-like sequence previously isolated by Venkatesan et al. (1986). DNA sequence analysis of the cDNA clone demonstrated the nonfibrillar nature of the encoded molecule--13 interruptions of the Gly-X-Y repeat motif were found in the 85-kDa open reading frame. The mRNA of approximately 9 kb accumulated specifically in mesenchyme cells of the embryo through development to the pluteus larva. Polyclonal antibodies generated against a Spcoll-beta-galactosidase fusion protein were utilized to identify and localize the native Spcoll. This collagen molecule of approximately 210 kDa was deposited into the blastocoel by the primary mesenchyme cells. When primary mesenchyme cells were cultured in vitro, Spcoll was secreted into the media and accumulated at sites of cell-substrate interaction. Addition of anti-Spcoll antibodies to primary mesenchyme cell cultures selectively inhibited spiculogenesis, whereas other antibodies had no inhibitory effect. Since collagen is not a component of the organic matrix of spicules (Benson et al., 1986), these results suggest that the autonomous production of Spcoll by differentiating mesenchyme cells in turn influences the point in differentiation at which these cell initiate biomineralization.

Cell interactions in the sea urchin embryo studied by fluorescence photoablation

In many organisms, interactions between cells play a critical role in the specification of cell fates. In the sea urchin embryo, primary mesenchyme cells (PMCs) regulate the developmental program of a subpopulation of secondary mesenchyme cells (SMCs). The timing of this cell interaction was analyzed by means of a fluorescence photoablation technique, which was used to specifically ablate PMCs at various stages of development. In addition, the PMCs were microinjected into PMC-depleted recipient embryos at different developmental stages and their effect on SMC fate was examined. The critical interaction between PMCs and SMCs was brief and took place late in gastrulation. Before that time, SMCs were insensitive to the suppressive signals transmitted by the PMCs.