Blastopore formation and dorsal mesoderm induction are independent events in early Cynops embryogenesis

The independent roles of blastopore formation and dorsal mesoderm induction in dorsal axis formation of the Cynops pyrrhogaster embryo were attempted to be clarified. The blastopore-forming (bottle) cells originated mainly from the progeny of the mid-dorsal C and/or D blastomeres of the 32-cell embryo, but were not defined to a fixed blastomere. It was confirmed that the isolated dorsal C and D blastomeres autonomously formed a blastopore. Ultraviolet-irradiated eggs formed an abnormal blastopore and then did not form a dorsal axis, although the lower dorsal marginal zone (LDMZ) still had dorsal mesoderm-inducing activity. Involution of the dorsal marginal zone was disturbed by the abnormal blastopore. These embryos were rescued by artificially facilitating involution of the dorsal marginal zone. Suramin-injected and nocodazole-treated blastulae did not have involution of the dorsal marginal zone, although the blastopore was formed. Neither embryos formed the dorsal axis. The dorsal mesoderm-inducing activity of the LDMZ in the nocodazole-treated gastrulae was still active. In contrast, the LDMZ of the suramin-injected embryos lost its dorsal mesoderm-inducing activity. bra expression was activated in the nocodazole-treated embryos but not in the suramin-injected embryos. The present study suggested that (i) the dorsal determinants consist of blastopore-forming and dorsal mesoderm-inducing factors, which are not always mutually dependent; (ii) both factors are activated during the late blastula stage; (iii) the dorsal marginal zone cannot specify to an organized notochord and muscle without the involution that blastopore formation leads to; and (iv) the localization of both factors in the same place is prerequisite for dorsal axis formation.     

Impaired intermediate formation in mouse embryos expressing reduced levels of Tbx6

Intermediate mesoderm (IM) is the strip of tissue lying between the paraxial mesoderm (PAM) and the lateral plate mesoderm that gives rise to the kidneys and gonads. Chick fate mapping studies suggest that IM is specified shortly after cells leave the primitive streak and that these cells do not require external signals to express IM‐specific genes. Surgical manipulations of the chick embryo, however, revealed that PAM‐specific signals are required for IM differentiation into pronephros—the first kidney. Here, we use a genetic approach in mice to examine the dependency of IM on proper PAM formation. In Tbx6 null mutant embryos, which form 7–9 improperly patterned anterior somites, IM formation is severely compromised, while in Tbx6 hypomorphic embryos, where somites form but are improperly patterned along the axis, the impact to IM formation is lessened. These results suggest that IM and its derivatives, the kidneys and the gonads, are directly or indirectly dependent on proper PAM formation. This has implications for humans harboring Tbx6 mutations which are known to have somite‐derived defects including congenital scoliosis.     

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral–aboral axis, involving local mesoderm–ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left–right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal–vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom–vestibule interaction that establishes the adult oral–aboral axis.     

Crossing the frontier: a hypothesis for the origins of meristic constraint in mammalian axial patterning

Serially homologous systems with high internal differentiation frequently exhibit meristic constraints, although the developmental basis for constraint is unknown. Constraints in the counts of the cervical and lumbosacral vertebral series are unique to mammals, and appeared in the Triassic, early in their history. Concurrent adaptive modifications of the mammalian respiratory and locomotor systems involved a novel source of cells for muscularization of the diaphragm from cervical somites, and the loss of ribs from lumbar vertebrae. Each of these innovations increased the modularity of the somitic mesoderm, and altered somitic and lateral plate mesodermal interactions across the lateral somitic frontier. These developmental innovations are hypothesized here to constrain the anteroposterior transposition of the limbs along the column, and thus also cervical and thoracolumbar count. Meristic constraints are therefore regarded here as the nonadaptive, secondary consequences of adaptive respiratory and locomotor traits.     

The lens: a classical model of embryonic induction providing new insights into cell determination in early development

The lens was the first tissue in which the concept of embryonic induction was demonstrated. For many years lens induction was thought to occur at the time the optic vesicle and lens placode came in contact. Since then, studies have revealed that lens placodal progenitor cells are specified already at gastrula stages, much earlier than previously believed, and independent of optic vesicle interactions. In this review, I will focus on how individual signalling molecules, in particular BMP, FGF, Wnt and Shh, regulate the initial specification of lens placodal cells and the progressive development of lens cells. I will discuss recent work that has shed light on the combination of signalling molecules and the molecular interactions that affect lens specification and proper lens formation. I will also discuss proposed tissue interactions important for lens development. A greater knowledge of the molecular interactions during lens induction is likely to have practical benefits in understanding the causes and consequences of lens diseases. Moreover, knowledge regarding lens induction is providing fundamental important insights into inductive processes in development in general.     

Frodo proteins: modulators of Wnt signaling in vertebrate development

The Frodo/dapper (Frd) proteins are recently discovered signaling adaptors, which functionally and physically interact with Wnt and Nodal signaling pathways during vertebrate development. The Frd1 and Frd2 genes are expressed in dynamic patterns in early embryos, frequently in cells undergoing epithelial-mesenchymal transition. The Frd proteins function in multiple developmental processes, including mesoderm and neural tissue specification, early morphogenetic cell movements, and organogenesis. Loss-of-function studies using morpholino antisense oligonucleotides demonstrate that the Frd proteins regulate Wnt signal transduction in a context-dependent manner and may be involved in Nodal signaling. The identification of Frd-associated factors and cellular targets of the Frd proteins should shed light on the molecular mechanisms underlying Frd functions in embryonic development and in cancer.     

Abnormal formation of sarcoplasmic reticulum networks and triads during early development of skeletal muscle cells in mitsugumin29-deficient mice

Recently, we detected a novel membrane protein, mitsugumin29 (MG29), in the triads in rabbit skeletal muscle cells and suggested important roles for this membrane protein in the formation of the sarcoplasmic reticulum (SR) networks and triads in muscle cells. In the present study, we examined the development of skeletal muscle cells in MG29-deficient mice to try to determine the roles played by MG29 in the formation of the SR networks and triads. Ultrastructural observations revealed some morphological abnormalities in these mice, such as incomplete formation of the SR networks, an irregular running of the transverse tubule and a partial defect in the triads at the A-I junctional region. These ultrastructural abnormalities occurred during early myogenesis and were preserved until the adult stage. The possible roles for MG29 in the formation of SR networks and triads in skeletal muscle cells are discussed in the light of these observations.     

Pattern formation in development of chondrichthyan dentitions: a review of an evolutionary model

The mode of tooth development displayed in Chondrichthyans (sharks, rays and holocephalans), one of frequent tooth replacement, was possible once a dental lamina had evolved, and since 1982 this has been known as the odontode regulation theory after Reif. Today, Reif's concepts need to be transformed into those of modern biology, the crosstalk between epithelium and mesenchyme, for the regulation of timing, spacing and shape of vertebrate teeth. Although Reif's proposed ‘primordial tissue’ may be the only site of progenitor cells, to restrict odontogenic potential to time-specific sites (protogerms), as has been suggested in the sequential addition tooth (SAT) model, very little data are available. Here, his model of alternate tooth replacement files has been interpreted as an integrated tooth addition unit of two adjacent files (SAT) unit for alternate replacement of teeth, regulated by putative, precisely timed gene expression for activation and inhibition. We have provided new data on patterns of tooth succession in dentitions of extant sharks and rays to compare with those of Reif. Using a phylogeny combined from molecular and morphological data, it is suggested that the alternate tooth addition and replacement model is derived within Chondrichthyes, and diversified from single file tooth addition of the stem chondrichthyans.     

Cell communication with the neural plate is required for induction of neural markers by BMP inhibition: evidence for homeogenetic induction and implications for Xenopus animal cap and chick explant assays

In Xenopus, the animal cap is very sensitive to BMP antagonists, which result in neuralization. In chick, however, only cells at the border of the neural plate can be neuralized by BMP inhibition. Here we compare the two systems. BMP antagonists can induce neural plate border markers in both ventral Xenopus epidermis and non-neural chick epiblast. However, BMP antagonism can only neuralize ectodermal cells when the BMP-inhibited cells form a continuous trail connecting them to the neural plate or its border, suggesting that homeogenetic neuralizing factors can only travel between BMP-inhibited cells. Xenopus animal cap explants contain cells fated to contribute to the neural plate border and even to the anterior neural plate, explaining why they are so easily neuralized by BMP-inhibition. Furthermore, chick explants isolated from embryonic epiblast behave like Xenopus animal caps and express border markers. We propose that the animal cap assay in Xenopus and explant assays in the chick are unsuitable for studying instructive signals in neural induction.     

Unshelled abalone and corrupted urchins: development of marine calcifiers in a changing ocean

The most fragile skeletons produced by benthic marine calcifiers are those that larvae and juveniles make to support their bodies. Ocean warming, acidification, decreased carbonate saturation and their interactive effects are likely to impair skeletogenesis. Failure to produce skeleton in a changing ocean has negative implications for a diversity of marine species. We examined the interactive effects of warming and acidification on an abalone (Haliotis coccoradiata) and a sea urchin (Heliocidaris erythrogramma) reared from fertilization in temperature and pH/pCO(2) treatments in a climatically and regionally relevant setting. Exposure of ectodermal (abalone) and mesodermal (echinoid) calcifying systems to warming (+2°C to 4°C) and acidification (pH 7.6-7.8) resulted in unshelled larvae and abnormal juveniles. Haliotis development was most sensitive with no interaction between stressors. For Heliocidaris, the percentage of normal juveniles decreased in response to both stressors, although a +2°C warming diminished the negative effect of low pH. The number of spines produced decreased with increasing acidification/pCO(2), and the interactive effect between stressors indicated that a +2°C warming reduced the negative effects of low pH. At +4°C, the developmental thermal tolerance was breached. Our results show that projected near-future climate change will have deleterious effects on development with differences in vulnerability in the two species.     

Early neural development in vertebrates is also a matter of calcium

The calcium (Ca²⁺) signaling pathways have crucial roles in development from fertilization through differentiation to organogenesis. In the nervous system, Ca²⁺ signals are important regulators for various neuronal functions, including formation and maturation of neuronal circuits and long-term memory. However, Ca²⁺ signals are mainly involved in the earliest steps of nervous system development including neural induction, differentiation of neural progenitors into neurons, and the neuro-glial switch. This review examines when and how Ca²⁺ signals are generated during each of these steps with examples taken from in vivo studies in vertebrate embryos and from in vitro assays using embryonic and neural stem cells. Also discussed is the highly specific nature of the Ca²⁺ signaling pathway and its interaction with the other signaling pathways involved in early neural development.     

Effect of protein phosphatase inhibitors on cleavage furrow formation in newt eggs: Inhibition of normal furrow formation and concomitant induction of furrow-like dents

The effects of three protein phosphatase inhibitors, okadaic acid, calyculin A and tautomycin, on the formation of cleavage furrows and the induction of furrow-like dents in the egg of the newt, Cynops pyrrhogaster , were examined. Solutions of the individual compound were injected into the animal hemisphere of one of the two presumptive blastomere regions of the embryo during the first cleavage. Injection of a solution containing any of the chemicals often disturbed the formation of a normal furrow in the injected blastomere at second cleavage. Injection with okadaic acid or calyculin A often induced furrow-like dents on the surface of the injected blastomere at the same time as second cleavage in control embryos, while that with tautomycin usually did not induce them. In an injected blastomere, formation of dents started in the animal half and moved towards the vegetal half as the furrow in its counterpart blastomere extended from the animal half towards the vegetal. Dents gradually became slightly deeper and formed cytoplasmic projections that later degenerated, leaving a surface scar. Cytological observations on blastomeres injected with calyculin A revealed that nuclear division occurred normally.     

A comparative ultrastructural analysis of exine pattern development in wild-typeArabidopsis and a mutant defective in pattern formation

Summary In order to identify factors necessary for the establishment of the reticulate pollen wall pattern, we have characterized a T-DNA tagged mutant ofArabidopsis thaliana that is defective in pattern formation. This study reports the results of an ultrastructural comparison of pollen wall formation in the mutant to wall development in wild-type plants. Pollen wall development in the mutant parallels that of wild-type until the early tetrad stage. At this point in wild-type plants, the microspore plasma membrane assumes a regular pattern of ridges and valleys. Initial sporopollenin deposition occurs on the ridges marking the beginning of probacula formation. In contrast, the plasma membrane in the mutant appears irregular with flattened protuberances and rare invaginations. As a result, the wild-type regular pattern of ridges and valleys is not formed. Sporopollenin is randomly deposited on the plasma membrane and aggregates on the locule wall; it is not anchored to the membrane. Our finding that the mutation blocks the normal invagination of the plasma membrane and disrupts the proper deposition of sporopollenin during wall formation suggests that the mutation could be in a gene responsible for pattern formation. These results also provide direct evidence that the plasma membrane plays a critical role in the establishment of the pollen wall pattern.     

Cell wall invertase as a regulator in determining sequential development of endosperm and embryo through glucose signaling early in seed development

Seed development depends on coordination among embryo, endosperm and seed coat. Endosperm undergoes nuclear division soon after fertilization, whereas embryo remains quiescent for a while. Such a developmental sequence is of great importance for proper seed development. However, the underlying mechanism remains unclear. Recent results on the cellular domain- and stage-specific expression of invertase genes in cotton and Arabidopsis revealed that cell wall invertase may positively and specifically regulate nuclear division of endosperm after fertilization, thereby playing a role in determining the sequential development of endosperm and embryo, probably through glucose signaling.     

Strongylocentrotus drobachiensis oocytes maintain a microtubule organizing center throughout oogenesis: implications for the establishment of egg polarity in sea urchins

Although it has been known for over a century that sea urchin eggs are polarized cells, very little is known about the mechanism responsible for establishing and maintaining polarity. Our previous studies of microtubule organization during sea urchin oogenesis described a cortical microtubule-organizing center (MTOC) present during germinal vesicle (GV) migration in large oocytes. This MTOC was localized within the future animal pole of the mature egg. In this study we have used electron microscopy and immunocytochemistry to characterize the structure of this MTOC and have established that this organelle appears prior to GV migration. We show that the cortical MTOC contains all the components of a centrosome, including a pair of centrioles. Although a centrosome proper was not found in small oocytes, the centriole pair in these cells was always found in association with a striated rootlet, a structural remnant of the flagellar apparatus present in precursor germinal cells (PGCs). The centrioles/striated rootlet complex was asymmetrically localized to the side of the oocyte closest to the gonadal wall. These data are consistent with the previously proposed hypothesis that in echinoderms the polarity of the PGCs in the germinal epithelium influences the final polarity of the mature egg.