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1.
Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal-vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.  相似文献   

2.
In Patella vulgata the 32-cell stage represents a pause in the mitotic activity prior to the differentiation of the mesentoblast mother cell 3D. At the onset of this stage, the embryo is radially symmetrical. Nevertheless, the plane of bilateral symmetry is indicated as it passes through the macromeres forming the vegetal cross-furrow. From the early beginning of the 32-cell stage, all four macromeres intrude far into the interior and touch the centrally radiating cells of the first quartet of micromeres. The two cross-furrow forming macromeres (3B and 3D) intrude the farthest and come into contact with the greatest number of micromeres. Finally, the contacts are extended significantly and maintained with only one of these macromeres. From that moment, this cell can be called the macromere 3D and the dorsoventral axis is determined. The evolution of the internal cell contacts between the micromeres of the first quartet and the macromeres indicates an essential role of the former in the determination of one of the latter as the mesentoblast mother cell, and thus in the determination of dorsoventral polarity.  相似文献   

3.
In mollusks with an equal four-cell stage, dorsoventral polarity becomes noticeable in the interval between the formation of the third and fourth quartet of micromeres, i.e., between the fifth and sixth cleavage. One of the two macromeres at the vegetal cross-furrow then partly withdraws from the surface and becomes located more toward the center of the embryonic cell mass than the other three macromeres. Only this specific macromere (3D) contacts the micromeres of the animal pole, divides with a delay, and develops into the stem cell of the mesentoblast (4d). After suppression of the normal contacts between micromeres and macromeres either by dissociation of the embryos or by deletion of first quartet cells, the normal differentiation of the macromeres fails to appear. By deleting a decreasing number of first quartet cells, an increasing percentage of embryos shows the normal differentiation pattern. Deletion of one of the cross-furrow macromeres does not preclude formation of the mesentoblast, which then originates by differentiation of an other macromere. It is concluded that initially the embryo is radially symmetrical and that the four quadrants have identical developmental capacities; mesentoblast differentiation from one macromere is induced through the contacts of the first quartet cells and that single macromere.  相似文献   

4.
Summary The dorsal-ventral axis inPatella vulgata embryos is established at the 32-cell stage by an inductive interaction between the animal micromeres and one vegetal macromere. This vegetal macromere, once induced, is called the 3D macromere, and marks the future dorsal side of the embryo. We examined the pattern of filamentous (F) actin in such embryos using fluorescent phalloidin and found that this dorsal 3D macromere contains more F-actin than the remainder of the cells. In addition, only one of its two daughter cells, i.e. the 4D macromere, retains this higher density. In embryos in which the establishment of the dorsal-ventral axis has been experimentally inhibited via treatment with monensin, such differences in F-actin were not found. These results suggest that the appearance of an increased density of F-actin in the dorsal 3D and 4D macromeres of normal embryos requires the inductive interactions that establish the dorsal-ventral axis. We therefore conclude that F-actin is an early marker for dorsal induction in thePatella embryo.  相似文献   

5.
Embryos of the gastropod snail Crepidula fornicata exhibit a typical spiral cleavage pattern. Although a small polar lobe is formed at the first and second cleavage divisions, the embryo of C. fornicata exhibits a mode of development similar to that of equal-cleaving spiralians in which the D quadrant is conditionally specified by inductive interactions involving the derivatives of the first quartet micromeres. This study demonstrates that mitogen activated protein kinases, MAPK, are initially activated in the progeny of the first quartet micromeres, just prior to the birth of the third quartet (e.g., late during the 16-cell and subsequently during the 20-cell stages). Afterwards, MAPK is activated in 3D just prior to the 24-cell stage, transiently in 4d and finally in a subset of animal micromeres immediately following those stages. This pattern of MAPK activation differs from that reported for other spiralians. Using an inhibitor of MAPK kinase (MEK), we demonstrated that activated MAPK is required for the specification of the 3D macromere, during the late 16-cell through early 24-cell stages. This corresponds to the interval when the progeny of the first quartet micromeres specify the D quadrant macromere. Activated MAPK is not required in 3D later during the 24-cell stage or in the embryonic organizer, 4d, for its normal activity. Likewise, activated MAPK is not required in the animal micromeres during subsequent stages of development. Additional experiments suggest that the polar lobe, though not required for normal development, may play a role in restricting the activation of MAPK and biasing the specification of the 3D macromere.  相似文献   

6.
Summary

In the spiralian embryos studied which display unequal-cleavage at the first two cleavages (either by a polar lobe or an asymmetric cleavage mechanism) the D quadrant is determined at the four cell stage by an unequal segregation of cytoplasmic stuffs. The normal formation of eyes, foot, and shell by overlying micromeres in these forms requires the inductive interaction with the D quadrant before the formation of the third quartet of micromeres. In equal-cleaving spiralians the D quadrant (3D macromere) becomes determined as a result of inductive interactions with first quartet derivatives (animal-vegetal interaction) sometime after the production of the third quartet of micromeres. This paper investigates the exact timing of D quadrant determination and the inductive role of third-order macromeres on the development of micromere derived structures in an equal-cleaving spiralian. Deletions of third-order macromeres, and their derivatives, were performed without rupturing the egg capsule membrane of the Lymnaea embryo with a UV laser microbeam. Virtually normal snails were produced when the 3A, 3B, 3C, or 4D macromere was irradiated. Juvenile snails lacking all mesodermal structures but possessing eyes, foot, and shell were obtained when the mesentoblast (4d) or its progenitor (3D) were deleted. Furthermore, ‘mesoderm-less’ snails were produced by deleting one of the two possible 3D candidates (cross furrow macromeres) as early as 20 min after third quartet formation. These results indicate that the 3D macromere begins to become determined at, or soon after, animal-vegetal interaction; before the 3D macromere becomes visibly distinguishable from the 3B macromere. The results also demonstrate that normal pattern formation in the overlying micromeres does not require the ‘prolonged’ interaction with an asymmetrically positioned 3D macromere. Possible adhesive differences between the 3D macromere and the remaining three macromeres are also revealed.  相似文献   

7.
Summary InLymnaea stagnalis, mesoderm induction occurs at the 24-cell stage, when the apical tip of the macromere 3D establishes a close contact with a number of micromeres. Via its tip, the macromere 3D is supposed to receive an inductive signal from the micromeres, resulting in the determination of the mesodermal stem cell 4d at the next division. In view of the possibility that transcellular ionic currents might somehow be involved, either in the processes that bring about this particular configuration of blastomeres or in the induction process itself, we mapped the electric field around the embryo during the 24-cell stage, using a vibrating probe. We detected a reversal of the current direction as compared to the uncleaved egg, whilst the polarity of the field along the animal-vegetal axis was maintained. We also mapped the localization of Ca2+-stimulated AT-Pase, an enzyme that drives the Ca2+-efflux from the cell. We found that this enzyme is localized exclusively along the cytoplasmic face of the apical plasma membrane of macromere 3D, and that its presence is restricted to the period from 110 to 135 min after the fifth cleavage, when there is close contact between macormere 3D and the micromeres. Since the localization of the Ca2+-stimulated ATPase coincides both in time and space with the induction of the mesoderm-mother cell, we suggest that localized calcium fluxes may play a role in this induction process.  相似文献   

8.
Embryos of many spirally cleaving species possess a characteristic cell type, the trochoblasts. These cells differentiate early in development into ciliated cells and give rise to the prototroch, the locomotory organ of the trochophore larva. As a necessary prelude to the investigation of the mechanisms that are responsible for specification of trochoblasts in the equally cleaving gastropod molluscPatella vulgata, the cell-lineage of the prototroch was studied. This was done by microinjection of the cell-lineage tracer lucifer yellow-dextran in trochoblasts and by scanning electron microscopical analysis of formation of the prototroch. The results show that trochoblasts that form the prototroch are of different clonal origin and that the four quadrants of the embryo have an unequal contribution to the prototroch. Since the four quadrants of the equally cleaving embryo are initially equipotent, some trochoblasts must become conditionally specified. Other trochoblasts seem to become autonomously specified. After initial ciliation some trochoblasts become deciliated and for some cells the choice between a larval and an adult cell fate is conditionally specified. Cell-lineage analysis demonstrates that the various autonomously and conditionally specified trochoblasts are organised according to the dorsoventral axis of the embryo. Possible mechanisms that can account for the conditional specification of trochoblasts — including a role for the 3D macromere, which forms the primary mesoderm and is responsible for the formation of the dorsoventral axis of the embryo — are discussed. Correspondence to: P. Damen  相似文献   

9.
At fourth cleavage of sea urchin embryos four micromeres at the vegetal pole separate from four macromeres just above them in an unequal cleavage. The micromeres have the capacity to induce a second axis if transplanted to the animal pole and the absence of micromeres at the vegetal pole results in the failure of macromere progeny to specify secondary mesenchyme cells (SMCs). This suggests that micromeres have the capacity to induce SMCs. We demonstrate that micromeres require nuclear beta-catenin to exhibit SMC induction activity. Transplantation studies show that much of the vegetal hemisphere is competent to receive the induction signal. The micromeres induce SMCs, most likely through direct contact with macromere progeny, or at most a cell diameter away. The induction is quantitative in that more SMCs are induced by four micromeres than by one. Temporal studies show that the induction signal is passed from the micromeres to macromere progeny between the eighth and tenth cleavage. If micromeres are removed from hosts at the fourth cleavage, SMC induction in hosts is rescued if they later receive transplanted micromeres between the eighth and tenth cleavage. After the tenth cleavage addition of induction-competent micromeres to micromereless embryos fails to specify SMCs. For macromere progeny to be competent to receive the micromere induction signal, beta-catenin must enter macromere nuclei. The macromere progeny receive the micromere induction signal through the Notch receptor. Signaling-competent micromeres fail to induce SMCs if macromeres express dominant-negative Notch. Expression of an activated Notch construct in macromeres rescues SMC specification in the absence of induction-competent micromeres. These data are consistent with a model whereby beta-catenin enters the nuclei of micromeres and, as a consequence, the micromeres produce an inductive ligand. Between the eighth and tenth cleavage micromeres induce SMCs through Notch. In order to be receptive to the micromere inductive signal the macromeres first must transport beta-catenin to their nuclei, and as one consequence the Notch pathway becomes competent to receive the micromere induction signal, and to transduce that signal. As Notch is maternally expressed in macromeres, additional components must be downstream of nuclear beta-catenin in macromeres for these cells to receive and transduce the micromere induction signal.  相似文献   

10.
Summary The significance of the first quartet of micromeres for the morphogenesis ofBithynia — a polar lobe-forming gastropod-has been studied by deletion experiments. After removal of the whole first quartet at the 8-cell stage a dorsoventrally organized veliger larva is formed. Apparently, an interaction between the animal micromeres and a vegetal macromere, which is essential for the origin of a dorsoventral organization in equally cleaving gastropods, is not required in polar lobe forming eggs. It is concluded that in these eggs dorsoventrality is determined by segregation of the polar lobe. The embryos, in which the first quartet has been removed, never develop head structures. This indicates that the capacity to form head structures is restricted to the first quartet of micromeres. Deletion of a specific first quartet micromere (1a, 1b, 1c or 1d) showed, however, that the individual cells of this quartet are not strictly determined right from their origin. Frequently regulative development was observed after removal of individual first quartet cells.  相似文献   

11.
Dorsoventral polarity in molluscan embryos can arise by two distinct mechanisms, where the mechanism employed is strongly correlated with the cleavage pattern of the early embryo. In species with unequal cleavage, the dorsal lineage, or "D quadrant", is determined in a cell-autonomous manner by the inheritance of cytoplasmic determinants. However, in gastropod molluscs with equal cleavage, cell-cell interactions are required to specify the fate of the dorsal blastomere. During the fifth cleavage interval in equally cleaving embryos, one of the vegetal macromeres makes exclusive contacts with the animal micromeres, and this macromere will give rise to the mesodermal precursor cell at the next division, thereby identifying the dorsal quadrant. This study examines D-quadrant determination in an equally cleaving species from a group of previously uninvestigated gastropods, the subclass Opisthobranchia. Blastomere ablation experiments were performed on embryos of Haminoea callidegenita to (i) determine the developmental potential of macromeres before and after fifth cleavage, and (ii) examine the role of micromere-macromere interactions in the establishment of bilateral symmetry. The results suggest that the macromeres are developmentally equivalent prior to fifth cleavage, but become nonequivalent soon afterward. The dorsoventral axis corresponds to the displacement of the micromeres over one macromere early in the fifth cleavage interval. This unusual cellular topology is hypothesized to result from constraints imposed on micromere-macromere interactions in an embryo that develops from a large egg and forms a stereoblastula (no cleavage cavity). Ablation of the entire first quarter of micromeres results in embryos which remain radially symmetrical in the vegetal hemisphere, indicating that micromere-macromere interactions are required for the elaboration of bilateral symmetry properties. Therefore, inductive interactions between cells may represent a general strategy for dorsoventral axis determination in equally cleaving gastropods.  相似文献   

12.
In annelids, molluscs, echiurans and sipunculids the establishment of the dorsal-ventral axis of the embryo is associated with D quadrant specification during embryogenesis. This specification occurs in two ways in these phyla. One mechanism specifies the D quadrant via the shunting of a set of cytoplasmic determinants located at the vegetal pole of the egg to one blastomere of the four cell stage embryo. In this case, at the first two cleavages of embryogenesis there is an unequal distribution of cytoplasm, generating one macromere which is larger than the others at the four cell stage. The D quadrant can also be specified by a contact mediated inductive interaction between one of the macromeres at the vegetal pole with micromeres at the animal pole of the embryo. This mechanism operates at a later stage of development than the cytoplasmic localization mechanism and is associated with a pattern of cleavage in which the first two cleavages are equal. An analysis of the phylogenetic relationships within these phyla indicates that the taxa which determine the D quadrant at an early cleavage stage by cytoplasmic localization tend to be derived and lack a larval stage or have larvae with adult characters. Those taxa where the D quadrant is specified by induction include the ancestral groups although some derived groups also use this mechanism. The pulmonate mollusc Lymnaea uses an inductive mechanism for specifying the D quadrant. In these embryos each of the four vegetal macromeres has the potential of becoming the D macromere; however under normal circumstances one of the two vegetal crossfurrow macromeres almost invariably becomes the D quadrant. Experiments are described here in which the size of one of the blastomeres of the four cell stage Lymnaea embryo is increased; this macromere invariably becomes the D quadrant. These experiments suggest that developmental change in relative blastomere size during the first two cleavages in spiralian embryos that normally cleave equally may have provided a route that has led to the establishment of the cytoplasmic localization mechanism of D quadrant formation.  相似文献   

13.
Gastropods are members of the Spiralia, a diverse group of invertebrates that share a common early developmental program, which includes spiral cleavage and a larval trochophore stage. The spiral cleavage program results in the division of the embryo into four quadrants. Specification of the dorsal (D) quadrant is intimately linked with body plan organization and in equally cleaving gastropods occurs when one of the vegetal macromeres makes contact with overlying micromeres and receives an inductive signal that activates a MAPK signaling cascade. Following the induction of the 3D macromere, the embryo begins to gastrulate and assumes a bilateral cleavage pattern. Here we inhibit MAPK activation in 3D with U0126 and examine its effect on the formation and patterning of the trochophore, using a suite of territory-specific markers. The head (pretrochal) region appears to maintain quadri-radial symmetry in U0126-treated embryos, supporting a role for MAPK signaling in 3D in establishing dorsoventral polarity in this region. Posterior (posttrochal) structures - larval musculature, shell and foot - fail to develop in MAPK inhibited trochophores. Inhibition of 3D specification by an alternative method - monensin treatment - yields similar abnormal trochophores. However, genes that are normally expressed in the ectodermal structures (shell and foot) are detected in U0126- and monensin-perturbed larvae in patterns that suggest that this region has latent dorsoventral polarity that is manifested even in the absence of D quadrant specification.  相似文献   

14.
In embryos of Patella vulgata at the 32-cell stage, one of the four vegetally located macromeres makes contacts with overlying animal micromeres. As a result, this macromere (designated 3D) divides significantly later than the other macromeres and forms the mesodermal stem cell 4d. Shortly before and during this interaction two types of extracellular matrix are present: a basal lamina-like layer on the tips of the micromeres and a loose fibrillar meshwork in the blastocoel. In this paper we examine the role of the matrix in cleavage delay and mesoderm determination. The microinjection of extracellular matrix-binding lectins, or of hyaluronidase, or of decasaccharide fragments of hyaluronate into the blastocoel results in embryos in which either no or two macromeres are delayed in cleavage and are presumably determined as mesodermal stem cells. We suggest that the fibrillar meshwork is needed for macromere elongation toward the micromeres and that the basal lamina-like layer is involved in the determination process itself.  相似文献   

15.
We investigated the inductive signals originating from the vegetal blastomeres of embryos of the sand dollar Peronella japonica, which is the only direct developing echinoid species that forms micromeres. To investigate the inductive signals, three different kinds of experimental embryos were produced: micromere-less embryos, in which all micromeres were removed at the 16-cell stage; chimeric embryos produced by an animal cap (eight mesomeres) recombined with a micromere quartet isolated from a 16-cell stage embryo; and chimeric embryos produced by an animal cap recombined with a macromere-derived layer, the veg1 or veg2 layer, isolated from a 64-cell stage embryo. Novel findings obtained from this study of the development of these embryos are as follows. Micromeres lack signals for endomesoderm specification, but are the origin of a signal establishing the oral–aboral (O–Ab) axis. Some non-micromere blastomeres, as well as micromeres, have the potential to form larval skeletons. Macromere descendants have endomesoderm-inducing potential. Based on these results, we propose the following scenario for the first step in the evolution of direct development in echinoids: micromeres lost the ability to send a signal endomesoderm induction so that the archenteron was formed autonomously by macromere descendants. The micromeres retained the ability to form larval spicules and to establish the O–Ab axis.  相似文献   

16.
Summary Spirally cleaving embryos in which the first two cleavages generate four equal-sized blastomeres remain radially symmetrical along their animal-vegetal axis until the interval between third and fourth quartet formation. At this time animal micromeres and vegetal macromeres contact each other as they elongate and occlude the central, fluid-filled cleavage cavity. The overlying micromeres focus their contacts onto one of the four macromeres, the presumptive 3D macromere, as it elongates to a central position within the embryo. We tested the hypothesis that this animal-vegetal interaction was causally involved in the determination of the symmetry properties in both the animal and vegetal hemispheres by reversibly inhibiting animal-vegetal contact at the 24 cell stage with cytochalasin-B. Embryos remained hollow throughout the treatment period and animal-vegetal interaction did not occur. After treatment, blastomere elongation occurred but no D quadrant macromere appeared and the vegetal hemisphere remained radialized. On the basis of cleavage and ciliation patterns of first quartet derivatives, treated embryos remained fully or partially radialized, showing a strong tendancy to develop as ventral quadrants. These results show that the quadrants of this equal-cleaving spiralian are not definitively determined until after the 24 cell stage and that animal-vegetal interaction is required for D quadrant determination. The mechanisms of symmetrization in the animal and vegetal hemispheres of equal-cleaving spiralians is also discussed.  相似文献   

17.
A small polar lobe forms at the first and second cleavage divisions in the gastropod mollusc Crepidula fornicata. These lobes normally fuse with the blastomeres that give rise to the D quadrant at the two- and four-cell stages (cells ultimately generating the 4d mesentoblast and D quadrant organizer). Significantly, removal of the small polar lobe had no noticeable effect on subsequent development of the veliger larva. The behavior of the polar lobe and characteristic early cell shape changes involving protrusion of the 3D macromere at the 24-cell suggest that the D quadrant is specified prior to the sixth cleavage division. On the other hand, blastomere deletion experiments indicate that the D quadrant is not determined until the time of formation of the 4d blastomere (mesentoblast). In fact, embryos can undergo regulation to form normal-appearing larvae if the prospective D blastomere or 3D macromere is removed. Removal of the 4d mesentoblast leads to highly disorganized, radial development. Removal of the first quartet micromeres at the 8-cell stage also leads to the development of radialized larvae. These findings indicate that the embryos of C. fornicata follow the mode of development exhibited by equal-cleaving spiralians, which involves conditional specification of the D quadrant organizer via inductive interactions, presumably from the first quartet micromeres.  相似文献   

18.
In embryos of the gastropod Ilyanassa obsoleta, the first-quartet micromeres of the A, B and C lineages (1a, 1b, and 1c) are each competent to form an eye in response to signaling from the 3D cell. The first-quartet micromere of the dorsal D lineage (1d) is smaller than the others, divides at a slower rate, and lacks the ability to form an eye. These properties of 1d all depend on inheritance of vegetal polar lobe cytoplasm by its mother cell D at second cleavage. I show that they depend also on the presence of cells adjacent to D during the late four-cell stage: after ablation of the A and/or C cells before this stage, 1d inherits more cytoplasm than normal, divides more rapidly, and frequently forms an eye. In non-D lineages, cleavage plane positioning and micromere division rates are relatively insensitive to cell contacts. Compressing whole embryos during third cleavage also leads to an increase in 1d volume correlated with abnormal eye formation; this suggests that the normal effect of cell contacts is to position the D cell cleavage furrow closer to the animal pole, and the enhanced division asymmetry of the D cell contributes to the suppression of eye development.  相似文献   

19.
Fourth cleavage of the sea urchin embryo produces 16 blastomeres that are the starting point for analyses of cell lineages and bilateral symmetry. We used optical sectioning, scanning electron microscopy and analytical 3-D reconstructions to obtain stereo images of patterns of karyokinesis and cell arrangements between 4th and 6th cleavage. At 4th cleavage, 8 mesomeres result from a variant, oblique cleavage of the animal quartet with the mesomeres arranged in a staggered, offset pattern and not a planar ring. This oblique, non-radial cleavage pattern and polygonal packing of cells persists in the animal hemisphere throughout the cleavage period. Contrarily, at 4th cleavage, the 4 vegetal quartet nuclei migrate toward the vegetal pole during interphase; mitosis and cytokinesis are latitudinal and subequatorial. The 4 macromeres and 4 micromeres form before the animal quartet divides to produce a 12-cell stage. Subsequently, macromeres and their derivatives divide synchronously and radially through 8th cleavage according to the Sachs-Hertwig rule. At 5th cleavage, mesomeres and macromeres divide first; then the micromeres divide latitudinally and unequally to form the small and large micromeres. This temporal sequence produces 28-and 32-cell stages. At 6th cleavage, macromere and mesomere descendants divide synchronously before the 4 large micromeres divide parasynchronously to produce 56- and 60-cell stages.  相似文献   

20.
The developmental potential of the animal cap (consisting of eight mesomeres) recombined with micromeres or of micromere progeny was examined in sea urchin embryos. The embryos derived from the animal cap recombined with a quartet of micromeres or their descendants developed into four-armed plutei. After feeding, the larvae developed into eight-armed plutei. The left-right polarity of the larvae, recognized by the location of the echinus rudiment, was essentially normal, regardless of the orientation of animal-vegetal polarity in micromeres combining with the animal cap. The larvae had sufficient potential to metamorphose into complete juvenile sea urchins with five-fold radial symmetry. Cell lineage tracing experiments showed that: (i) macromere progeny were not required for formation of the typical pattern of primary mesenchyme cells derived exclusively from large micromeres; (ii) the progeny of large micromeres did not contribute to cells in the endodermal gut with three compartments of normal function; (iii) the presumptive ectoderm had the potential to differentiate into endodermal gut and mesodermal secondary mesenchyme cells, from which pigment cells likely differentiated; and (iv) behavior of the progeny of small micromeres was the same as that in normal embryos through the gastrula stage. These results indicate that the mesomeres respecify their fate under the inductive influence of micromeres so perfectly that complete juvenile sea urchins are produced.  相似文献   

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