Cell-cell interactions determine the dorsoventral axis in embryos of an equally cleaving opisthobranch mollusc

Embryonic organizer specification in the mud snail Ilyanassa obsoleta depends on intercellular signaling

In early embryos of the caenogastropod snail Ilyanassa obsoleta, cytoplasmic segregation of a polar lobe is required for establishment of the D quadrant founder cell, empowering its great-granddaughter macromere 3D to act as a single-celled organizer that induces ectodermal pattern along the secondary body axis of the embryo. We present evidence that polar lobe inheritance is not sufficient to specify 3D potential, but rather makes the D macromere lineage responsive to some intercellular signal(s) required for normal expression of 3D-specific phenotypes. Experimental removal of multiple micromeres resulted in loss of organizer-linked MAPK activation, complete and specific defects of organizer-dependent larval organs, and progressive cell cycle retardation, leading to equalization of the normally accelerated division schedule of 3D (relative to the third-order macromeres of the A, B and C quadrants). Ablation of the second-quartet micromere 2d greatly potentiated the effects of first micromere quartet ablation. Our findings link organizer activation in I. obsoleta to the putative ancestral spiralian mechanism in which a signal from micromeres leads to specification of 3D among four initially equivalent macromeres.

Functional evidence that FGFR regulates MAPK signaling in organizer specification in the gastropod mollusk Lottia peitaihoensis

The D-quadrant organizer sets up the dorsal-ventral (DV) axis and regulates mesodermal development of spiralians. Studies have revealed an important role of mitogen-activated protein kinase (MAPK) signaling in organizer function, but the related molecules have not been fully revealed. The association between fibroblast growth factor receptor (FGFR) and MAPK signaling in regulating organizer specification has been established in the annelid Owenia fusiformis. Now, comparable studies in other spiralian phyla are required to decipher whether this organizer-inducing function of FGFR is prevalent in Spiralia. Here, we indicate that treatment with the FGFR inhibitor SU5402 resulted in deficiency of organizer specification in the mollusk Lottia peitaihoensis. Subsequently, the bone morphogenetic protein (BMP) signaling gradient and DV patterning were disrupted, suggesting the roles of FGFR in regulating organizer function. Changes in multiple aspects of organizer function (the morphology of vegetal blastomeres, BMP signaling gradient, expression of DV patterning markers, etc.) indicate that these developmental functions have different sensitivities to FGFR/MAPK signaling. Our results reveal a functional role of FGFR in organizer specification as well as DV patterning of Lottia embryos, which expands our knowledge of spiralian organizers.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-023-00194-x.

Nonmuscle Myosin II is Required for Larval Shell Formation in a Patellogastropod

The molecular mechanisms underlying larval shell development in mollusks remain largely elusive. We previously found evident filamentous actin (F-actin) aggregations in the developing shell field of the patellogastropod Lottia goshimai, indicating roles of actomyosin networks in the process. In the present study, we functionally characterized nonmuscle myosin II (NM II), the key molecule in actomyosin networks, in the larval shell development of L. goshimai. Immunostaining revealed general colocalization of phosphorylated NM II and F-actin in the shell field. When inhibiting the phosphorylation of NM II using the specific inhibitor blebbistatin in one- or 2-h periods during shell field morphogenesis (6–8 h post-fertilization, hpf), the larval shell plate was completely lost in the veliger larva (24 hpf). Scanning electron microscopy revealed that the nascent larval shell plate could not be developed in the manipulated larvae (10 hpf). Further investigations revealed that key events in shell field morphogenesis were inhibited by blebbistatin pulses, including invagination of the shell field and cell shape changes and cell rearrangements during shell field morphogenesis. These factors caused the changed morphology of the shell field, despite the roughly retained “rosette” organization. To explore whether the specification of related cells was affected by blebbistatin treatments, we investigated the expression of four potential shell formation genes (bmp2/4, gata2/3, hox1 and engrailed). The four genes did not show evident changes in expression level, indicating unaffected cell specification in the shell field, while the gene expression patterns showed variations according to the altered morphology of the shell field. Together, our results reveal that NM II contributes to the morphogenesis of the shell field and is crucial for the formation of the larval shell plate in L. goshimai. These results add to the knowledge of the mechanisms of molluskan shell development.

Early shell field morphogenesis of a patellogastropod mollusk predominantly relies on cell movement and F-actin dynamics

Background

The morphogenesis of the shell field is an essential step of molluscan shell formation, which exhibits both conserved features and interlineage variations. As one major gastropod lineage, the patellogastropods show different characters in its shell field morphogenesis compared to other gastropods (e.g., the pulmonate gastropod Lymnaea stagnalis), likely related to its epibolic gastrulation. The investigation on the shell field morphogenesis of patellogastropods would be useful to reveal the lineage-specific characters in the process and explore the deep conservation among different molluscan lineages.

Results

We investigated the early shell field morphogenesis in the patellogastropod Lottia goshimai using multiple techniques. Electron microscopy revealed distinct morphological characters for the central and peripheral cells of the characteristic rosette-like shell field. Gene expression analysis and F-actin staining suggested that the shell field morphogenesis in this species predominantly relied on cell movement and F-actin dynamics, while BrdU assay revealed that cell proliferation contributed little to the process. We found constant contacts between ectodermal and meso/endodermal tissues during the early stages of shell field morphogenesis, which did not support the induction of shell field by endodermal tissues in general, but a potential stage-specific induction was indicated.

Conclusions

Our results emphasize the roles of cell movement and F-actin dynamics during the morphogenesis of the shell field in Lo. goshimai, and suggest potential regulators such as diffusible factors and F-actin modulators. These findings reflect the differences in shell field morphogenesis of different gastropods, and add to the knowledge of molluscan larval shell formation.

CLEAVAGE PATTERNS AND MESENTOBLAST FORMATION IN THE GASTROPODA: AN EVOLUTIONARY PERSPECTIVE

The larger gastropod taxa are characterized by distinctive cleavage patterns. The cell stage at which the mesentoblast is formed appears to be crucial. In none of the taxa is it formed earlier than the 24- and not later than the 63-cell stage. A heterochronic shift from late to early mesentoblast formation appears to coincide with successive steps in gastropod evolution. Comparison of the early cleavage patterns appears to be a powerful method for investigating the evolutionary relations between major gastropod taxa.

Allocation of cytoplasm to macromeres in embryos of annelids and molluscs is positively correlated with egg size

Evolutionary transitions between feeding and nonfeeding larval development have occurred many times in marine invertebrates, but the developmental changes underlying these frequent and ecologically important transitions are poorly known, especially in spiralians. We use phylogenetic comparative methods to test the hypothesis that evolutionary changes in egg size and larval nutritional mode are associated with parallel changes in allocation of cytoplasm to macromere cell lineages in diverse annelids and molluscs. Our analyses show that embryos of species with large eggs and nonfeeding larvae tend to allocate relatively more embryonic cytoplasm to macromeres at 3rd cleavage than do embryos of species with small eggs and feeding larvae. The association between egg size and allocation to macromeres in these spiralians may be driven by constraints associated with mitotic spindle positioning and size, or may be a result of "adaptation in cleavage" to maintain rapid cell cycles in micromeres, position yolk in cell lineages where it can be most efficiently used, or adjust allocation to ectoderm to accommodate changes in embryonic surface area/volume ratio.

Deviating from the Norm: Peculiarities of Aplysia cf. californica Early Cleavage Compared to Traditional Spiralian Models

Spiralia represents one of the main clades of bilaterally symmetrical metazoans (Bilateria). This group is of particular interest due to the remarkable conservation of its early developmental pattern despite of the high diversity of larval and adult body plans. Variations during embryogenesis are considered powerful tools to determine ancestral and derived characters under a phylogenetic framework. By direct observation of embryos cultured in vitro, we analyzed the early cleavage of the euopisthobranchs Aplysia cf. californica. We used tubulin immunocytochemistry to stain mitotic spindles during early cleavages, and followed each division with the aid of an autofluorescent compound inside yolk platelets, which differed from the characteristic pink-brownish pigment of the vegetal cytoplasm in zygotes and early embryos. We found that this species exhibits an unequal cleavage characterized by ooplasmic segregation, oblique inclination of mitotic spindles, and differences in size and positioning of the asters in relation to the cellular cortex. Furthermore, we detected asynchrony in cleavage timing between the two large macromeres C and D, which increases the number of cleavage rounds required to reach a particular cell stage in comparison to other spiralians. Here, we report the presence of a transient and previously undescribed U-shaped embryo in this species. The present detailed description of A. californica early development deviates considerably from stereotypical patterns described in other spiralians. Our observations demonstrate that early spiralian development can be more plastic than previously thought.

The slipper snail, Crepidula: an emerging lophotrochozoan model system

Recent developmental and genomic research focused on "slipper snails" in the genus Crepidula has positioned Crepidula fornicata as a de facto model system for lophotrochozoan development. Here we review recent developments, as well as earlier reports demonstrating the widespread use of this system in studies of development and life history. Recent studies have resulted in a well-resolved fate map of embryonic cell lineage, documented mechanisms for axis determination and D quadrant specification, preliminary gene expression patterns, and the successful application of loss- and gain-of-function assays. The recent development of expressed sequence tags and preliminary genomics work will promote the use of this system, particularly in the area of developmental biology. A wealth of comparative information on phylogenetic relationships, variation in mode of development within the family, and numerous studies on larval biology and metamorphosis, primarily in Crepidula fornicata, make these snails a powerful tool for studies of the evolution of the mechanisms of development in the Mollusca and Lophotrochozoa. By bringing a review of the current state of knowledge of Crepidula life histories and development together with some detailed experimental methods, we hope to encourage further use of this system in various fields of investigation.

MAPK activation and the specification of the D quadrant in the gastropod mollusc, Crepidula fornicata

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.

The role of MAPK signaling in patterning and establishing axial symmetry in the gastropod Haliotis asinina

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.

Cell specification and the role of the polar lobe in the gastropod mollusc Crepidula fornicata

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.

Specification of polarity of teloblastogenesis in the oligochaete annelid Tubifex: cellular basis for bilateral symmetry in the ectoderm

Ectodermal teloblastogenesis in the oligochaete annelid Tubifex is a spatiotemporally regulated process that gives rise to four bilateral pairs of ectoteloblasts (N, O, P, and Q) that assume distinct fates. Ectoteloblasts on either side of the embryo arise from an invariable sequence of asymmetric cell divisions of a proteloblast, NOPQ, which occur with a defined orientation with respect to the embryonic axes: the N teloblast is generated first and located ventralmost, and the Q teloblast, which is generated next, is located dorsalmost; finally, the O and P teloblasts are generated by almost equal division of their precursor cell, OP. Polarity of teloblastogenesis on one side of the embryo is a mirror image of the other; this mirror symmetry of ectoteloblasts about the embryo's midline gives rise to the bilaterally symmetric organization of the ectoderm. In this study, we examined whether cellular interactions are involved in specification of polarity of asymmetric cell divisions in NOPQ cells. A set of cell transplantation experiments demonstrated that NOPQ cells are initially uncommitted in terms of division pattern and cell fates: If a left NOPQ cell is transplanted to the right side of a host embryo, it exhibits a polarity comparable to that of right NOPQ cells. The results of another set of cell transplantation experiments suggest that contact between NOPQ cells serves as an external cue for their polarization, irrespective of their position in the embryo, and that in the absence of host NOPQ cells, transplanted NOPQ cells can be polarized according to positional information residing in the host embryo. The competence of NOPQ cells to respond to external cues tapers down before their division into N and OPQ. A set of cell ablation experiments demonstrated that neighboring cells such as posteriorly located M teloblasts and anterolaterally located micromeres play a role in controlling spatial aspects of NOPQ's behavior that gives rise to their division along the dorsoventral axis. These results suggest that NOPQ cells, which do not initially have a rigidly fixed polarity, become polarized through external cues. Possible sources of signals for this polarizing induction are discussed in the light of the present results.

Persistent ancestral feeding structures in nonfeeding annelid larvae

Evolutionary loss of the requirement for feeding in larvae of marine invertebrates is often followed by loss of structures involved in capturing and digesting food. Studies of echinoderms suggest that larval form evolves rapidly in response to loss of the requirement for feeding, but a lack of data from other taxa makes it difficult to assess the generality of this result. I show that many members of a large clade of annelids, the Sabellidae, retain ancestral systems for particle capture despite loss of the need and ability to feed. In at least one species, Schizobranchia insignis, an opposed-band system of prototrochal, food-groove, and metatrochal ciliary bands can concentrate suspended particles and transport them to the mouth, but captured particles are invariably rejected because larvae lack a functional gut. The persistence of particle capture systems in larvae of sabellids suggests that they have lost larval feeding very recently, that opposed bands are inexpensive to construct and operate, or that opposed bands have some alternative function. These observations also suggest a hypothesis on how the ability to feed is lost in larvae of annelids and other spiralians following increases in egg size.

Conserved mechanism of dorsoventral axis determination in equal-cleaving spiralians

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.

Cell-lineage and clonal-contribution map of the trochophore larva of Patella vulgata (mollusca)

Molluscan development is characterised by its extremely regular cleavage pattern. In numerous molluscs the fate of various early-cleavage stage blastomeres has been determined and fate maps have been constructed. On the basis of similarities between these fate maps, a generalised molluscan cell-lineage map has been constructed. Recently, the validity of this map has been challenged. In this study, the cell-lineage of the first-, second-, and third-quartet micromeres and third-generation macromeres of the equally-cleaving gastropod mollusc Patella vulgata was studied by fluorescent cell-lineage tracer injection followed by epifluorescence microscopy and confocal laser scanning microscopy. For the first time, a complete cell-lineage map, in the form of a clonal-contribution map of the trochophore, has been constructed with the use of fluorescent cell-lineage tracers. This map both agrees and differs in a number of respects with the generalised cell-lineage map of molluscs. The most important deviation is that the micromere 2d, formerly referred to as the first somatoblast, is not the only cell that forms the foot and shell gland in Patella.

Organiser role of the stem cell of the mesoderm in prototroch patterning in Patella vulgata (Mollusca, Gastropoda)

During early development of the gastropod mollusc Patella vulgata, the stem cell of the mesoderm (3D-macromere) is induced. As a result of this induction, the embryo becomes dorsoventrally organised. At about the same time in development, ciliated cells, so-called trochoblasts, are formed. Later in development, some trochoblasts deciliate and, together with the ciliated trochoblast, form the dorsoventrally organised prototroch, the locomotory organ of the larva. In order to study the role of the 3D-macromere in the specification of trochoblasts and in the induction of the dorsoventral organisation of the prototroch, induction of 3D has been prevented in various ways. it is shown that preventing 3D-induction results in the formation of a radially symmetrical prototroch. The trochoblasts of all four quadrants developed like corresponding trochoblasts of the A-quadrant. Somewhere between 30 and 120 min after fifth cleavage the 3D-macromere induces the formation of specific trochoblasts and organises the dorsoventral pattern of the prototroch. Besides a role of the 3D-macromere, a role of other cells has been demonstrated in the conditionally specified deciliation of trochoblasts.

Modifications of cell fate specification in equal-cleaving nemertean embryos: alternate patterns of spiralian development

The nemerteans belong to a phylum of coelomate worms that display a highly conserved pattern of cell divisions referred to as spiral cleavage. It has recently been shown that the fates of the four embryonic cell quadrants in two species of nemerteans are not homologous to those in other spiralian embryos, such as the annelids and molluscs (Henry, J. Q. and Martindale, M. Q. (1994a) Develop. Genetics 15, 64–78). Equal-cleaving molluscs utilize inductive interactions to establish quadrant-specific cell fates and embryonic symmetry properties following fifth cleavage. In order to elucidate the manner in which cell fates are established in nemertean embryos, we have conducted cell isolation and deletion experiments to examine the developmental potential of the early cleavage blastomeres of two equal-cleaving nemerteans, Nemertopsis bivittata and Cerebratulus lacteus. These two species display different modes of development: N. bivittata develops directly via a non-feeding larvae, while C. lacteus develops to form a feeding pilidium larva which undergoes a radical metamorphosis to give rise to the juvenile worm. By examining the development of certain structures and cell types characteristic of quadrant-specific fates for each of these species, we have shown that isolated blastomeres of the indirect-developing nemertean, C. lacteus, are capable of generating cell fates that are not a consequence of that cell's normal developmental program. For instance, dorsal blastomeres can form muscle fibers when cultured in isolation. In contrast, isolated blastomeres of the direct-developing species, N. bivittata do not regulate their development to the same extent. Some cell fates are specified in a precocious manner in this species, such as those that give rise to the eyes. Thus, these findings indicate that equal-cleaving spiralian embryos can utilize different mechanisms of cell fate and axis specification. The implications of these patterns of nemertean development are discussed in relation to experimental work in other spiralian embryos, and a model is presented that accounts for possible evolutionary changes in cell lineage and the process of cell fate specification amongst these protostome phyla.

Cell-lineage analysis of the prototroch of the gastropod molluscPatella vulgata shows conditional specification of some trochoblasts

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.

Cell lineage in molluscan development

Cell lineage specification in molluscs is brought about by two mechanism: the segregation of morphogenetic plasms and inductive cell interactions. The evidence for the existence of morphogenetic plasms is largely circumstantial, but in one species, Bithynia, such a plasm has been identified in the polar lobe that forms at first cleavage. Inductive cell interactions are thought to be a prerequisite for the development of a large number of tissues and organs. The most extensively studied example is the specification of the mesodermal stem cell in Lymnaea and Patella, which occurs between 5th and 6th cleavage through an interaction between one macromere and a large number of micromeres. Both segregation and induction are tuned to the animal-vegetal polarity of the egg, at least during early development. This polarity probably arises during oogenesis and is manifest in regional differentiations of the surface architecture of the egg, in the distribution of inner membrane particles in the plasma membrane, in membrane fluidity characteristics, in ionic conductance properties of the plasma membrane, etc. All these phenomena have in common that they represent properties of the egg surface, suggesting that the polarity of the egg is somehow imprinted into the plasma membrane and the cortex of the egg during oogenesis.

An investigation of the specification of unequal cleavages in leech embryos

Leech embryos develop via stereotyped cell divisions, many of which are unequal. The first division generates identifiable cells, blastomeres AB and CD, which normally follow distinct developmental pathways. When these two cells are dissociated and cultured in isolation, their fates remain distinct and are reminiscent of normal development, but their typical cleavage patterns are disrupted; cell AB undergoes relatively few cell divisions, giving rise to a variable number of macromeres and micromeres, while cell CD cleaves many times, usually forming a poorly organized set of macromeres, embryonic stem cells (teloblasts), and micromeres. We have investigated the hypothesis that the abnormal cleavage pattern of isolated CD blastomeres is due to removal of mechanical constraints normally imposed by cell AB. We find that when cell CD is constrained in vitro to mimic its in vivo shape, it cleaves more normally.

Localized activity of Ca2+-stimulated ATPase and transcellular ionic currents during mesoderm induction in embryos ofLymnaea stagnalis (Mollusca)

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.