Expression of epidermis-specific antigens during embryogenesis of the ascidian, Halocynthia roretzi

We have produced two monoclonal antibodies (Epi-1 and Epi-2) which specifically recognize epidermal cells and their derivative, the larval tunic, of developing embryos of the ascidian Halocynthia roretzi. The antigens, examined by indirect immunofluorescence staining, first appear at the early tailbud stage and are present until at least the swimming larval stage. There were distinct and separate puromycin and actinomycin D sensitivity periods for each antigen. Aphidicolin, a specific inhibitor of DNA synthesis, prevented the appearance of each antigen when embryos were exposed to the drug continuously from cleavage stages. These results suggest that the antigens are synthesized during embryogenesis by developing epidermal cells and that several rounds of DNA replication are required for the antigen expression. Early cleavage stage embryos, including fertilized but unsegmented eggs, in which cytokinesis had been blocked with cytochalasin B expressed the antigens, and blastomeres exhibiting the antigens were always of the epidermis lineage. In partial embryos produced by four separated blastomere pairs of the 8-cell embryos, the expression of antigens was seen only in those developed from the animal blastomere pairs, which are progenitors of epidermal cells. These observations indicate that differentiation of epidermal cells in ascidian embryos takes place in a typical "mosaic" fashion.     

An 83-kDa embryonic-type nuclear antigen is detected within the germinal vesicles of oocytes of the ascidianHalocynthia roretzi

Summary Monoclonal antibodies were raised against germinal vesicles which were isolated from fully grown oocytes of the ascidianHalocynthia roretzi. Immunoblot analyses revealed that one of the antibodies, designated Hgv-2, recognized a single band with a molecular weight of about 83 kDa. The antibody, visualized by indirect immunohistochemistry, reacted only with the germinal vesicles of oocytes and did not react with test cells, follicle cells, and other somatic cells of the gonad. During embryogenesis the antigenicity was found in interphase nuclei of all embryonic cells. The antibody did not react with chromosomes or the mitotic apparatus. The antigenicity was retained by interphase nuclei of larval cells, but it disappeared from nuclei of juveniles about 7 days after metamorphosis.     

The morphology of the cement gland apparatus of Larval Pterophyllum scalare Cuv. & Val. (Cichlidae,Teleostei)

Summary The cement gland apparatus of newly hatched Pterophyllum scalare Cuv. & Val. was examined by histology, scanning and transmission electron microscopy. The whole organ is composed of three pairs of endoepithelial, ductless glands, which cause prominent elevations on the larval head and are found in a specific arrangement. Each single gland is represented by an aggregation of elongated, tubular secretory cells surrounding a pyriform acinus. It overlies a basal lamina and is covered by the outer layer of the bilaminar embryonic epidermis.Two different types of secretory cells can be distinguished. One type is restricted to the bottom of the cavity. It is characterized by multiform cytoplasmic protrusions, which project into the gland's cavity. The secretory granules contain a network of light filamentous material. The second type constitutes the side wall of the acinus. It does not develop any protrusions. The contents of the secretory granules is of very high and homogeneous electron density. The mechanism of extrusion is discussed for both cell types. All secretory cells show a strong PAS-reaction. In SEM a circular microridge pattern with attached mucus globules can be recognized on the larval epithelial surface.Dedicated to Prof. Dr. H. Leonhardt on the occasion of his 60th birthday     

Expression of murine early embryonic antigens, SSEA-1 and antigenic determinant of EMA-1, in embryos and ovarian follicles of a teleost medaka (Oryzias latipes)

Stage-specific embryonic antigen-1 (SSEA-1) and the antigenic determinant of monoclonal antibody EMA-1 are expressed in a stage-specific manner in mouse early embryos. To study whether these antigens generally exist in fish, expression of the antigens was examined in embryos, ovarian follicles, and adult tissues of a teleost medaka (Oryzias latipes), using immunohistochemical techniques. In 1-cell-stage embryos, these carbohydrate antigens were found in numerous cytoplasmic granules in the blastodisc and the cortical cytoplasm. These granules gradually decreased in number as the embryos developed. In 4-cell-stage embryos, the antigens appeared on the cleavage planes and were located on the cleavage planes within the blastoderm in the following cleavage stages. In blastula-stage embryos, the expression was ubiquitously found on the cell surface of blastomeres. At the mid-gastrula stage, the antigens were restricted to the enveloping layer, yolk syncytial layer, and cortical cytoplasm, but were rarely found in deep cells that contribute to formation of the embryonic body. In later-stage embryos and adult fish, the antigens were located in various tissues. In ovarian follicles, the antigens were found in granules of oocytes and granulosa cells. These observations were basically consistent with those in mice; however, expression in 1-cell-stage embryos and ovarian follicles has not been observed in mice. This unexpected finding suggests that the antigens are produced in granulosa cells and transferred to 1-cell-stage embryos via oocytes, and that the antigens involved in the early developmental process are maternally prepared in teleosts.     

Larval antigen molecules recognized by adult immune cells of inbred Xenopus laevis: two pathways for recognition by adult splenic T cells

During anuran metamorphosis, larval cells of the tadpole are completely eliminated and replaced by adult cells in the corresponding tissues of the frog for the adaptation to terrestrial life from an aquatic life. Before the metamorphic climax, most of the cells have already transformed from larval cells into adult-type cells, but the tail cells remain as larval cells even at the climax stages of metamorphosis. In our previous works, we demonstrated that larval skin grafts are rejected by an inbred strain of adult Xenopus and that the larval cells are recognized and made apoptotic by splenocytes obtained from adults and/or metamorphosing tadpoles in vitro (Y. Izutsu and K. Yoshizato, 1993, J. Exp. Zool. 266, 163-167; Y. Izutsu et al., 1996, Differentiation 60, 277-286). In the present study, it was found that there were two types of larval epidermal cells, classified according to the presence of major histocompatibility complex (MHC); one is the apical cell expressing both MHC classes I and II, and the other is the skein cell, which expresses no MHC. By a Percoll gradient, we were able to separate these two types of cells and examined the proliferative response of adult T cells to each of them. It was revealed that the apical cells (MHC-positive) were recognized directly by adult splenic T cells, whereas the skein cells (MHC-negative) were recognized by the T cells via the antigen presentation by adult splenocytes. Both of these proliferative responses were restricted to MHC class II. This is the first report showing how the larval-specific antigens present in different forms in epidermal cells are recognized as immunological targets by syngeneic adult T lymphocytes.     

Monoclonal antibodies recognizing larval- and pupal-specific cuticular proteins of Tenebrio molitor (Insecta,Coleoptera)

To study the sequential expression of insect epidermal cells during metamorphosis, a library of monoclonal antibodies (MABs) was prepared against the water-soluble proteins from preecdysial pupal cuticle of Tenebrio molitor. Six selected MABs recognizing only larval and pupal cuticular proteins (CPs) in immunoblot analysis were classified into three types. Type 1 recognized a 21.5 and a 22 kDa polypeptide, type 2, a 26 kDa polypeptide, and type 3, three polypeptides of 18.5, 19.5 and 21.5 kDa. They did not immunoreact with any protein of fat bodies or haemolymph from pharate pupae, suggesting that the antigens originate from the epidermis. The stage-specificity was confirmed by electron microscopic immunogold labelling. Type 1 and 3 MABs recognized antigens characterizing larval and pupal preecdysial sclerotized cuticles, while the antigens recognized by type 2 were localized in the first few lamellae of unsclerotized postecdysial cuticle. When the expression of the adult programme was inhibited by application of a juvenile hormone analogue, the larval-/pupal-specific CPs were detected in the supernumerary pupal cuticle. These results suggest that the genes encoding these proteins are juvenile hormone dependent. These MABs should be useful tools to isolate pupal-specific genes whose regulation sems to be different from that of the adult-specific ones.     

Larval antigen molecules recognized by adult immune cells of inbred Xenopus laevis: partial characterization and implication in metamorphosis

It has been shown that larval skin (LS) grafts are rejected by an inbred strain of adult Xenopus, which suggests a mechanism of metamorphosis by which larval cells are recognized and attacked by the newly differentiating immune system, including T lymphocytes. In an attempt to define the larval antigenic molecules that are targeted by the adult immune system, anti-LS antibodies (IgY) were produced by immunizing adult frogs with syngeneic LS grafts. The antigen molecules that reacted specifically with this anti-LS antiserum were localized only in the larval epidermal cells. Of 53 and 59-60 kDa acidic proteins that were reactive with anti-LS antibodies, a protein of 59 kDa and with an isoelectric point of 4.5 was selected for determination of a 19 amino acid sequence (larval peptide). The rat antiserum raised against this peptide was specifically reactive with the 59 kDa molecules of LS lysates. Immunofluorescence studies using these antisera revealed that the larval-specific molecules were localized in both the tail and trunk epidermis of premetamorphic larvae, but were reduced in the trunk regions during metamorphosis, and at the climax stage of metamorphosis were detected only in the regressing tail epidermis. Culture of splenocytes from LS-immunized adult frogs in the presence of larval peptide induced augmented proliferative responses. Cultures of larval tail pieces in T cell-enriched splenocytes from normal frogs or in natural killer (NK)-cell-enriched splenocytes from early thymectomized frogs both resulted in significant destruction of tail pieces. Tissue destruction in the latter was enhanced when anti-LS antiserum was added to the culture. These results indicate that degeneration of tail tissues during metamorphosis is induced by a mechanism such that the larval-specific antigen molecules expressed in the tail epidermis are recognized as foreign by the newly developing adult immune system, and destroyed by cytotoxic T lymphocytes and/or NK cells.     

The fate of larval flagellated cells during metamorphosis of the sponge Halisarca dujardini

Sponge larval flagellated cells have been known to form the external layer of larva, but their subsequent fate and morphogenetic role are still unclear. It is actually impossible to follow flagellated cell developmental fate unless a specific marker is found. We used percoll density gradient fractionation to separate different larval cell types of Halisarca dujardini (Demospongiae, Halisarcida). A total of 5 fractions were obtained which together contained all cell types. Fraction 1 contained about 100% FC and its polypeptide composition was very different to that of the other fractions. Of all larval cell types, flagellated cells displayed the lowest in vitro aggregation capacity. We raised a polyclonal antibody against a 68 kDa protein expressed by larval flagellated cells. Its specificity was tested on total protein extract from adult sponges by Western blotting and proved to be suitable for immunofluorescence. By means of double immunofluorescence using both this polyclonal antibody and commercial anti-tubulin antibodies, we studied the distribution of the 68 kDa protein in larval flagellated cells and its fate at successive stages of metamorphosis. In juvenile sponges just after metamorphosis the choanocytes and the upper pinacoderm were labelled with both antibodies. In larval flagellated cells, the 68 kDa protein was found all over the cytoplasm appearing as granules, while in adult sponges, it was present in the apical part of choanocytes in the vicinity of collars. Direct participation of the larval flagellated cells in the development of definitive structures was demonstrated.     

Electron microscopy studies of ciliated cells in the larval epidermis of Rana t. temporaria (L.) (Amphibia, Anura)

Examination of the epidermis of Rana temporaria in various stages of development revealed the presence of densely ciliated cells from the late neurula until shortly before metamorphosis. Unlike the other epidermal cells, these ciliated cells do not divide; once formed, they are constant in number until they disappear. In shape, size, and structure, however, they vary depending on stage of development and on their position in the body. In older larval stages and young tadpoles they are fully differentiated and strongly basophilic. Their function is to improve the hydrodynamic properties of the larval body, and hence ultimately to optimize the energy balance during locomotion.     

Stage-specific embryonic antigens (SSEA-3 and -4) are epitopes of a unique globo-series ganglioside isolated from human teratocarcinoma cells.

Two monoclonal antibodies (MC631 and MC813-70) raised against 4- to 8-cell stage mouse embryos and a human teratocarcinoma cell line, respectively, detect the stage-specific embryonic antigens, the previously defined SSEA-3 and SSEA-4, described herein. These antibodies were both reactive with a unique globo-series ganglioside with the structure shown below: (formula; see text) The antibodies were found to recognize sequential regions of this ganglioside, i.e., MC813-70 recognizes the terminal 'a' structure whereas antibody MC631 recognizes the internal 'b' structure. Thus, a set of two antibodies defines this unique embryonic antigen. During differentiation of human teratocarcinoma 2102Ep cells, the globo-series glycolipids defined by these antibodies decrease and the lacto-series glycolipids, reacting with the SSEA-1 antibody appear. This antigenic conversion suggests that a shift of glycolipid synthesis from globo-series to lacto-series glycolipids occurs during differentiation of human teratocarcinoma and perhaps of pre-implantation mouse embryos.     

Cytochemical investigations on tunic morphogenesis in the sea peach Halocynthia papillosa (Tunicata, Ascidiacea). 1: demonstration of polysaccharides

The distribution of carbohydrates was demonstrated in the embryonic, larval, and juvenile tunics of Halocynthia papillosa. An enzyme-gold marker (cellobiohydrolase-Au) was used to identify cellulose on ultrathin sections. This is the first time this biopolymer has been detected in the embryonic or larval tunic of an ascidian. Cellulose is present from the initial tail-bud stage onwards, as soon as the outer compartment of the tunic appears. Both compartments of the larval tunic also contain non-cellulosic polysaccharides, as demonstrated by the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method. Our observations point to two types of cellulose synthesis. One occurs during the embryonic and larval stages, when glycogen-like material is stored in epidermal intracellular lacunae and discharged into the tunic where it is presumably used to synthesize cellulose throughout the depth of the tunic. The second occurs from the onset of metamorphosis onwards, just above the apical plasmalemma of epidermal cells, like cellulose biogenesis in plants.     

Normal human epidermal keratinocytes express in vitro specific molecular forms of (pro)filaggrin recognized by rheumatoid arthritis-associated antifilaggrin autoantibodies.

BACKGROUND: The so-called antikeratin antibodies and the antiperinuclear factor are the most specific serological markers of rheumatoid arthritis (RA). They were recently shown to be largely the same autoantibodies and to recognize human epidermal filaggrins and profilaggrin-related proteins of buccal epithelial cells (collectively referred to as (pro)filaggrin). MATERIALS AND METHODS: To further characterize the target antigens, we investigated their expression by normal human epidermal keratinocytes cultured in differentiating conditions, using immunofluorescence and immunoblotting with RA sera and three different monoclonal antibodies to (pro)filaggrin. RESULTS: On the cornified, stratified epithelial sheets obtained in vitro, RA sera with anti(pro)filaggrin autoantibodies (AFA) produced granular staining of the stratum granulosum and diffuse staining of the stratum corneum. The antigens recognized by RA sera strictly colocalized with (pro)filaggrin in keratohyalin granules. Following sequential extraction of the proteins from the epithelial sheets, the RA sera and the three monoclonal antibodies to (pro)filaggrin, recognized a series of low-salt-soluble molecules, including a neutral/acidic isoform of filaggrin and several proteins with sizes and pI intermediates between this isoform and profilaggrin. They also recognized urea-soluble high-molecular-weight profilaggrin-related molecules. CONCLUSIONS: These results show that in vitro epidermal keratinocytes express various molecular forms of (pro) filaggrin that bear epitopes targeted by AFA of RA sera, and that some of these are absent from epidermis. Moreover, these epitopes, which are present on the keratohyalin granules of buccal epithelial cells but not on those of epidermal cells, are present on the granules of the cultured keratinocytes. This work completes the molecular characterization of the proteins targeted by AFA.     

A Novel Monoclonal Antibody disrupting Cell Type Specific Substratum Adhesion of Frog (Xenopus laevis) Epithelial Cells and Endothelial Cells

We isolated a mouse monoclonal antibody (FAD-II) that disrupts cell-substratum adhesion of amphibian ( Xenopus laevis ) epithelial cells and endothelial cells. The effect of the antibody was cell-type specific, and the antibody had no effect on fibroblastic cells while fibronectin peptide blocked cell-substratum adhesion of all the cell types examined. In developing frog embryos, the epitopes recognized by the antibody were detected in pronephrotic ducts and in other tissue cells of embryos (from stage 33/34 afterwards). In adult tissues, the antibody mainly recognized antigens in extracelluar matrices. The antigens recognized by the antibody seems to be novel glycoepitopes in frog cells.     

Actin-mediated retraction of the larval epidermis during metamorphosis of the sand dollar,Dendraster excentricus

Summary During the first 15 to 20 min of metamorphosis the larval arms are retracted and resorbed into the aboral surface of the juvenile. Arms excised from metamorphosing larvae will undergo a sequence of contraction and histolysis that is identical to that occurring in intact larvae. Prior to and during metamorphosis, epidermal cells contain bundles of 5 to 7-nm microfilaments in arrays radiating apically from the base of the cells. Sparse microfilaments also occur near the plasmalemma of epidermal cells and some mesenchymal cells in larvae fixed during metamorphosis. Contraction of excised arms is reversibly inhibited by treatment with cytochalasin B, and microfilaments bind myosin subfragment-l. Indirect immunofluorescence of larval arms using an antibody against chicken-muscle actin and staining with the F-actin specific probe, NDB phallacidin indicate that the arms contain actin distributed in a manner consistent with ultrastructural findings. It is proposed that retraction of the larval arms during metamorphosis is produced by an actin-mediated change in shape of the epidermal cells.     

Loss of reactivity to pan-cadherin antibody in epidermal cells as a marker for metamorphic alteration of Xenopus skin

Pan-cadherin antibodies recognize the conserved C-terminal region of the family of cell-cell adhesion molecules, cadherins, and have a broad spectrum of reactivity to the molecules. In the present study, by immunohistochemistry using an anti-pan cadherin monoclonal antibody (mAb), expression dynamics of cadherins in epidermal tissues were analyzed during metamorphosis of Xenopus laevis. At early stages of development, the anti-pan cadherin mAb detected signals at cell-cell boundaries and in the cytoplasm of both trunk and tail epidermal cells. During metamorphosis, the immunoreactivity decreased in the trunk skin tissue but remained in the tail. At the climax stage, immunoreactivity was observed only in the regressing tail epidermis. The signals disappeared completely from the trunk epidermis, which had already transformed into adult-type tissue. This observation was confirmed by western blot analysis. A specific band was detected in the larval skin, but not in the adult lysate, at approximately 135 kDa in molecular size, corresponding to the molecular mass of cadherins. This different immunoreactivity in larvae and adults was observed in the epidermis of the skin, but not in any other tissues examined, that is, brain, kidney and liver. The immunoreactivity seen in larval epidermal cells was drastically downregulated by thyroid hormone treatment in vitro. These changes of immunoreactivity were specific for the C-terminal region of cadherins, suggesting intracellular alteration of the molecules during metamorphosis, and the anti-pan cadherin mAb can be a marker for larval-type epidermal cells that is applicable to analysis of Xenopus metamorphosis.     

Embryos and Larvae of a Lancelet,Branchiostoma floridae,from Hatching through Metamorphosis: Growth in the Laboratory and External Morphology

Florida lancelets were raised in laboratory cultures from the egg to the juvenile stage. At frequent intervals during development, elongation of the embryonic and larval body was measured at room temperature (22.5°C) and at the approximate temperature of the natural environment (30°C). Development was slower at the lower temperature, with metamorphosis commencing during the fifth week as compared to the third week at the higher temperature. Scanning electron microscopy (SEM) was used to describe a frequently sampled series of hatched embryos, pre-metamorphic larvae, metamorphic larvae, and juveniles. The advent (and sometimes subsequent disappearance) of the following structures was determined from the SEM data: general epidermal ciliation, peroral pit, mouth, primary gill slits, ciliary tuft, external opening of the club-shaped gland, sense cells, anus, metapleural folds, and preoral cirri. Our SEM did not substantiate the claims of van Wijhe for a transitory larval mouth near the anteriovental end of the larvae. The general epidermal cilation, which is uniformly distributed in the embryos, becomes somewhat reduced in the pre-metamorphic larvae and then disappears almost entirely during metamorphosis. The epidermis includes two distinct sense cell types (I and II) and possibly a third type (the ventral pit cells, to which an adhesive role has alternatively been attributed). The anus first opens on the right-hand side and only later migrates across the mid-ventral line to assume a position on the left-hand side of the larva; this is contrary to the established view that the anus of the larval lancelets opens on the left-hand side and remains there.     

Spatial and temporal variations in cuticle proteins as revealed by monoclonal antibodies. Immunoblotting analysis and ultrastructural immunolocalization in a beetle, Tenebrio molitor

A library of monoclonal antibodies (Mabs) against adult cuticle of Tenebrio was used to visualize the secretion of cuticular antigens during metamorphosis. Immunoblots of water- and urea-soluble proteins, and high resolution immunogold labelling has shown that, except in one clone, the Mabs recognize antigens in the three developmental stages. However, the MW of larval and pupal antigens are different from the adult ones, though sharing common epitopes. Blots of cuticle proteins (CPs) bound to different lectins shown few water-soluble glycosylated proteins weakly or not recognized by the Mabs, suggesting that the majority of the Mabs do not recognize glycosylated epitopes. The immunolocalization of the different antigens suggests a molecular basis for both developmental and regional variations in cuticular architecture and to the modifications due to sclerotization, which differ between pre- and postecdysial cuticles of the three developmental stages.     

Monoclonal Antibodies against Differentiating Mesenchyme Cells in Larvae of the Ascidian Halocynthia roretzi

Mechanisms of cell specification of mesenchyme during ascidian embryogenesis are poorly understood. This is because no good molecular markers have been available to evaluate differentiation of the mesenchyme cells. To obtain molecular markers of mesenchyme differentiation, we established monoclonal antibodies, Mch-1 and Mch-3, that recognize antigens present in the mesenchyme cells of the larva of Halocynthia roretzi. The antigens recognized by both antibodies start to be detectable in the mesenchyme cells at the late tailbud stage. The Mch-3 antibody specifically recognized all mesenchyme cells of the larva, whereas the Mch-1 antibody stained the cells only in the anterior portions of mesenchyme clusters in the trunk region of the larva. The Mch-1 antibody also stained trunk lateral cells. In addition, both antibodies recognized the mesenchyme cells in the ventro-lateral boundary between endoderm and epidermis that are migrating to the anterior head region of the larva. The partial embryos that originated from the mesenchymelineage cells at the 8-cell stage expressed the Mch-1 and Mch-3 antigens. The Mch-1 and Mch-3 antibodies will be useful as immunological probes for studying the specification mechanisms of mesenchyme cells.     

Intracytoplasmic ciliary elements in epidermal cells of Syndesmis echinorum and Paravortex cardii (Platyhelminthes, Dalyellioida).

Epidermal cells of Syndesmis echinorum and Paravortex cardii contain many intracytoplasmic ciliary components: clusters of centrioles disorganized and incomplete short axonemes composed of loosely organized microtubules of irregular lengths, fully formed axonemes though some with fewer than nine doublets, and ciliary rootlets. Furthermore, conspicuous dense granules are found in solitary groups in the cytoplasm. Clusters of dense granules are also closely associated with Golgi complexes and developing axonemal microtubules. Since the dense granules decrease in number as the axonemes increase, it is likely that the granules are involved in the formation of axonemal microtubules. Ciliary elements are especially abundant in epidermal cells of Paravortex cardii embryos, some of them resembling those previously described by several authors in differentiating ciliated cells engaged in centriologenesis and ciliogenesis. Attention has been focused on the relative proportion and position of these elements, as well as the different morphology and several assembling states that they exhibit in epidermal cells of adult S. echinorum and adults and embryos of P. cardii. A functional interpretation of some of the findings is given, which allows us to suggest a sequence of ciliogenetic events that occur in epidermal cells of both species.     

MHC class I antigens as surface markers of adult erythrocytes during the metamorphosis of Xenopus

An alloantiserum produced against Xenopus MHC class I antigens has been used to distinguish different erythrocyte populations at metamorphosis. By analysis using a fluorescence-activated cell sorter (FACS) analyzer, tadpole (stage 55) and adult erythrocytes have distinct volume differences and tadpole cells have no MHC antigens on the cell surface. Both tadpole and adult erythrocytes express a "mature erythrocyte" antigen marker, recognized by its monoclonal antibody (F1F6). During metamorphosis, immature erythrocytes, at various stages of differentiation, which express adult levels of cell-surface MHC antigens by 12 days after tail resorption, are found in the bloodstream. These immature cells are biosynthetically active, produce adult hemoglobin, and mature by 60 days after the completion of metamorphosis. Percoll gradient-density fractionation has shown that all of the cells in the new erythrocyte series express adult levels of MHC antigens but there is only a gradual increase in the amount of "mature erythrocyte" antigen. Tadpole erythrocytes, which are biosynthetically active during larval stages, produce small amounts of surface MHC antigens before the metamorphic climax and then become metabolically inactive. They are completely cleared from the circulation by 60 days after metamorphosis. Erythrocytes from tadpoles arrested in their development for long periods of time express intermediate levels of MHC antigens, suggesting a "leaky" expression of these molecules in the tadpole cells. The most abundant erythrocyte cell-surface proteins from tadpoles and adults, as judged by two-dimensional gel electrophoresis, are very different.