The distribution of cell surface glycoconjugates during mouse secondary neurulation

Summary During secondary neurulation in the mouse, the neural tube develops from the tail bud by caudal extension of the primary neurocoele. The mesenchymal cells of the tail bud become radially arranged around the neurocoele and undergo a mesenchymal to epithelial transformation to form a neuroepithelium. In order to study the expression of glycoconjugates during the morphogenesis of the secondary neural tube, 14 lectins were applied to serial sections of tail buds at various stages of development. In general, binding was fairly homogeneous during the early stages of tail bud development. However, as development progressed, several lectins became localized to specific structures. The changes were observed to parallel the ongoing development of the secondary neuraxis. sWGA, which is N-acetylglucosamine (GlcNAc) specific, bound mainly to the luminal surface of the secondary neurocoele and to a lesser extent, the notochord. WGA, which has both GlcNAc and sialic acid specificities, showed most intense binding at the luminal and abluminal surfaces of the secondary neurocoele. Binding by the lectin PNA was restricted to the extracellular matrix around the developing secondary neural tube. A comparison of the lectin binding patterns in mouse with those previously reported in chick, demonstrates a less elaborate pattern of lectin binding in murine embryos. This may suggest a less complex expression of glycoconjugates in rodents, in keeping with their comparatively simpler mechanism of secondary neurulation.     

Distribution of cell surface glycoconjugates during secondary neurulation in the chick embryo

Lectin histochemistry was used to examine the expression of cell surface glycoconjugates during secondary neurulation in chick embryos. Fourteen lectins were applied to serial sections of the caudal region of embryos at the various stages of tail bud development. The lectins Bandeiraea simplicifolia, Dolichos biflorus agglutinin, Phaseolus vulgaris leukoagglutinin, soybean agglutinin, Sophora japonica agglutinin, Ulex europaeus agglutinin and succinylated wheat germ agglutinin (sWGA) showed very light or no binding to the developing medullary cord of the tail bud. With the other lectins, staining occurred throughout the early tail bud and solid medullary cord. During cavitation, however, differential expression of cell surface glycoconjugates by different cell populations was observed. The lectins concanavalin A, Lens culinaris agglutinin, Pisum sativum agglutinin, Phaseolus vulgaris erythroagglutinin, Ricinus communis agglutinin and WGA showed basic similarities in the distribution of lectin binding. Of these, the binding pattern of WGA was the most striking. As the medullary cord cells were separating into central mesenchymal and peripheral epithelial populations, WGA bound preferentially to the epithelial cells and the notochord. The lectin PNA, however, became preferentially bound to the mesenchymal cells. Heavy staining by WGA (specific for N-acetylglucosamine and sialic acid) where sWGA staining (specific for N-acetylglucosamine only) was faint suggested that WGA binding was due to the presence of sialic acid containing glycoconjugates.     

Studies of neural tube development in the chicken embryo tail

Summary The tail regions of chick embryos between stages 21 to 46 were studied by light microscopy using paraffinand epoxyembedded serial sections. The embryonic tail attains its maximum length at about stage 22. The present study examined the morphogenesis of the caudal neural tube during the reduction and remodelling processes of the embryonic tail. Between stages 21 and 28, the embryonic tail became markedly shorter, and the neural tube, with a single central canal, merged caudally with the short medullary cord and tail bud. Between stages 29 and 31, the neural tube elongated and curved rostrally, while the caudal end of the notochord and the tail bud disappeared. Between stages 32 and 39, after showing various structural changes such as dilatation or rupture and abnormal elongation of its marginal zone the neural tube became shorter. By stage 40, development of the caudal neural tube was essentially complete and the neural tube was shorter than the notochord. The neural tube opened dorsally, as in the adult chicken. The caudal opening may be newly-formed as the open portion was found to contain numerous macrophages.     

Morphological features of tail bud development in truncate mouse mutants

A key malformation in the homozygous truncate mouse mutants is a partial lack of the notochord in the embryo tail. In order to analyze if tail bud development was affected by the truncate (tc) mutation, serial semithin sections of tails of the homozygous mutant embryos were compared to the wild-type controls. In the wild-type embryos morphologically uniform mesenchyme of the tail bud was continuous via the medullary cord to the secondary neural tube, and via the tail cord to the notochord and the gut. In truncate embryos the tail cord was not continuous to the notochord, but to the additional lumen of the tail gut resulting in tail gut duplication. Toward the base of the tail two tail guts subsequently fused together or the additional one disappeared. If present in the tip of the tail, the notochord in truncate embryos ended near the ventral border of the secondary neural tube. The rest of the tail notochord was fragmented and the posterior ends of the fragments were frequently adjacent or even continuous to the neural tube. We suggest that the improper regionalization of the tail bud, where notochord is associated with the neural tube rather than with the tail gut, is related to the subsequent segmental lack of the notochord in truncate mutants.     

Origin of the avian glycogen body: I. Effects of tail bud removal in the chick embryo.

The tail bud was removed from chick embryos at stage 16-17 in a first study directed to learn of the origin of the glycogen body in the lumbosacral spinal cord of birds. Results of tail bud removal and chorio-allantoic grafting of caudal portions of the embryo containing the tail bud or the neural tube suggest that the glycogen body does not arise from the tail bud, but from the preexisting neural tube craniad or anterior to the tail bud. The stem cells of the glycogen body are most likely among those components of the anterior portion of the lumbosacral neural tube derived from primary neurulation.     

Differentiation of the chick embryo floor plate

Summary In a number of species, the floor plate of the developing neural tube and spinal cord has been ascribed specialized functions associated with the patterning of neuronal differentiation. The differentiation of the floor plate itself is believed to be closely related to the presence of the underlying notochord. Grafting experiments have previously shown that in the chick embryo an implanted segment of notochord is capable of inducing the adjacent host neural plate or neural tube to produce an additional floor plate, although the inductive effect diminishes with increasing age of the host. We have examined the potential of notochord to promote the appearance of floor plate-like structures from neural tube tissue in culture. To facilitate this, it was necessary initially to examine the immunoreactivity of the early neural tube and floor plate in situ and in vitro with a panel of antibodies to identify a suitable marker for floor plate differentiation in vitro. In situ, the differentiation of the floor plate was characterized by a lack of immunoperoxidase staining with antibody to neurofilaments and the monoclonal antibody HNK-1 throughout the period examined. This distinguished the floor plate from other regions of the neural tube, and was in contrast to its conspicuous affinity for antibodies to N-CAM and highly sialylated N-CAM, which also stained several closely adjacent regions of the neural tube over the period examined. We also found that oligodendrocytes occurred both in the floor plate and in the flanking ventral neural tube, and that astrocytes were too poorly represented throughout the neural tube at the stages examined to be useful markers of floor plate differentiation. We therefore concluded that only the anti-neurofilament and the HNK-1 antibodies were potentially useful markers for floor plate differentiation. When these antibodies were tested on cells in culture, neural tube tissue showed the presence of neurofilament and HNK-1-positive neurites, while floor plate cultures showed few of these. These distributions were consistent with those demonstrated in situ. However, cells staining positively for N-CAM, sialylated N-CAM and the glial cell markers were relatively sparse in floor plate cultures, suggesting that these epitopes were not retained or were masked in cultured cells. As a result of these experiments, we selected the absence of neurofilament-positive cells as a marker for floor plate differentiation in culture. Co-culture of pieces of neural tube from 3-day embryos with notochord segments resulted in the suppression of neurofilament-positive neurite outgrowth from the former, and the consequent production of tissue with floor plate-like characteristics. The absence of neurites was most marked on the side of the neural tube tissue that was apposed to the notochord. Co-culture of neural tube with other tissues did not produce this effect. We suggest that the neurite suppression by notochord in vitro is analogous to its activity in situ, and that neural tubes from 3-day embryos are still competent to respond to notochordal tissue.     

Morphological diversity of dying cells during regression of the human tail

During normal human development a number of transient structures form and subsequently regress completely. One of the most prominent structures that regress during development is the human tail. We report here a histological and ultrastructural study of cell death in the cranial and caudal (tail) parts of the neural tube in 4 to 6-week-old human embryos. Initially, the human tail is composed of tail bud mesenchyme which differentiates into caudal somites, secondary neural tube, notochord and tail gut. Later on, these structures gradually regress by cell death. During the investigated period, we observed two morphologically distinct types of dying cells. The well-described apoptotic type of cell death was observed only in the cranial neural tube that forms during primary neurulation. The other type of cell death characterized by necrotic morphology was observed in the tail mesenchyme and in the caudal neural tube that forms during secondary neurulation. This morphological diversity suggests that besides differences in origin and fate there are different mechanisms of developmental cell death between two parts of the human neural tube. We can speculate that the apoptotic type of cell death is associated with the precise control of cell numbers and that the other morphologically distinct type of cell death is responsible for the massive removal of transitory structures.     

Glycoconjugate distribution in early human notochord and axial mesenchyme

Glycosylation patterns of cells and tissues give insights into spatially and temporally regulated developmental processes and can be detected histochemically using plant lectins with specific affinities for sugar moieties. The early development of the vertebral column in man is a process which has never been investigated by lectin histochemistry. Therefore, we studied binding of several lectins (AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA) in formaldehyde-fixed sections of the axial mesenchyme of 5 human embryos in Carnegie stages 12-15. During these developmental stages, an unsegmented mesenchyme covers the notochord. Staining patterns did not show striking temporal variations except for SBA which stained the cranial axial mesenchyme only in the early stage 12 embryo and for PNA, of which the staining intensity in the mesenchyme decreased with age. The notochord appeared as a highly glycosylated tissue. Carbohydrates detected may correspond to adhesion molecules or to secreted substances like proteoglycans or proteins which could play an inductive role, for example, for the neural tube. The axial perinotochordal unsegmented mesenchyme showed strong PNA binding. Therefore, its function as a PNA-positive "barrier" tissue is discussed. The endoderm of the primitive gut showed a lectin-binding pattern that was similar to that of the notochord, which may correlate with interactions between these tissues during earlier developmental stages.     

Effects of complete tail bud extirpation on early development of the posterior region of the chick embryo

The tail bud was completely extirpated down to the yolk from 65 embryos at stages 13-17 to determine whether the posterior part of the noto-chord originates from the tail bud or from a more anterior region (i.e., prospective notochordal region). About 40% of the 44 surviving embryos developed near-normal tails, con?taining a localized defective region beginning near the base of the tail and extending a short distance posteriorly, about 15% developed truncated, cone-shaped tails, containing a defective region beginning near the base of the tail and extending towards the tip, and about 45% developed short, ventral tail remnants, containing a localized defective region beginning near the base of the tail and extending a short distance posteriorly. The tail was absent in only one embryo. These differences were probably due primarily to variation in the amount of healing and regeneration that occurred, and were independent of the stage at which the operation took place. The tail region has a tremendous capacity for regeneration since a near-normal tail frequently developed. The location of the beginning of the defective region near the base of the tail suggests that the tail bud primarily gives rise to tail structures. All embryos had neural tube defects, about 30% developed large, midline somites within the defective region, and about 25% developed an ourenteric outgrowth. The notochord was always normal within the defective region. These results are consistent with the view that the tail bud contributes cells to the posterior part of the neural tube, but not to the notochord.     

Catecholamines and morphogenesis of the chick neural tube and notochord

Catecholamines in the early chick embryo (days 2–3) were studied using the Falck-Hillarp technique. After treating specimens with nor-epinephrine, specific catecholamine fluorescence was localized in the early neural tube of the youngest embryos (stages 7–9). In older embryos (stages 10–12) exposed to exogenous norepinephrine the fluorescence was seen in both the neural tube and notochord and by stage 13 fluorescence was brighter in the notochord than neural tube. Endogenous notochord catecholamine fluorescence was demonstrable in stages 13 through 17 without exposure to exogenous amines although treatment with a monoamine oxidase inhibitor (nialamide) or norepinephrine intensified the fluorescence. Treatment of embryos with nialamide or an inhibitor of catecholamine synthesis (α-Methyltyrosine) caused numerous archencephalic anomalies, ventral flexure of the head without lateral rotation, and spina bifida. The results suggest that catecholamines in the early neural tube and notochord may provide the motive force for morphogenetic movements, including closure of the neural folds and torsion of the embryo.     

Histological and ultrastructural observations of tail bud formation in the chick embryo

Tail bud formation was studied in chick embryos by light and electron microscopy. The caudal part of the neural groove at stage 11 is flanked by widely separated neural folds and merges posteriorly with the shallow primitive groove. The neural groove and primitive streak partially overlap. The depth of the neural groove gradually decreases antero-posteriorly within this overlap zone while the dorso-ventral thickness of the streak progressively increases. The anterior end of the streak begins to form a spherical accumulation of mesenchymal cells, the incipient tail bud, concomitant with closure of the posterior neuropore. Formation of the posterior body fold results in consolidation of the remainder of the streak into the definitive tail bud. The overlap zone between neural groove and primitive streak is retained as the tail bud forms. Thus the posterior end of the neural tube and anterior end of the tail bud overlap. The latter undergoes cavitation to form the ventral part of the spinal cord within this overlap region. The tail bud is initially continuous with an overlying, flattened layer of ectoderm and an underlying, columnar layer of endoderm. A bilaminar ectodermal epithelium forms directly above the developing neural tube as the dorsal portion of the tail bud undergoes cavitation. Most of the endodermal cells are displaced from the ventral surface of the tail bud by the posterior body fold and condensed into a disk-shaped region which ultimately gives rise to tail gut.     

Early stages of development in the caudal neural tube of the golden Syrian hamster (Mesocricetus auratus)

Secondary neurulation is the morphogenetic process whereby the caudal segments of the neural tube are derived from cells in the embryonic tail bud. Comparative studies have demonstrated similar characteristics in the mechanism of secondary neurulation among tailless species, which are thought to be due to the evolutionary reduction in tail length (Hughes and Freeman, 1974). In order to explore this hypothesis further, light and scanning electron microscopy was used to study early stages of neurulation in the tail buds of hamster embryos. The golden Syrian hamster is a relatively common laboratory rodent with a reduced tail. In this species, secondary neurulation first became apparent in embryos with approximately 17 pairs of somites. This was well before closure of the posterior neuropore which occurred at the 21-somite stage. The lumen of the neural tube appeared to extend into the tail bud in an even and progressive fashion accompanied by reorientation and rearrangement of tail-bud cells. The mechanism appeared to be similar to that reported in long-tailed rodents.     

Age-related changes induced by tunicamycin in wheat germ agglutinin binding to chick embryo fibroblasts

The specific roles of N-acetylglucosaminyl and sialyl residues were investigated in the binding of wheat germ agglutinin (WGA) to fibroblasts from 8- and 16-day chick embryos. Cells from the 8-day embryos exhibited two classes of WGA binding site, whereas fibroblasts from 16-day embryos only displayed one. Neuraminidase treatment of fibroblasts from 8-day embryos raised the number of WGA binding sites with a high affinity constant and reduced the number of sites with a low affinity constant. In 16-day cells, neuraminidase treatment reduced the number of WGA binding sites. The data presented here suggest that WGA binds to cell-surface glycoproteins containing sialic acid residues. This tendency was more marked in 16-day than in 8-day cells and corresponded to the finding that neuraminidase released more sialic acid from the surface of 16-day cells than of 8-day cells. Glycosylation of cell-surface N-linked glycoproteins did not seem essential in determining the WGA binding capacity, as shown by the effect of tunicamycin on fibroblasts from both 8- and 16-day embryos. In the absence of N-glycosylated binding sites, WGA bound to O-glycosylated structures and the older tunicamycin-treated embryo cells exhibited a larger number of WGA binding sites than the younger tunicamycin-treated cells, in relation to the increase of the amount of O-glycosylated structures during embryo development.     

Somitomeres in the chick tail bud: an SEM study

Summary In the chick embryo the final number of somites is achieved at about stage 22 of Hamburger and Hamilton. By this time the neural tube and notochord have reached the tip of the tail bud but some paraxial mesoderm remains unsegmented. In this study using scanning electron microscopy we show that somitomeres are present in this mesoderm. This indicates that the terminal paraxial mesoderm of the tail bud may have the potential to form supplementary somites, though we do not as yet know what prohibits the completion of segmentation to the tip.     

Monoamines in the early chick embryo: Demonstration of serotonin synthesis and the regional distribution of serotonin-concentrating cells during morphogenesis

Monoamine compounds, such as serotonin (5-HT), have been previously suggested to regulate the early development of the chick embryo. Possible roles for 5-HT in chick morphogenesis were investigated further by examining the distribution of sites that concentrate 5-HT as well as the ability of embryos to synthesize 5-HT during this period. Following the in vitro incubation of chick embryos with 5-HT, cells accumulating 5-HT were localized by the formaldehyde-induced fluorescence (FIF) technique; these sites of 5-HT concentration were mapped and studied with respect to changes in their developmental patterns during morphogenesis. Locations of 5-HT FIF included two regions of the brain within the floor plate of the mesencephalon and caudal myelencephalon, and within scleratome cells of somites advanced in differentiation. Additionally, two separate zones of 5-HT FIF appeared in the neural tube caudal to the somites in conjunction with a small band of intense FIF in the caudal end of the notochord. Once established, the capabilities for 5-HT concentration in the brain and somites persisted at the same locations during development, whereas these capabilities in caudal portions of the embryo were transient and appeared to move down the embryo as the embryo lengthened. These changes in the patterns of 5-HT FIF in caudal segments of the neural tube and notochord correlated spatio-temporally with the progression of caudal neuropore closure. Additional experiments, involving the use of anti-5-HT immunocytochemistry, were undertaken to examine the characteristics of 5-HT concentration. The mechanism(s) of 5-HT concentration at all locations appeared to have a high affinity and specificity for 5-HT; however, in contrast to the transient mechanism situated more caudally in the embryos, the uptake mechanism at rostral locations required energy and was blocked by a 5-HT uptake inhibitor (fluoxetine). The ability of embryos to synthesize 5-HT was demonstrated by the generation of 5-HT (localized by FIF and immunocytochemistry) in embryos treated with the 5-HT precursors, 5-hydroxytryptophan or L-tryptophan. Confirming evidence of 5-HT synthesis was provided by a marked reduction in immunostaining obtained with either precursor in the presence of 5-HT synthesis inhibitors (R04–4602 or para-chlorophenylalanine). Although the sites of synthesis remain uncertain, endogenous 5-HT was detected (after monoamine oxidase inhibition) at all sites of 5-HT concentration. These results provide further support for the suggestion that 5-HT may be involved in various aspects of morphogenesis at specific sites within the chick embryo.     

Analysis of the embryonic phenotype of Bent tail, a mouse model for X-linked neural tube defects

Neural tube defects, mostly believed to result from closure defects of the neural tube during embryonic development, are frequently observed congenital malformations in humans. Since the etiology of these defects is not well understood yet, many animal models for neural tube defects, either arising from spontaneous mutations or generated by gene targeting, are being studied. The Bent tail mouse is a model for X-linked neural tube defects. This mutant has a characteristic short and kinked tail. Exencephaly occurs in Bent tail embryos with a frequency of 11–16%. Laterality defects also belong to the phenotypic spectrum. In this study, we analyzed the embryonic phenotype in further detail using scanning electron microscopy during the stages of neurulation. We observed a number of defects in both wild type and Bent tail embryos, including a kinked neural tube, tight amnion, delay in axial rotation and even malrotation. The severity or frequency of most defects, the delay in axial rotation excluded, was significantly higher in Bent tail embryos compared to wild type embryos. Other abnormalities were seen in Bent tail embryos only. These defects were related to anterior and posterior neural tube closure and resulted in exencephaly and a closure delay of the posterior neuropore, respectively. The exencephalic phenotype was further analyzed by light microscopy in ED14 embryos, showing disorganization and overgrowth in the mesencephalon and rhombencephalon. In conclusion, the anterior and posterior neural tube closure defects in the Bent tail are strictly linked to the genetic defect in this mouse. Other phenotypic features described in this study also occur in the wild type genetic background of the Bent tail strain. Apparently, the genetic background contains elements conducive to these developmental abnormalities.     

Binding pattern of ferritin-labeled lectins (RCAI and WGA) during neural tube closure in the bantam embryo

Summary Cell surface sugar residues in neurulating ectoderms of bantam chick embryos of stage 4–11 were examined using ferritin-labeledRicinus communis (RCA_I) and Wheat germ agglutinin (WGA). RCA_I binding sites densely covered the apical surfaces of the basal plate cells during the neural plate stage (1,322.8±28.8 ferritin particles/m²). As neural tube formation advanced, the number of receptors decreased as a result of an increase in the extent of the sparsely covered regions. The decrease in receptors for WGA occurred in a similar manner but more rapidly. By the stage of development at which the opposite sides of the neural ridges meet at the dorsal midline, the receptors for WGA were reduced to about half. After this period, the two lectin receptors did not show significant changes. This result suggests that sugar residues or the sugar-chain skeleton on basal plate cells are altered during neurulation.     

Changes in glycoconjugate expression during early chick embryo development: A lectin-binding study

A selection of lectins was used to investigate developmentally regulated changes in the distribution of cell surface oligosaccharides during the gastrulation and neurulation stages of early chick embryo development. Lectins from three specificity classes were used: glucose/mannose specificity (concanavalin A [Con A], Lens culinaris agglutinin [LCA], Pisum sativum agglutinin [PSA]); N-acetylglucosamine specificity (Lycopersicon esculentum agglutinin [LEA], wheat germ agglutinin [WGA], succinylated WGA [sWGA]); N-acetylgalactosamine/galactose specificity (Dolichos biflorus agglutinin [DBA], soybean agglutinin [SBA], Sophora japonica agglutinin [SJA], Bandeiraea (Griffonia) simplicifolia lectin I [BSL I], peanut agglutinin [PNA], Artocarpus integrifolia lectin [Jacalin], Ricinus communis agglutinin-1 [RCA-1], Erythrina cristagalli lectin [ECL]). At gastrulation stages, patterns of lectin binding could be distinguished in the epiblast, mesoderm, and endoderm cell layers. The primitive streak failed to bind any of the lectins, but LEA and WGA bound to the epiblast in regions lateral to the streak, indicating the loss of some glucosamine residues medially in preparation for the ingression movements of gastrulation. Several lectins showed marked binding to the mesoderm cells after their passage through the primitive streak; these were LCA, PSA, WGA, sWGA, BSL, and most particularly PNA. Therefore, the epithelial-mesenchymal transformation from epiblast to mesoderm at the primitive streak is accompanied by cell surface oligosaccharide changes in the epiblast and mesoderm that involve all classes of lectins including the PNA-binding sequence Galβ1-3GalNAc. Ultrastructurally, PNA was shown to bind extracellularly to matrix fibrils. Jacalin, having the same sugar specificity as PNA, but binding to serine/threonine linked chains rather than asparagine linked chains showed no binding to the mesoderm. The endoderm layer most clearly bound WGA and BSL. At neurulation stages, medio-lateral domains in the ectoderm could again be demonstrated. Neural plate cells bound only PNA, although the hinge point of the neural plate, the future floor plate, failed to bind PNA unless pre-treated with neuraminidase to remove sialic acid residues. Caudally, where the primitive streak persisted, all mesoderm cells reacted very strongly with PNA, but rostrally this binding became more restricted to the mesodermal regions immediately adjacent to the streak. This mesodermal PNA-binding was abolished by hyaluronidase pre-treatment, suggesting extracellular matrix association, whereas the neural plate binding was unaffected by this treatment, suggesting a more intimate developmentally regulated association with the cell surface of early neural cells. Neuraminidase treatment and sWGA-binding indicated patterns of sialylation on cells of several tissues at the later stages of development. These sialic acid residues had the effect of masking both PNA and ECL reactivity. The latter, specific for sequences of the poly-N-lactosamine series, (Galβ1-4GlcNAc)_n, bound to mesoderm only after removal of sialic acids. Basement membranes bound lectins of glucose/mannose and galactose specificities at both stages, and RCA-binding was localized ultrastructurally to the fibronectin-rich interstitial bodies of the lamina densa.     

Polysialic acid, a unique glycan that is developmentally regulated by two polysialyltransferases, PST and STX, in the central nervous system: From biosynthesis to function

Polysialic acid is a developmentally regulated carbohydrate composed of a linear homopolymer of a-2,a-linked sialic acid residues. This unique glycan is mainly attached to the neural cell adhesion molecule (N-CAM) and implicated in many morphogenic events of the neural cells by modulating the adhesive property of N-CAM. Recently, the cDNA that encodes polysialyltransferase, which is responsible for the polysialylation of N-CAM, was successfully cloned from three mammalian species. This review focuses on the molecular cloning of human polysialyltransferase, designated PST. it then describes the number of enzymes actually required for the polysialylation of N-CAM using an in vitro polysialyltransferase assay. Comparisons between PST and another polysialyltransferase, sialyltransferase X (STX), are made and it Is demonstrated that both enzymes can independently form polysiatic acid In vitro , but that during neural development they coordinately but distinctly synthesize polysialic acid on N-CAM. The role of polysialic acid in the central nervous system is also discussed. Finally, evidence that the two polysialyltransferases, PST and STX, apparently have distinct roles in the development of neural cells is provided by using a neurite outgrowth assay.     

Lectin-binding patterns in the embryonic human paraxial mesenchyme

The paraxial mesenchyme in seven human embryos aged between Carnegie stages 12 and 17 was studied by lectin histochemistry with the lectins AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA. The paraxial mesenchyme was found to be segmented into sclerotomes by intersegmental vessels and from late stage 12 by intrasclerotomal clefts dividing each sclerotome into a cranial and caudal half. The lectins Con A, GSA II, LFA, LTA, SBA and SNA did not react at all in the paraxial mesenchyme. Staining for AIA, PNA, RCA I and WGA was found in the developing sclerotomes. However, no differences in the staining pattern between the two sclerotomal halves could be seen. It was striking that in contrast to the chick embryo no differences in binding for PNA between the cranial and caudal sclerotomal parts was observed. These findings reveal that PNA-binding sites do not play the same functional role in segmented axonal outgrowth and neural crest immigration into cranial sclerotomal halves in the human embryo, as found in chick embryonic development. Beginning with the stage 16-embryo, the already condensed caudal sclerotomal halves express Con A-, RCA- and PNA-binding sites. The staining for PNA in particular marked the differentiation of chondrogenous structures developing in this half. From the late stage 12 or stage 13, the walls of intersegmental and other vessels showed binding sites for AIA, PNA, RCA I, SNA and WGA.