Regulation of paraxis expression and somite formation by ectoderm- and neural tube-derived signals

During vertebrate embryogenesis, the paraxial mesoderm becomes segmented into somites, which form as paired epithelial spheres with a periodicity that reflects the segmental organization of the embryo. As a somite matures, the ventral region gives rise to a mesenchymal cell population, the sclerotome, that forms the axial skeleton. The dorsal region of the somite remains epithelial and is called dermomyotome. The dermomyotome gives rise to the trunk and limb muscle and to the dermis of the back. Epaxial and hypaxial muscle precursors can be attributed to distinct somitic compartments which are laid down prior to overt somite differentiation. Inductive signals from the neural tube, notochord, and overlying ectoderm have been shown to be required for patterning of the somites into these different compartments. Paraxis is a basic helix-loop-helix transcription factor expressed in the unsegmented paraxial mesoderm and throughout epithelial somites before becoming restricted to epithelial cells of the dermomyotome. To determine whether paraxis might be a target for inductive signals that influence somite patterning, we examined the influence of axial structures and surface ectoderm on paraxis expression by performing microsurgical operations on chick embryos. These studies revealed two distinct phases of paraxis expression, an early phase in the paraxial mesoderm that is dependent on signals from the ectoderm and independent of the neural tube, and a later phase that is supported by redundant signals from the ectoderm and neural tube. Under experimental conditions in which paraxis failed to be expressed, cells from the paraxial mesoderm failed to epithelialize and somites were not formed. We also performed an RT-PCR analysis of combined tissue explants in vitro and confirmed that surface ectoderm is sufficient to induce paraxis expression in segmental plate mesoderm. These results demonstrate that somite formation requires signals from adjacent cell types and that the paraxis gene is a target for the signal transduction pathways that regulate somitogenesis.

Expression of the avian VEGF receptor homologues Quek1 and Quek2 in blood-vascular and lymphatic endothelial and non-endothelial cells during quail embryonic development

We have studied the expression of Quek1 and Quek2 (VEGFR-2 and VEGFR-3, respectively) in quail embryos from day 2 to day 16 by in situ hybridization with digoxigenin-labelled riboprobes on whole-mounts and paraffin sections. Parallel sections were also stained with the QH1 antibody to detect all endothelial cells and with an antibody against alpha-smooth-muscle-actin to reveal the media of blood vessels. Quek1/VEGFR-2 is a marker of blood-vascular and lymphatic endothelial cells throughout development. In 2-day-old embryos, it is expressed in the intra-embryonic vascular plexus, in cells (most probably angioblasts) located in the paraxial head mesoderm and in the somites, and caudo-laterally from Hensen's node. Thereafter, until about day 9, Quek1 is expressed in all endothelial cells. Cells positive and negative for Quek1 can later be found within the same vessel. Quek1 is additionally expressed in lymphatic endothelial cells. Occasionally, some non-endothelial cell types express Quek1. Quek2/VEGFR-3 is also a marker of endothelial cells; however, its expression pattern differs from that of Quek1. In 2-day-old embryos, Quek2 is expressed in the notochord and the intra-embryonic vascular plexus. Whereas all endothelial cells are Quek2-positive in 3-day-old embryos, expression is subsequently reduced to a subset of endothelial cells: arteries become Quek2-negative and then expression of Quek2 is limited to a few vessels that appear to be lymphatic. Endothelial cells of lymph nodes and the periaortal lymphatic vessels are Quek2-positive in later stages. A few non-endothelial cells express Quek2.

The fate of the first avian somite

We have studied the derivatives of the first somite using the quail-chick marking technique. After transplantation of the somite, the chick embryos were reincubated for periods ranging from 4 h to 11 days. Coronal and sagittal sections of the embryos were prepared for parallel staining with Feulgen-reaction, anti-quail antibody, anti-desmin antibody and QH-1 antibody. The first somite consists of an epithelial envelope surrounding somitocoele cells. Like other somites, it forms sclerotome, dermatome and myotome. Cells contribute to the occipital and parasphenoid bone, the meninges, the dermis in the occipital region and the pharyngeal connective tissue. The contribution of the first somite to bones, meninges, dermis and pharyngeal connective tissue is characterised by sharp anterior and posterior boundaries. In contrast, other derivatives such as connective tissue surrounding the vagus nerve, the carotid artery, and jugular vein exceed 10 to 18 segments. This is also true for myogenic cells participating in the formation of the cucullaris capitis muscle that extends from the temporal bone to the shoulder. In one third of the embryos, myocytes of the intrinsic laryngeal muscles are derived from the grafted first somite. Moreover, endothelial cells originate from this somite and migrate into the head (hind-brain, meninges, dermis), neck (pharynx, connective tissue surrounding the vagus nerve, carotid artery and jugular vein) and thorax. With respect to differentiation and derivatives the first somite is similar to other somites.

Location and growth of epaxial myotome precursor cells

The skeletal muscle progenitor cells of the vertebrate body originate in the dermomyotome epithelium of the embryonic somites. To precisely locate myotome precursor cells, fluorescent vital dyes were iontophoretically injected at specific sites in the dermomyotome in ovo and the fates of dye-labeled cells monitored by confocal microscopy. Dye-labeled myotome myofibers were generated from cells injected along the entire medial boundary and the medial portion of the cranial boundary of the dermomyotome, regions in close proximity to the dorsal region of the neural tube where myogenic-inducing factors are thought to be produced. Other injected regions of the dermomyotome did not give rise to myotome fibers. Analysis of nascent myotome fibers showed that they elongate along the embryonic axis in cranial and caudal directions, or in both directions simultaneously, until they reach the margins of the dermomyotome. Finally, deposition of myotome fibers and expansion of the dermomyotome epithelium occurs in a lateral-to-medial direction. This new model for early myotome formation has implications for myogenic specification and for growth of the epaxial domain during early embryonic development.

Insertion of musculus tibialis posterior into musculus peroneus (fibularis) longus

We report on a muscular variation in the human foot. The tendon of the tibialis posterior muscle was attached to the peroneus longus tendon by means of a trapezoid tendinous connection. This variation was observed only in the left foot.

Identification and characterization of differentiation-dependent Schwann cell surface antigens by novel monoclonal antibodies: introduction of a marker common to the non-myelin-forming phenotype

In an attempt to identify and characterize novel Schwann cell surface molecules with putative functions during development, maintenance, and regeneration of the peripheral nervous system (PNS), we have produced monoclonal antibodies against viable neonatal rat Schwann cells. Using a sensitive live cell ELISA protocol, three monoclonal antibodies reactive with cultured Schwann cells, designated 27B10, 26F2, and 27C7 were isolated. The 27B10 and 26F2 antibodies specifically labelled forskolin-stimulated secondary Schwann cells in vitro as determined by live cell ELISA implying that the expression of the antigens in situ is regulated by axonal contact. The observation that the antigens seemed to be associated with both Schwann cell phenotypes clearly discriminated them from the well characterized myelin proteins as well as from molecules known to be confined to the non-myelin-forming phenotype. Interestingly, both antigens were found to be concentrated at the nodes of Ranvier. Further studies therefore have to show whether the identified antigens share structural or functional homology with adhesion or channel molecules, which display a similar distribution. Following transection of the adult sciatic nerve, the 26F2 antigen was rapidly down-regulated in the distal nerve stump. The 27C7 antibody reacted with an 80 kDa cell surface molecule common to non-myelin-forming Schwann cells. No differences in expression of the antigen between forskolin-treated and untreated Schwann cells in vitro were found, suggesting that the antigen is expressed independently from axonal contact. Two weeks after nerve transection in the absence of myelinating Schwann cells, the antigen was associated with S-100-positive Schwann cells of the distal nerve stump. The antigen was found to be expressed also by non-neuronal tissues, the level of the protein declined towards the adult stage. Comparison of the 27C7 antigen with previously described marker molecules suggests that we have identified a novel Schwann cell surface antigen of the non-myelin-forming phenotype.

Mesoderm-derived cells proliferate in the embryonic central nervous system: confocal microscopy and three-dimensional visualization

In the chick and quail embryo, two cell populations migrate into the neural tube from the surrounding mesodermal tissues during the fourth day of incubation: individual cells which represent macrophages, and endothelial cells which remain continuous with the extraneural vessels. We report here on the proliferative capacity of these mesoderm-derived cells. A double-immunofluorescence protocol for two monoclonal antibodies of subtype IgG1, the endothelial cell/macrophage marker QH1, and the S-phase marker bromodeoxyuridine, was developed. With confocal laser scanning microscopy of thick microtome sections, labeling indices of intraneural individual QH1-positive cells (12%) and of endothelial cells (10%) were determined. In contrast, the labeling index of extraneural endothelial cells was 25%. With three-dimensional visualization of confocal data, the variable morphology of macrophages was shown. Our results indicate that: (1) proliferative activity of intraneural capillary endothelial cells is less than expected and that it is absent from sprouts; (2) both spheroidal and ramified macrophages proliferate inside the neural tissues; and (3) ramified macrophages frequently make contact with capillary endothelial cells. We conclude that most embryonic microglia may be derived from the early invasive QH1+ macrophages.

On the bifurcation of blood vessels--Wilhelm Roux's doctoral thesis (Jena 1878)--a seminal work for biophysical modelling in developmental biology

Wilhelm Roux's doctoral thesis described the relationship between the angle and diameter of bifurcating blood vessels. We have re-read this work in the light of biophysics and developmental biology and found two remarkable aspects hidden among a multitude of observations, rules and exceptions to these rules. First, the author identified the major determinants involved in vascular development; genetics, cybernetics, and mechanics; moreover, he knew that he could not deal with the genetic and regulatory aspects, and could hardly treat the mechanical part adequately. Second, he was deeply convinced that the laws of physics determine the design of organisms, and that a necessity for optimality was inherent in development. We combined the analysis of diameter relationships with the requirement for optimality in a stochastic biophysical model, and concluded that a constant wall-stress condition could define a minimum wall-tissue optimum during arterial development. Hence, almost 120 years after Wilhelm Roux's pioneering work, our model indicates one possible way in which physical laws have determined the evolution of regulatory and structural properties in vessel wall development.

Diastematomyelia and spina bifida can be caused by the intraspinal grafting of somites in early avian embryos

OBJECTIVE

In this experimental study, an embryological model was created to reproduce diastematomyelia and spina bifida and to investigate new aspects of the origin of spinal cord malformations.

METHODS

A somite was implanted from a donor quail embryo into the neural tube of a 2-day-old chick embryo. The somite was chosen because the septum that characteristically separates the two hemicords consists exclusively of mesodermal derivatives.

RESULTS

After 2 days of reincubation, diastematomyelia, spina bifida, or a normal embryo without a graft was observed. If the graft persisted in the neural tube, it formed a septum between the floor and roof plates but never made contact with the lateral walls of the tube. Otherwise, the graft was extruded from the neural tube. In this case, the quail cells often were found in dorsal or dorsolateral positions in the surrounding tissue. Sometimes, the wall of the neural tube formed an extrusion in the direction of the eliminated graft. On many occasions, however, spina bifida aperta was produced and no quail cells could be found in the host.

CONCLUSION

The results suggest that diastematomyelia may be the result of abnormal mesodermal invasion of the neural tube. The development of a septum in the neural tube after implantation of a somite may mimic the process during spontaneous diastematomyelia formation, which could be the consequence of abnormal gastrulation, the process by which the two early germ layers of the blastodisc are converted into the three definitive germ layers.

Impairment by interleukin 1 beta and tumour necrosis factor alpha of the glucagon-induced increase in phosphoenolpyruvate carboxykinase gene expression and gluconeogenesis in cultured rat hepatocytes

The influence of the inflammatory mediators interleukin 1 beta (IL1 beta) and tumour necrosis factor alpha (TNF alpha) on the glucagon-induced expression of phosphoenolpyruvate carboxykinase (PCK) and on glucose formation via gluconeogenesis was investigated in cultured rat hepatocytes. Gene expression was monitored by determination of mRNA levels and of enzyme activity. Glucose formation was estimated with newly synthesized radioactive glucose derived from a radiolabelled lactate precursor. Glucagon (0.1 or 1 nM) induced PCK mRNA transiently to a maximum 2 h after its application. In the presence of recombinant human (rh) IL1 beta or rhTNF alpha the increase in PCK mRNA levels was totally inhibited at 0.1 nM glucagon, whereas at 1 nM glucagon the maximal increase was inhibited by only 25%. Glucagon (0.1 or 1 nM) induced PCK activity to a maximum after 4 h (4-fold and 6-fold over prestimulatory activity respectively). In the presence of rhIL1 beta or rhTNF alpha the maximal increase was inhibited by approx. 50%. Addition of rhIL1 beta or rhTNF alpha 2 h after glucagon, at the maximal glucagon-induced PCK mRNA levels, accelerated the decay of PCK mRNA. Glucagon (1 or 10 nM) [corrected] increased glucose formation from lactate by 1.3-fold and 1.7-fold respectively over unstimulated rates. In the presence of rhIL1 beta or rhTNF alpha this increase in glucose formation was inhibited by 60-90%. At 0.1 nM, glucagon doubled the intracellular cAMP concentration. This increase was prevented by rhIL1 beta or rhTNF alpha. At 1 nM, glucagon increased cAMP concentrations by 10-fold. In the presence of rhIL1 beta or rhTNF alpha this increase was inhibited by 70%. From the results it is suggested that rhIL1 beta and rhTNF alpha prevented glucagon-stimulated PCK gene expression and gluconeogenesis at least in part by inhibition of the glucagon-stimulated increase in cAMP concentrations.

Scatter factor/hepatocyte growth factor (SF/HGF) induces emigration of myogenic cells at interlimb level in vivo

The initiating event in the migration of myogenic cells to the limb buds is an epitheliomesenchymal transformation of cells located at the lateral edge of the dermomyotome. Recently, a targeted mutation of c-met in mice demonstrated an essential role of this tyrosine kinase receptor and its ligand, scatter factor/hepatocyte growth factor (SF/HGF), in the migration of myogenic cells to the limb buds. Here, we show that ectopic application of exogenous SF/HGF induces emigration of Pax-3-positive myogenic cells into the lateral plate mesoderm. During this process, the lateral portions of the dermomyotomes deepithelialize and the basement membrane disintegrates. Detaching myogenic cells do not lose N-cadherin from their surfaces. We conclude that an HGF/SF- and c-met-mediated signal detaches myogenic precursor cells from the somites and thus plays a necessary role in the initiation of myoblast migration.

Participation of individual brachial somites in skeletal muscles of the avian distal wing

In this paper we investigate the somitic origin of the individual muscles of the forearm and hand using quail-chick chimeras. Our results show that only somites 16-21 give rise to wing muscle, but they take part in muscle formation to different extents. Somite 21 does not always participate in the formation of muscle of the forearm and hand. The most cranial somite (16) takes part in the radial muscles and the most caudal somites (20, 21) in the ulnar muscles, reflecting their position with respect to the limb bud. The centrally located somites (17, 18, 19) are involved in all (18) or most (17, 19) muscle primordia. This pattern of distribution is clearest in the forearm, whereas the participation of somites in particular muscle groups is not so distinct in the hand. Hand muscles are mainly made up of cells from somites 18-20. All brachial somites participate in dorsal (extensor) as well as ventral (flexor) muscles of the forearm and hand. Each somite takes part in more than three muscle primordia in a reproducible fashion, and every muscle primordium is derived from at least three somites. Especially the M. ulnimetacarpalis ventralis takes origin from all somites involved in limb muscle formation (16-21). Apart from muscle cells, endothelial cells also and a few fibroblasts of quail origin are found in the limb bud after somite grafting.

Expression of avian Pax1 and Pax9 is intrinsically regulated in the pharyngeal endoderm, but depends on environmental influences in the paraxial mesoderm

Pax1 and Pax9 represent a subfamily of paired-box-containing genes. In vertebrates, Pax1 and Pax9 transcripts have been found specifically in mesodermal tissues and the pharyngeal endoderm. Pax1 expression in the sclerotomes has been shown to be indispensable for proper formation of the axial skeleton, but expression of Pax1 in the endoderm has not been studied in detail. We have cloned the chick homologue of the murine Pax9 gene. Our results show that transcripts of Pax1 and Pax9 are first detectable in the prospective foregut endoderm of headfold-stage avian embryos. Endodermal expression correlates with the highly proliferative zones of the folding foregut and evaginating pharyngeal pouches. In later stages, Pax1 and Pax9 are expressed in overlapping but distinct patterns within the developing sclerotomes and limb buds. From grafting experiments we conclude that activation of pharyngeal Pax1 and Pax9 expression is an intrinsic property of the endoderm, not requiring midline structures or head mesoderm. In contrast, notochord is required to induce Pax1 in competent sclerotomes. Here we show that in vitro there is a cranio-caudal gradient of inductive capacity in the notochord. This coincides with the graded expression of Pax1 and Pax9 along the cranio-caudal axis in 2- to 3-day-old embryos. Furthermore, paraxial head mesoderm shows no competence to express Pax1. Finally, in vitro we find counteracting influences on notochord signaling by lateral tissues (lateral plate, intermediate mesoderm), leading to an inhibition of Sonic hedgehog (Shh) expression in notochord and floor plate, as well as Pax1 and Pax9 expression in sclerotomes. Taken together, our results demonstrate that different mechanisms regulate expression of Pax1 and Pax9 in foregut and sclerotome, but suggest a common function for both genes in the two tissues that is promoting proliferation and preventing fusion of neighboring blastemas.

The expression and regulation of follistatin and a follistatin-like gene during avian somite compartmentalization and myogenesis

We report on the normal and experimentally altered expression of two structurally related genes, Follistatin and Follistatin-like (Flik), in the somites of avian embryos. In normal chick embryos, Follistatin expression can first be seen in the cells of the dorsolateral somite quarter. During somite maturation, the cells of the dorsomedial quarter also express this gene. Within the dermomyotome it seems that only the muscle precursors are Follistatin-positive. The migrating precursors of limb and tongue muscle as well as the myotome cells show Follistatin expression. The manipulation experiments reveal that the expression of Follistatin in the somites can be inhibited by notochord signals. This effect can be mimicked by sonic hedgehog protein. Flik is expressed in the dorsomedial compartment of the somite and later on in the myotome. Unlike Follistatin, Flik expression requires signals emanating from the neural tube. Notochordal influences do not alter Flik expression. The expression of both genes does not depend on signals of intermediate or lateral mesoderm. Since the products of both genes are proposed to antagonize TGF-beta superfamily proteins during gastrulation and neuralization, we postulate that during myogenesis follistatin and flik counteract inhibiting effects of related molecules on muscle differentiation.

N-cadherin is involved in myoblast migration and muscle differentiation in the avian limb bud

Limb muscle formation involves invasion of the limb bud mesoderm by myogenic precursor cells from the dermomyotomes at limb bud level. Directed cell migration, homing, and differentiation of myogenic cells are controlled by the stationary cells of the limb bud mesoderm. At the level of the extracellular matrix, the molecular basis of migration control has been suggested to be exerted by the distribution of hyaluronan. Here, we demonstrate that N-cadherin-mediated interactions play a role at cell-membrane level in myoblast distribution and differentiation. N-cadherin is strongly expressed by myogenic cells in the chick limb bud and more moderately expressed by stationary mesodermal cells in the myogenic zones and progress zone. After in vivo injection of antibodies and Fab-fragments against the homophilic binding site of N-cadherin into the wing bud mesoderm, aggregates of myoblasts are found predominantly in the dorsal myogenic zone 36 hr after injection apparently due to immobilization. In the same position, areas of myf-5-positive cells are also observed. In injected limb buds, Pax-3-positive cells are less evenly distributed than in uninjected limbs. They are found to spread up to the epidermis and also form loosely arranged aggregates. After prolonged reincubation periods, injected limbs show ectopic myoblasts that are rich in desmin and areas of strongly desmin-expressing myoblasts within muscle blastemas. These effects were not observed after application of antibodies against other parts of the N-cadherin molecule. We conclude that N-cadherin is involved in myoblast migration in the limb buds via homophilic interactions and that it plays a role in signal transduction during myogenesis.

Fibroblast growth factor receptor 1 in skeletal and heart muscle cells: expression during early avian development and regulation after notochord transplantation

Basic fibroblast growth factor (bFGF, FGF-2) mediates several biological functions during embryonic development. With regard to skeletal muscle formation, it has been suggested that FGF-2 is involved in the growth and differentiation of myogenic precursor cells. To identify the FGF-responsive cells we studied the expression of FGF receptor type I (FGFR-1) during early embryonic development of the chick. FGFR-1 immunoreactivity is present at all stages examined (embryonic day [E] 2-E5). Expression of FGFR-1 is found in the somite myotome, limb bud muscle cells, eye and tongue muscle cells, and myocardium. Transplantation of an additional notochord into the paraxial mesoderm, which prevents the formation of a myotome, reveals the absence of FGFR-1 immunoreactivity on the operated side. The distinct expression pattern of FGFR-1 in migrating and differentiating muscle cells indicates that in addition to the stimulation of proliferation of myoblasts, FGF-2 exerts other (nonmitogenic) effects on postmitotic myocytes.

VEGF121 induces proliferation of vascular endothelial cells and expression of flk-1 without affecting lymphatic vessels of chorioallantoic membrane

We have studied the effect of VEGF(121) homodimer and VEGF(121/165) heterodimer on the chorioallantoic membrane (CAM) of 13-day-old chick embryos. The factors were applied in doses of 2-4 micrograms and the effects were evaluated macroscopically after 2 and 3 days. Histological studies were performed on semi- and ultrathin sections. Proliferation was studied according to the BrdU-anti-BrdU method on whole mounts and sections. The labeling density was quantified in whole mounts. The fractal dimension, D, of the vascular tree was assessed as a value for vascular bifurcation density. Both forms of VEGF induce brush-like vessel formation in the precapillary region. New capillaries are found in the stroma of the CAM, which normally does not contain capillaries. Our results show that VEGF(121) is a specific endothelial cell mitogen. A fourfold increase of BrdU-labeled endothelial cells is found after VEGF(121) application. The fractal dimension of the vascular tree increases from 1.26 in the controls to 1.44 (VEGF(121)) and 1.41 (VEGF(121/165)). The endothelial cells of the newly formed capillaries possess many mitochondria and micropinocytotic vesicles, but no fenestrations. These capillaries are obviously formed by intussusceptive microvascular growth. Signs of sprouting are almost absent. An effect on the lymphatic vessels of the CAM is not detectable. Compared to VEGF(165) and VEGF(121/165), VEGF(121) diffuses over a slightly greater distance. Using in situ hybridization, VEGF receptor-2 (flk-1/Quek1) and the homologous flt-4 (Quek2) receptor were studied in the CAM of normal quail embryos and after VEGF(121) application on the CAM of 11-day-old quail embryos. During normal development, flk-1 expression becomes restricted to vascular endothelial cells of large vessels in the stroma of the CAM. VEGF(121) application induces expression of flk-1 in capillaries that normally do not express the receptor. In the normal development of the CAM, flt-4 becomes restricted to endothelial cells of vessels that appear to be lymphatic vessels. Application of VEGF(121) does not alter flt-4 expression.

Molecular cloning, sequencing and expression of the cDNA of the mitochondrial form of phosphoenolpyruvate carboxykinase from human liver

In human liver, phosphoenolpyruvate carboxykinase (PCK; EC 4.1.1.32) is about equally distributed between cytosol and mitochondria in contrast with rat liver in which it is essentially a cytosolic enzyme. Recently, the isolation of the gene and cDNA of the human cytosolic enzyme has been reported [Ting, Burgess, Chamberlian, Keith, Falls and Meisler (1993) Genomics 16, 698-706; Stoffel, Xiang, Espinosa, Cox, Le Beau and Bell (1993) Hum. Mol. Genet. 2, 1-4]. It was the goal of this investigation to isolate the cDNA of the human mitochondrial form of hepatic PCK. A human liver cDNA library was screened with a rat cytosolic PCK cDNA probe comprising sequences from exons 2 to 9. A cDNA clone was isolated which had overall a 68% DNA sequence and a 70% deduced amino acid sequence identity with the human cytosolic PCK cDNA. Without the flanking 270 bases (=90 amino acids) each at the 5' and 3' end, the sequence identity was 73% on the DNA and 78% on the amino acid level. The isolated cDNA had an open reading frame of 1920 bp; it was 54 bp (equivalent to 18 amino acids) longer than that of human or rat cytosolic PCK cDNA. The isolated cDNA was cloned into the eukaryotic expression vector pcDNAI and transfected into human embryonal kidney cells HEK293; PCK activity was increased by 3-fold in the mitochondria, which normally contain 70% of total PCK activity, but not in the cytosol. The isolated cDNA was also transfected into cultured rat hepatocytes; again, PCK activity was enhanced by about 40-fold in the mitochondria, which normally possess only 10% of total PCK activity, but not in the cytosol. In the rat hepatocytes only the endogenous cytosolic PCK and not the transfected mitochondrial PCK was induced 3-fold with glucagon. Comparison of the amino acid sequences deduced from the isolated cDNA with human and rat cytosolic PCK showed that the additional 18 amino acids were located at the N-terminus of the protein and probably constitute a mitochondrial targeting signal. Northern-blot analyses revealed the human mitochondrial PCK mRNA to be 2.25 kb long, about 0.6 kb shorter than the mRNA of the cytosolic PCK. Primer extension experiments showed that the 5'-untranslated region of mitochondrial PCK mRNA was 134 nucleotides in length.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

Axial structures control laterality in the distribution pattern of endothelial cells

In the midline of the embryo an invisible barrier exists that keeps endothelial cells from migrating to the contralateral side. Interspecific grafting experiments between chick and quail were carried out in order to investigate the role of the axial structures in maintaining this barrier. The quail endothelial cells of the graft were therefore stained with QH1 antibody. In all experimental series quail paraxial mesoderm was used as a source of endothelial cells. First, a quail somite was transplanted either ipsilaterally or contralaterally. The results not only show the existence of laterality in the distribution pattern, but also demonstrate that the laterality does not depend on the origin of the graft but on the environment of the host embryo. Laterality in the distribution pattern of endothelial cells means that the endothelial cells of the two body halves migrate independently and do not change from one side to the other. Single cells do not know whether they are cells from the right or from the left half of the body. In the next series of experiments axial structures were removed in order to modify the barrier. In addition, paraxial mesoderm was exchanged with the corresponding quail tissue in order to determine the migration behaviour of the grafted endothelial cells. The removal of the neural tube does not influence the barrier. After notochordectomy, however, the endothelial cells exhibited a balanced distribution pattern over both halves of the embryo. We concluded that the notochord forms a barrier for endothelial cells that presumably operates on the basis of chemical substances. It is conceivable that our results can explain the lateralization of illnesses of the vascular system, as the Klippel-Trénaunay syndrome or the Sturge-Weber syndrome.

An ovomucin-like protein on the surface of migrating primordial germ cells of the chick and rat

A mucin was discovered on the surface of migratory primordial germ cells (PGCs) from chick and rat embryos by means of two monoclonal antibodies. The protein was found to be identical or closely related to ovomucin, a 600 X 10(3) relative molecular mass glycoprotein, and a major constituent of the vitelline membrane of the avian yolk. Based on its resemblance to ovomucin it is referred to as ovomucin-like protein (OLP). The OLP was expressed on PGCs from E3 to E7 female, and from E3 to E12 male chick embryos as the PGCs migrate and colonize the gonadal ridges. After the PGCs have settled in the gonads, they no longer express OLP. In tissue cultures of dissociated cells from E6 gonads, OLP was present only on cells that were positive for PAS staining, the standard histological method to identify PGCs in the chick embryo. Since unfixed PGCs were recognized by the antibodies, at least part of the OLP is localized on the cell surface. The anti-OLP antibodies also stained PGCs in the gonads of the rat embryo, showing that the expression of this antigen on PGCs is phylogenetically conserved. Ovomucin isolated from vitelline membrane prevented adhesion of fibroblasts but not PGCs when used a as a substratum in vitro. The anti-adhesive quality of the mucin resides in the sialic acid residues of the carbohydrate side chains. We propose that OLP has a similar anti-adhesive quality as the ovomucin from vitelline membrane, and that this anti-adhesive property is important to prevent precocious adhesion of migrating PGCs to blood vessel walls and to connective tissue in the mesentery as they migrate toward the gonadal ridges.

The formation of somite compartments in the avian embryo

The somites develop from the unsegmented paraxial mesoderm that flanks the neural tube. They form in an intrinsic process which lays down the primary segmental pattern of the vertebrate body. We review the processes of somitogenesis and somite differentiation as well as the mechanisms involved in these developmental events. Long before overt differentiation occurs, different compartments of the still epithelial somites give rise to special cell lines and to particular derivatives. By means of isotypic grafting between quail and chick embryos, it is possible to follow the fate of groups of somitic cells. In this way, the development of the myotome and the back dermis from the dorsomedial quadrant and of the hypaxial body wall and limb musculature from the dorsolateral quadrant was established. The two ventral quadrants and the somitocoele give rise to the chondrogenic/fibroblastic lineage of the sclerotome and form the vertebral column. Somite compartments can first be visualized by the expression pattern of Pax genes. Pax-3 is expressed in the dorsal part of the epithelial somite, while the ventral two thirds express Pax-1, a marker of sclerotome development. Pax-3 expression is retained also in the premitotic myogenic cells that migrate into the limb buds. In differentiating myoblasts, Pax-3 expression is turned down and taken over by the activation of MDF's. This initial event in myogenesis occurs in the absence of local signals, whereas the expression of Pax-1 in the sclerotome can be shown to be induced by signals from the notochord and floor-plate of the neural tube. Epaxial myotome differentiation is supported by the neural tube, after the neural tube has received patterning signals from the notochord. The hypaxial musculature and limb musculature differentiate independently of the axial structures. The myogenic cells migrating within the limb buds respond to signals of the lateral plate mesoderm which guide their distalward migration and pattern the muscle.

First blood vessels in the avian neural tube are formed by a combination of dorsal angioblast immigration and ventral sprouting of endothelial cells

We studied the early pattern of neural tube (NT) vascularization in quail embryos and chick-quail chimeras. Angioblasts appeared first in the dorsal third at Hamburger and Hamilton (HH) stage 19 as single, migrating cells. Their distribution did not correspond to a segmental pattern. After this initial dorsal immigration, endothelial sprouts invaded the NT on either side of the floor plate (HH stage 21). These cells remained continuous with their arterial vascular sources, connected to the venous perineural vascular plexus at HH-stage 22, and formed the first perfused vessels of the NT at HH-stage 23. The same pattern of angiotrophic vascularization was observed in a craniocaudal sequence starting caudal to the rhombencephalic NT. Extremely long filopodia were observed on sprouting cells, extending toward the central canal and the mantle layer. The exclusively extraneuroectodermal origin of angioblastic cells was demonstrated with chick-quail chimeras. Following replacement of quail NT by chick NT graft, angioblast and sprout distribution in chimeras was the same as in controls. We conclude that the NT receives its first blood vessels by a combination of two different processes, dorsal immigration of isolated migrating angioblastic cells and ventral sprouting of endothelial cells, which derive from perfused vessels. The dorsal invasive angioblasts contribute to the developing intraneural vascular plexus after having traversed the neural tube. The initial distribution of blood vessels within the neuroepithelium corresponds to intrinsic random motility of angioblastic cells; a more regular pattern is seen later. The floor plate apparently prohibits connections between sprouts in both NT sides, whereas in the dorsal NT, such a separating effect on the migrating angioblasts does not exist.

Function of somite and somitocoele cells in the formation of the vertebral motion segment in avian embryos

We have studied the distribution of thoracic somite and somitocoele-derived cells using homotopical grafting between quail and chicken embryos and reincubation periods of 2-6 days. Serial sections were evaluated with antibodies against quail cells, quail hemangiopoietic cells and desmin. With the exception of neural crest cells in the cranial sclerotome half, all cells of the operated segment are quail cells derived from a single somite. These cells differentiate into sclerotome, myotome and the anlage of the dermis of the back. After longer reincubation periods, the somite-derived quail cells form the neighboring halves of 2 adjacent vertebral bodies and the intervening (disc-homologous) tissue. Resegmentation is furthermore visible in the lamina and the spinous process. Somite cells also form the articular and transverse processes, and the intertransverse muscle including its insertion to the next cranial transverse process. One thoracic somite forms the proximal part of 1 rib. In more distal parts, 1 somite forms the cranial half of 1 rib and the caudal half of the next cranial rib, and the intercostal muscle and part of the connective tissue. Somite-derived quail cells are found in muscle that bridges over 2 segments cranial and caudal from the operated segment. The craniocaudal distribution of endothelial cells is approximately the same. Somitocoele cells that are located centrally in the epithelial somite express the sclerotome-markers Pax-1 and Pax-9. After 2-3 days of reincubation, grafted thoracic somitocoele cells are found mainly in the cranial part of the caudal sclerotome half. They form an area representing the anlagen of the intervertebral disc and the rib. After longer reincubation periods, the grafted quail somitocoele cells form the intervertebral disc-homologous tissue and the proximal part of the rib. In more distal parts of the rib they are located in the cranial half of 1 rib and the caudal half of the next cranial rib. The somitocoele cells also form the surface of the intervertebral joint, and give rise to a small number of endothelial cells that are found up to 1 segment cranial and caudal to the operation site. Our studies show that resegmentation is found in most parts of the vertebra and in the distal ribs. One somite forms the origin and insertion of the segmental muscle. Therefore, the somite can be regarded as the ancestor of the vertebral motion segment. Somitocoele cells are located centrally both in the epithelial somite and in the vertebral motion segment.

Halves of epithelial somites and segmental plate show distinct muscle differentiation behavior in vitro compared to entire somites and segmental plate

Medial and lateral halves of the somite are known to differ with respect to their developmental fates: Cells from the medial half of the somite give rise to the epaxial muscle of the back and cells from the lateral half of the somite give rise to the skeletal muscles of the limbs and the ventrolateral body wall. To get a better insight into myogenic determination of somite hemispheres, isolated entire somites as well as medial and lateral parts of somites and of segmental plate from 2 day chick embryos were explanted in vitro. These parts of the paraxial mesoderm were also cocultured in contact with somite surrounding tissues such as neural tube lacking floorplate, neural tube including notochord-floorplate complex, and intermediate mesoderm, which were examined with respect to their muscle promoting or inhibiting influences. Skeletal muscle differentiation was monitored by the use of anti-myosin heavy chain antibody (MF20). It is shown that medial and lateral halves of segmental plate and epithelial somites are capable of undergoing myogenesis in the absence of axial organs. In contrast, cultures of intact segmental plate and epithelial somites from the same levels did not show muscle differentiation. Neural tube lacking floorplate promoted muscle differentiation in the medial halves especially of epithelial somites and also of segmental plate, but not in the lateral halves of the paraxial mesoderm at these levels. Intermediate mesoderm was found to inhibit muscle differentiation in medial and lateral halves of segmental plate and of epithelial somites. We further demonstrate that the arrangement of the myoblasts within tissue cultures is influenced by the presence or absence of axial organs.