FGF2 promotes skeletogenic differentiation of cranial neural crest cells

The cranial neural crest gives rise to most of the skeletal tissues of the skull. Matrix-mediated tissue interactions have been implicated in the skeletogenic differentiation of crest cells, but little is known of the role that growth factors might play in this process. The discovery that mutations in fibroblast growth factor receptors (FGFRs) cause the major craniosynostosis syndromes implicates FGF-mediated signalling in the skeletogenic differentiation of the cranial neural crest. We now show that, in vitro, mesencephalic neural crest cells respond to exogenous FGF2 in a dose-dependent manner, with 0.1 and 1 ng/ml causing enhanced proliferation, and 10 ng/ml inducing cartilage differentiation. In longer-term cultures, both endochondral and membrane bone are formed. FGFR1, FGFR2 and FGFR3 are all detectable by immunohistochemistry in the mesencephalic region, with particularly intense expression at the apices of the neural folds from which the neural crest arises. FGFRs are also expressed by subpopulations of neural crest cells in culture. Collectively, these findings suggest that FGFs are involved in the skeletogenic differentiation of the cranial neural crest.

Coordinated expression of 3' hox genes during murine embryonal gut development: an enteric Hox code

BACKGROUND & AIMS

Hox genes are highly conserved developmental control genes that may be organized and expressed in the form of a code required for correct morphogenesis. Little is known about their control of the embryonal gut. However, Hox paralogues 4 and 5, which are expressed at the sites of origin of vagal neural crest cells and splanchnic mesoderm, are likely to be important. We have studied the expression domains of these genes in the gut both spatially and temporally.

METHODS

CD1 mice embryos of embryonic days E8.5-E17.5 were studied. The spatial and temporal expression patterns of messenger RNA of Hoxa4, b4, c4, d4, a5, c5, and b5 homeoprotein were determined by in situ hybridization and immunohistochemistry in whole embryos, whole gastrointestinal tracts, and vibratome sections.

RESULTS

There were different spatial, temporal, and combinatorial expression patterns in different morphological regions: foregut, prececal gut, cecum, and postcecal gut. Two dynamic gradients, rostral and caudal, were coordinated with nested expression domains along the gut primordium. Region-specific domains were present in the stomach and cecum.

CONCLUSIONS

The expression patterns of genes in paralogous groups 4 and 5 suggest that they are organized to form a specific enteric Hox code required for correct enteric development.

Detailed characterization of the human aorta-gonad-mesonephros region reveals morphological polarity resembling a hematopoietic stromal layer

The definitive long-term repopulating human hematopoietic stem cell, which seeds the adult blood system, was previously thought to derive from the extra-embryonic yolk sac. However, there is now considerable evidence that in both avian and murine systems, yolk sac hematopoietic cells are largely a transient, embryonic population and the definitive stem cell, in fact, derives from a distinct region within the embryonic mesoderm, the aorta-gonad-mesonephros region. In the human embryo, an analogous region has been found to contain a cluster of cells distinct from, but closely associated with, the ventral endothelium of the dorsal aorta, the appearance of which is restricted both spatially and temporally. We have used antibodies recognising hematopoietic regulatory factors to further characterise this region in the human embryo. These studies indicate that all factors examined, including vascular endothelial growth factor and its receptor FLK-1, Flt-3 ligand and its receptor STK-1, and stem cell leukemia transcription factor, are expressed by both hematopoietic cells in the cluster and endothelial cells. However, there is some discontinuity in cells directly underlying the cluster. Furthermore, we have identified a morphologically distinct region of densely-packed, rounded cells in the mesenchyme directly beneath the ventral wall of the dorsal aorta, and running along its entire length. In the preumbilical AGM region, directly underlying the hematopoietic cluster, but not at more rostral and caudal levels, this region of mesenchyme expresses tenascin-C, an extracellular matrix glycoprotein known to facilitate cell-cell interactions and migration. This region of cells may therefore provide the microenvironmental support for the intraembryonic development of definitive hematopoietic stem cells, a process in which tenascin-C may play a pivotal role.

Pleiotropic features of syndromic craniosynostoses correlate with differential expression of fibroblast growth factor receptors 1 and 2 during human craniofacial development

Mutations in FGFR1, -2, and -3 are linked to five human craniosynostosis syndromes. In addition to premature fusion of cranial sutures, nonskeletal manifestations in skin, and teeth together with CNS abnormalities, reflect widespread effects of these mutations. To understand this pleiotropy, we have assessed craniofacial FGFR1 and -2 expression in the human embryo from 6 wk postfertilization. We found that both genes are expressed in sheets of condensed mesenchyme before overt chondrogenic differentiation and that distinct patterns of expression are established by 8 wk. Thus, FGFR2(BEK) is expressed evenly throughout developing cartilage and bone, whereas FGFR1 transcripts predominate in perichondria and periostea. Complementary patterns of FGFR1 and FGFR2(BEK and KGFR) expression are also observed in the enamel epithelium and papilla mesenchyme of the tooth germ, at a stage when morphogenetic tissue interactions ensue. Both genes are expressed in the cortical layer of the brain, but expression levels vary significantly within the choroid plexus and wall of the fourth ventricle. Similarly, tissue-specific differences in receptor expression are found in both the skin and salivary glands. These expression data are consistent with the pleiotropic manifestations of syndromic craniosynostoses and provide the basis for a new paradigm to explain the associated CNS problems.

Stability and plasticity of neural crest patterning and branchial arch Hox code after extensive cephalic crest rotation

The extent to which the spatial organisation of craniofacial development is due to intrinsic properties of the neural crest is at present unclear. There is some experimental evidence supporting the concept of a prepattern established within crest while contiguous with the neural plate. In experiments in which the neural tube and premigratory crest are relocated within the branchial region, crest cells retain patterns of gene expression appropriate for their position of origin after migration into the branchial arches, resulting in skeletal abnormalities. But in apparent conflict with these findings, when crest is rerouted by late deletion of adjacent crest, infilling crest alters its pattern of gene expression to match its new location, and a normal facial skeleton results. In order to reconcile these findings thus identify processes of relevance to the course of normal development, we have performed a series of neural tube and crest rotations producing a more extensive reorganisation of cephalic crest than has been previously described. Lineage analysis using DiI labelling of crest derived from the rotated hindbrain reveals that crest does not migrate into the branchial arch it would have colonised in normal development, rather it simply populates the nearest available branchial arches. We also find that crest adjacent to the grafted region contributes to a greater number of branchial arches than it would in normal development, resulting in branchial arches containing mixed cell populations not occurring in normal development. We find that after exchange of first and third arch crest by rotation of r1-7, crest alters its expression of hoxa-2 and hoxa-3 to match its new location within the embryo resulting in the reestablishment of the normal branchial arch Hox code. A facial skeleton in which all the normal components are present, with some additional ectopic first arch structures, is formed in this situation. In contrast, when second and third arch crest are exchanged by rotation of r3 to 7, ectopic Hox gene expression is stable, resulting in the persistence of an abnormal branchial arch Hox code and extensive defects in the hyoid skeleton. We suggest that the intrinsic properties of crest have an effect on the spatial organisation of structures derived from the branchial arches, but that exposure to increasingly novel environments within the branchial region or "community effects" within mixed populations of cells can result in alterations to crest Hox code and morphogenetic fate. In both classes of operation we find that there is a tight link between the resulting branchial arch Hox code and a particular skeletal morphology.

Optic disc anomalies and frontonasal dysplasia

AIMS

To document the optic disc abnormalities in patients with frontonasal dysplasia in association with basal encephalocele.

METHODS

Names and hospital numbers of patients with midline clefts were obtained from the ophthalmology and genetics database. Six patients were identified who had the following common findings: midline facial cleft with midline cleft lip and palate; hypertelorism; absent corpus callosum; basal (sphenoethmoidal) encephalocele; and pituitary deficiency (five out of six cases). Ophthalmic examination was performed with fundal photography where possible.

RESULTS

Two patients had unilateral and one a bilateral peripapillary staphyloma. Two patients had bilateral optic disc hypoplasia and one appeared to have a peripapillary staphyloma in one eye and a morning glory disc in the other.

CONCLUSION

Optic disc abnormalities were found in all patients with this constellation of clinical findings. This association appears to represent a distinct subgroup within the spectrum of frontonasal dysplasia. The presence of midline facial anomalies and any dysplastic disc should alert the physician as to the presence of an encephalocele.

Segmentation, crest prespecification and the control of facial form

The early development of the vertebrate head is dependent on the formation of two series of segmented structures, the rhombomeres of the hindbrain and the branchial arch series. The initial formation of these two systems is closely linked, as the principal source of branchial arch mesenchyme is the neural crest, which derives from the lateral edge of the neural plate at the time of rhombomere formation. The subsequent development of the two systems maintains a close level of integration, as specific spatial relationships between skeletal, muscle and neural elements arising from the same axial level are established. Given the level of conservation of these anatomical relationships in vertebrates, it is likely that they are a reflection of a key mechanism in early facial and pharyngeal development. One model, in part based on these findings, proposed that the neural crest acquires an axial-level specific combination of gene expression while part of the neural plate. This prepattern is then maintained throughout the crest's subsequent development. In the model, this combination of gene expression would then specify the form of the facial and pharyngeal structures that the crest would give rise to. In this review we evaluate recent evidence on whether early facial development involves a crest prespecification of this type, and conclude that it is not the case.

The one bone spine: a failure of notochord/sclerotome signalling?

Three cases of extensive vertebral fusions with absent clivo-axial angle are presented. The 'bone-within-bone' appearance in two patients with almost complete fusion of the spine suggested ossification of the notochord and perinotochordal sheath. On the basis of the radiological appearances and the results of recent molecular genetic studies on vertebrate embryos, the suggested time of segmentation failure along the axis of the craniovertebral junction and between vertebrae is the third to fifth week of gestation. The possible roles of the Pax-1 gene and of signalling between notochord and sclerotome are discussed, concluding that an early defect of the notochord may be responsible for this type of failure of segmentation. Indications for surgery in these cases included cord compression with brachialgia and 'chin-on-chest' deformity causing severely restricted visual fields. A critical review of clinical lessons learned in the operative treatment is presented.

Expression of the transcription factor slug correlates with growth of the limb bud and is regulated by FGF-4 and retinoic acid

The slug gene encodes a zinc finger transcription factor expressed by neural crest cells (Nieto et al., Science 264: 835-839, 1994) and by certain non-crest derived mesenchymal cell populations, such as lateral mesoderm and sclerotome (Mayor et al., Development 121: 767-777, 1995; Buxton et al., Dev. Biol. 183: 150-165, 1997). We report here that slug is also expressed in developing chick limbs. The slug expression domain in the limb bud expands from posterior to anterior and marks cells that are predominantly destined to become chondrocytes but have not yet differentiated. Its expression is maintained in connective tissue, but is never observed in the premuscle masses. We show that removal of the apical ectodermal ridge results in loss of slug expression which can be arrested by the addition of an FGF-4 bead. Retinoic acid bead implants lead to down-regulation of slug expression, again accompanied by abolition of limb outgrowth. Dual bead implants demonstrate antagonism between these two factors, suggesting that a localized antagonistic effect between endogenous RA and FGF-4 on slug expression underlies the molecular mechanism controlling the transition between undifferentiated and differentiated state during normal limb development. The fact that slug expression pattern correlates with areas of growth in the limb, and is maintained by FGF-4 and down-regulated by retinoic acid, indicates that slug-expressing cells play a crucial role in growth and patterning of the chick limb. We propose that slug expression provides the best correlation to date between a molecular marker and the physical concept of the progress zone, defined as "a labile region where new positional values are successively engendered in the course of growth" (Summerbell et al., Nature 244: 492-496, 1973).

Retinoic acid and retinoic acid receptors in craniofacial development

Interest in retinoids and craniofacial development originated independently from nutritional and teratological studies; however, the site of action of retinoids in normal development remains contentious. Recent transgenic strategies have shown that retinoic acid and nuclear retinoid receptors are required for the morphogenetic specification of cranial neural crest cells and their mesenchymal derivatives during craniofacial development. Interestingly, while some aspects of the RA teratogenicity have been shown to be receptor-mediated, there is as yet no clear evidence that this is the case for the embryonic head and face. Hox genes are one important set of targets for RA in the developing neural primordium and cranial neural crest, but it remains unclear as to how retinoid-mediated regulation of such targets is realized as the morphogenetic specification of cell fate.

A role for midline closure in the reestablishment of dorsoventral pattern following dorsal hindbrain ablation

The cellular and molecular study of dorsal neural tube ablation reported here demonstrates a critical role for midline closure in hindbrain repatterning. This was revealed by detailed analysis of the transcriptional response of two genes, Pax-3 and slug, during repair of the neural tube following ablation. The reexpression of Pax-3 appears to rely on a single surface ectoderm/neuroepithelial contact, while this is insufficient for reexpression of slug. In fact, slug up-regulation only occurred upon midline closure and, strikingly, corresponded to down-regulation of Pax-3. We examined whether a candidate dorsalizing molecule, Bmp-4, was responsible for this reciprocal regulation of Pax-3 and slug at midline closure. However, Bmp-4 was not reexpressed following ablation, indicating not only that it is not responsible for the observed repatterning but that it lies in regulatory pathways distinct from Pax-3 and slug. We additionally examined the expression of Pax-6, which, together with assessment of the pattern of cranial ganglia, roof plate morphology, and positioning of branchiomotor exit points, demonstrates that neural crest regeneration is accompanied by reestablishment of a normal dorsoventral pattern within the neural tube. Thus, both local and longer range patterning appears to be restored following ablation, which is reliant dorsally on midline closure of the neural tube.

Branchial HOX gene expression and human craniofacial development

Members of the Antennapedia class of homeobox genes, known as Hox genes, are believed to be pivotal in vertebrate craniofacial development. Here we show that eight members of paralogous groups 1, 2, 3, and 4 are expressed in the human embryonic hindbrain and branchial arches at 4 weeks of development. The combinatorial patterns of expression of genes representing the first three paralogous groups parallel the patterns described for their homologues in various animal models, demonstrating a high degree of conservation of the branchial Hox code. Arch expression of group 4 genes is defined for the first time in any vertebrate. Furthermore, as development proceeds, individual paralogues of a single paralogous group (group 3), which initially share a common expression domain, are differentially down-regulated in a tissue-, organ-, or site-specific fashion.

Abnormal patterning of the aortic arch arteries does not evoke cardiac malformations

Ablation of the cardiac neural crest results in abnormal development of the aortic arch arteries leading to altered patterning of the great arteries. The cardiac outflow tract is also affected after neural crest ablation because normally a subset of neural crest cells migrates from the pharyngeal region to form the outflow septum. Using neural crest ablation, it has not been possible to separate the occurrence of aortic arch maldevelopment from cardiac outflow tract dysmorphogenesis. In order to determine whether normal aortic arch artery development is a prerequisite for normal outflow tract development, we have used a combination of antisense treatment with backtransplantation of cardiac neural folds to produce abnormal patterning of the aortic arch arteries. Paralogous groups of Hox messages with their anterior expression domains in pharyngeal arches 3, 4 and 6 were targeted. Antisense targeted to paralogous group 3 Hox message caused aortic arch 3 located within the pharyngeal arch to regress in a manner similar to aortic arch 2, while antisense targeted to paralogous group 5 Hox message caused the appearance of an additional pharyngeal arch containing a novel and completely independent aortic arch artery. Antisense treatment targeting paralogous group 4 Hox message led to no detectable cardiovascular phenotype in the first 6 days of development. While regression of arch artery 3 was associated with abnormal branching patterns of the aorta and pulmonary trunk, this did not involve abnormal separation of the aorta and pulmonary trunks, the semilunar valves or the subvalvular region of the outflow tract. Because none of these changes in pharyngeal or aortic arch artery development was accompanied by abnormal development of the cardiac outflow tract, it appears that normal patterning of the aortic arch arteries is not a prerequisite for normal heart development. Using reverse transcription polymerase chain reaction (RT-PCR) we were unable to detect changes in any of the Hox messages except group 4, thus, using this particular experimental strategy, we are unable to demonstrate or refute that expression of hox genes by cardiac neural crest cells controls aortic arch patterning. Development of the cardiac outflow tract was normal in each instance. This suggests that abnormal aortic arch patterning does not necessarily lead to cardiac malformations.

Regeneration of lower and upper jaws in urodeles is differentially affected by retinoic acid

The vitamin A derivative retinoic acid (RA) is a powerful teratogen which can induce severe craniofacial and limb malformations if administered at certain stages of gestation. In addition this compound has been shown to affect patterning in regenerating systems. A classical example is the induction of supernumerary structures along the proximodistal axis of the regenerating amphibian limb. We have investigated the effect of RA on other regenerating systems, the amphibian lower and upper jaws, both in developing and adult animals. We report here that RA does not induce formation of extra structures either in the lower or in the upper jaw of adult newts under experimental conditions where duplications of the regenerating limb occur. However, RA selectively induces severe malformations in the upper jaw regenerate that resemble those induced in avian and mammalian embryos. Analysis of the expression of the newt retinoic acid receptors RAR alpha and delta in upper and lower jaws showed that RAR alpha was expressed at a significant level in the wound epidermis, but not in blastemal cells, whereas no RAR delta could be detected in the regenerate either by in situ hybridization or by using an anti-RAR delta antibody. Therefore, unlike in the limb, in jaws RAR delta is not up-regulated following amputation, and this difference in expression may be causally related to the different effects induced by RA on jaws and limbs. In order to establish whether retinoids affected regeneration of developing jaws in a similar fashion, their effects were studied in animals whose jaws had been amputated at different developmental stages. Under the experimental conditions used overall growth retardation and head defects were observed in the majority of embryos which had been amputated and treated with retinol palmitate (RP) between stages 26-28 and 38-39. In contrast, patterning of upper jaw regenerates in larvae amputated at stage 26-28 and 38-39. In contrast, patterning of upper jaw regenerates in larvae amputated at stage 45 was not significantly affected by the treatment, although the early phase of regeneration was slower than in controls. The different responses to retinoids of regenerating facial structures in embryos, larvae and adults will be discussed.

Bone morphogenetic protein-2 and -4 expression during murine orofacial development

In the developing orofacial region, epithelial-mesenchymal interactions induce a differentiation cascade leading to bone and cartilage formation. Although the nature of this interaction is unknown, bone morphogenetic proteins (BMP)-2 and -4 have been suggested as putative signalling molecules. Using 35S-labelled cDNA probes, the expression patterns of BMP-2 and -4 mRNA were examined in murine perioral tissues preceding, during and following the time of the epithelial-mesenchymal interaction leading to mandibular formation. At embryonic age (e) 9.5 days, a restricted pattern of BMP-4 mRNA was expressed in the epithelium of the developing facial processes. This decreased rapidly, with little or no signal on E10.5 or E11.5. By E13.5, BMP-4 signal was restricted to the dental lamina, follicle and papilla. BMP-2 expression was not prominent in the developing face until E13.5. At this stage, signal was widespread throughout mesenchyme of neural-crest, but not somatic origin. Different domains of expression were present in the developing epithelium: for example, there was strong signal in the floor of the mouth and the ventral tongue, in contrast to that of the dorsum of the tongue and primary palate, which were negative. These results support the role of BMP-2 and -4 as regulators of orofacial development and demonstrates different fields of BMP-2 expression in developing oral mucosal epithelium.

KAL, a gene mutated in Kallmann's syndrome, is expressed in the first trimester of human development

Kallmann's syndrome (KS) is characterised by the association of anosmia and isolated hypogonadotrophic hypogonadism (IHH). Mutations of the KAL gene which is located at Xp22.3 cause X-linked KS (XKS). In this study we used the reverse transcriptase polymerase chain reaction and in situ hybridisation to examine the developmental expression of KAL in the first trimester of pregnancy, the earliest stage of human gestation examined thus far. At 45 days after fertilisation KAL mRNA was detected in the spinal cord, the mesonephros and metanephros but not in the brain. Later in gestation, at 11 weeks, the gene was expressed in the developing olfactory bulb, retina and kidney. This expression pattern correlates with the clinical findings in XKS since olfactory bulb dysgenesis with subsequent defective neural migration causes anosmia and IHH. Additionally, renal agenesis occurs in 40% of patients. Therefore this study provides strong evidence that KAL expression is required for the normal development of the olfactory bulb and kidney in the first trimester of human pregnancy.

Restoration of normal Hox code and branchial arch morphogenesis after extensive deletion of hindbrain neural crest

Among the derivatives of the cephalic neural crest is the ectomesenchyme which subsequently constitutes most of the craniofacial skeleton. There is evidence to suggest that the skeletogenic fate of the hindbrain neural crest is specified before emigration from the neural tube and that Antennapedia class Hox genes are involved in that process. To explore the putative causal link between Hox expression and craniofacial morphology, we produced a specific series of bilateral crest deletions in chick embryos and assessed branchial arch morphology, Hox gene expression, and patterning of skeletal structures in the postoperative embryo. Surprisingly, we found that deletion of the bulk of the rhombencephalic crest and substantial portions of the dorsal rhombencephalon did not prevent normal branchial arch morphogenesis and normal patterns of Hox gene (-A3 and -B4) expression 48 h after operation. Neural crest-like cells have been identified on crest migration pathways at the level of the original ablation, further confirming that ablated cephalic neural crest is replaced by regeneration from the cut edge of the neuroepithelium. Furthermore, in such embryos ectomesenchyme from regenerated crest is able to form a facial skeleton in which the mandible and hyoid apparatus are normal in size and organization. These findings demonstrate that the cranial neuroepithelium has more extensive regenerative capacities than was previously thought, which has important implications for investigations of craniofacial development.

Distribution of, and a putative role for, the cell-surface neutral metallo-endopeptidases during mammalian craniofacial development

Endopeptidase-24.11 (neutral endopeptidase, neprilysin, ‘enkephalinase’, EC 3.4.24.11) and endopeptidase-24.18 (endopeptidase-2, meprin, EC 3.4.24.18) are cell-surface zinc-dependent metallo-endopeptidases able to cleave a variety of bioactive peptides including growth factors. We report the first study of the cellular and tissue distribution of both enzymes and of the mRNA for NEP during embryonic development in the rat. Endopeptidase-24.11 protein was first detected at E10 in the lining of the gut and, at E12, the enzyme was present on the notochord, medial and lateral nasal processes, otocyst, mesonephros, heart and neuroepithelium. In contrast, at this time endopeptidase-24.18 was present only on the apical surface of the neuroepithelial cells. By E14 and E16, NEP was also detected in a wide range of craniofacial structures, notably the palatal mesenchyme, the choroid plexus, tongue and perichondrium. The distribution of endopeptidase-24.18 at these stages was restricted to the inner ear, the nasal conchae, and ependymal layer of the brain ventricles and the choroid plexus. Although endopeptidase-24.11 had been detectable in the craniofacial vasculature at E12 and E14, this was no longer apparent at E16. Significantly, the distribution of endopeptidase-24.11 mRNA closely matched the immunolocalization of the protein at all stages investigated. In order to explore the functional role of these enzymes, inhibition studies were carried out using two selective inhibitors of endopeptidase-24.11, phosphoramidon and thiorphan. E9.5 and E10.5 embryos exposed to either inhibitor displayed a characteristic, asymmetric abnormality consisting of a spherical swelling, possibly associated with a haematoma, predominantly on the left side of the prosencephalon, and the severity of this defect appeared to be a dose-dependent phenomenon. This study suggests that these enzymes play previously unrecognized roles during mammalian embryonic development.

Patterns of cell behaviour underlying somitogenesis and notochord formation in intact vertebrate embryos

We have made a detailed analysis of cell behaviour using high resolution time-lapse microscopy of the earliest cellular interactions taking place during morphogenesis of the notochord and somites in intact teleost embryos. Notochord formation is typified by active intercalation of paraxial mesenchyme cells into the lateral surfaces of the primordium. Following this recruitment phase, complete immiscibility develops between cells of the notochord and the presomitic mesenchyme. Dorso-ventral and rostro-caudal expansion of the notochord is characterised by translocation of cells within dorso-ventral planes of section and is supported by elongation of the remaining cells and reduction in width across its latero-medial axis. A lateral palisading of paraxial mesenchyme against the lateral aspects of the notochord precedes overt segmentation. Intersomitic furrows form by localised de-adhesion at small foci at the nascent intersomitic planes, which are consolidated by coalescence of such areas by de-adhesion to produce the interface. It is not possible to predict precisely where cells would initiate de-adhesion since there is a stochastic element to the phenomenon. Once formed, boundaries between somites are stable and provide no opportunity for mixing, except across the first formed furrow, which disintegrates at the 4-6 somite stage. The first ten somites form at a constant rate of 2.3 somites/hr, during which time we recorded constant relative displacement of the segmental plate against the rostro-caudally elongating notochord. Unlike teleost epiboly and gastrulation, no large-scale movements of individual cells can be detected during elaboration of the embryonic axis.

Behaviour of fibroblasts during initial attachment to a glass-ceramic implant material in vitro: a time-lapse video-micrographic study

The in vitro behaviour of 3T3 fibroblasts plated on a glass-ceramic implant material, Apoceram, and on tissue culture polystyrene (TCP) was monitored using phase-contrast, time-lapse video microscopy and scanning electron microscopy. Significant differences were observed in the short-term cellular response to TCP and Apoceram in the timing of the fibroblasts' morphological changes, with earlier spreading and stabilization of the cells occurring on TCP. Cell surface ruffling activity was greater on the glass-ceramic, reflecting weaker adhesions to the substratum. The experimental method used facilitates direct analysis of the dynamic activity of cells attaching to artificial substrata.

Regenerative capability of upper and lower jaws in the newt

The regenerating amphibian jaw represents an important model for studying pattern formation and the mechanisms underlying regeneration of facial structures. We have studied regeneration of upper and lower jaws in the urodele amphibian, Notophthalmus viridescens, using whole mount preparations stained for bone and cartilage, scanning electron microscopy and immunocytochemistry to further characterize these regenerating systems. In addition, we have investigated whether lower jaws of adults and larvae display similar regenerative ability. Although in adult animals the original shape of both the lower and upper jaws is rather faithfully reproduced following amputation, and the teeth and oral mucosa with its specialized sensory organs fully regenerate, significant differences in the regenerative ability of the various skeletal elements are observed. In fact, only tooth-bearing skeletal elements ossify, while the other elements of the regenerated skeleton remain cartilaginous for as long as 5 months after amputation. In contrast, a regenerated lower jaw in the larva is indistinguishable from an unamputated one at the same stage of development. Interestingly, regenerating adult jaws form directly bicuspid teeth, which are the type of teeth normally found in the adult, rather than the monocuspid teeth characteristic of larval jaws, indicating that jaw regeneration is not a recapitulation of development, in that an adult jaw blastema directly regenerates an adult jaw. Finally, we have studied the expression of tissue specific markers in normal and regenerating upper and lower jaws to establish whether the blastemal cells, which will form the missing part of the jaw, express any of these markers of the differentiated state, or are undifferentiated as suggested by their morphological appearance. Under our experimental conditions, no expression of markers of the differentiated state, such as those for muscle, cartilage and glands is detectable in early regenerates. On the contrary, the mesenchymal marker 22/31, whose expression in normal jaws is restricted to dermal fibroblasts and the dental pulp, is expressed in at least a half of the blastemal cells. The significance of these observations in relation to the origin of blastemal cells in the jaw will be discussed.

Heads and tales: recent advances in craniofacial development

In the last few years, our understanding of the complex series of events governing craniofacial morphogenesis has significantly increased. A variety of experimental approaches, such as cell lineage marking, monoclonal antibodies, organ culture and recombinant DNA techniques, have been fundamental to this progress. The neural crest contribution to the head and face of the embryo has been mapped, the significance and mechanisms of epithelial-mesenchymal interactions are better understood, and a number of regulatory molecules, which are likely to be causally involved in mediating the morphogenesis of the different craniofacial regions, have been identified. Although the information presently available on the molecules involved is far from complete, and the experimental strategies need further refinement, the existence of specific combinatorial gene-codes (ie the homeobox-containing gene code) has become evident. These findings are proving invaluable to our understanding of the mechanisms underlying craniofacial development, and of certain syndromes affecting craniofacial structures in humans.

Harderian gland hyperplasia in c-mos transgenic mice

Transgenic mice carrying the mouse mos proto-oncogene linked to a retroviral LTR develop hyperplasia of the Harderian glands. Enlargement of the glands is evident as early as 18 weeks after birth, with glands reaching up to 10 times their normal weight. Approximately 65% of the cases of hyperplasia occur bilaterally, and the majority of mice affected are male (66%). Elevated levels of mos expression are found in all Harderian glands of mice from the affected transgenic line, but not in glands of normal mice or a non-affected transgenic line, indicating that hyperplasia is dependent on mos expression. Histological examination of the tissue reveals a general involvement of the entire gland epithelium in hyperplastic growth, with no evidence of focal or malignant tumours. These observations show that in addition to neu, myc, ras and ret transgenes, mos, a member of the protein-serine/threonine kinase family of oncogenes, can induce Harderian gland hyperplasia, thus revealing an unusual response by this organ to various classes of oncogenes. Analysis of fos, jun, myc and ets oncogene RNA in mos-induced hyperplastic Harderian glands shows that there are no consistent changes in the level of expression of these oncogenes, suggesting that mos acts via a mechanism other than by increasing the expression of these genes.

Hypermedia in biomedical education: a case study

The origination, development and application of a hypermedia system for use in the life sciences is described. Implemented in the Apple Macintosh HyperCard environment, the project has been designed to make use of commercially produced laser discs as visual archives and has included the production of authoring and navigational tools. We report here on our experience in authoring stacks for undergraduate teaching, monitoring of student use and assessment of effectiveness as a self-teaching resource. Future developments and applications within the biomedical sciences are discussed.

Monoclonal antibodies raised against pre-migratory neural crest reveal population heterogeneity during crest development

In order to address the problem of when heterogeneity arises within premigratory and early migratory neural crest cell populations, mouse monoclonal antibodies were raised against quail premigratory neural crest. Due to the limited availability of immunogen an intrasplenic route for immunization was used. Three monoclonal antibodies (referred to as LH2D4, LH5D3 and LH6C2) were subsequently isolated which recognized subpopulations in 24 h cultures of both quail and chick mesencephalic and trunk neural crest in immunocytochemical studies. Subsequent investigations using a range of six antibodies, including LH2D4, LH5D3 and LH6C2, showed that population heterogeneity (which was not cell cycle related) could be detected as early as 15 h following mesencephalic crest explantation, a stage at which all the neural crest cells were morphologically identical. However, premigratory neural crest from the same axial level of origin was homogeneous, as judged by immunoreactivity patterns with these antibodies. Significant differences were found in the proportion of immunoreactive cells between populations of mesencephalic and trunk neural crest cultures. Double immunofluorescence studies revealed the existence of at least four separate cell populations within individual crest cultures, each identified by their unique antibody reactivity pattern, thus providing some insight into the underlying complexity of subpopulation composition within the neural crest. Immunocytochemical studies on quail embryos from stages 7-22 showed that the epitopes detected by LH2D4, LH5D3 and LH6C2 were not necessarily confined to the neural crest or to cells of crest derivation. All three epitopes displayed a spatiotemporal regulation in their expression during early avian ontogeny. Since the differential epitope expression described in this investigation was detectable as early as 15 h after premigratory neural crest explantation, took place in vitro in the absence of any other cell type and changed progressively with time, we conclude that a certain degree of population heterogeneity can be generated very early in neural crest ontogeny and independently of the tissue interactions that normally ensue in vivo.

The transient expression of type II collagen at tissue interfaces during mammalian craniofacial development

Using immunocytochemical techniques, the spatiotemporal distribution of the major collagen isoform of cartilage, type II collagen, has been investigated during early craniofacial development in the mouse embryo. Early and transient expression was associated with the otic and optic vesicles, the ventrolateral surfaces of the developing brain, olfactory conchi, endocardial and mesocardial tissues, the lateral and basal surfaces of the pharyngeal endoderm and beneath the ectoderm of the branchial arches. A number of these locations are sites of epithelial-mesenchymal tissue interaction believed to generate the component parts of the chondrocranium; here, type II collagen appears transiently in advance of overt chondrogenesis in the mesenchyme. At such sites, immunofluorescence is typically localised along the basal surface of the epithelial partner, with the strongest reaction detected between the basal aspects of the otic and rhombencephalic epithelia. Immunoelectron microscopy, using pre-embedding immunostaining and a protein G-gold technique, reveals that the type II collagen is adjacent to, but not integral with, the basal laminae. Gold particles are clearly associated with 10–15 nm fibrils of the extracellular matrix in the reticulate lamina region. The pattern of type II collagen expression in the mouse closely correlates with that demonstrated previously in the quail, indicating a high degree of phylogenetic conservation between these two vertebrate species. These findings are consistent with the hypothesis that the pattern of epithelial secretion of type II collagen, or a coexpressed matrix molecule, constitutes a morphogenetic signal, realised as a matrix-mediated tissue interaction, and specifying the form of the vertebrate chondrocranium. Three-dimensional reconstruction of early type II collagen distribution, and of the subsequent chondrocranial cartilages, reveals that chondrocranial form can be derived from a ‘pre-pattern’ of epithelially derived type II collagen expressed at epithelial-mesenchymal tissue interfaces.

Preface

How a single cell, the fertilized egg, gives rise to a new individual is one of the most challenging problems in Biology. In a remarkable effort, it has been possible to trace the fate of every cell during development of the determinate embryo of the nematode, which is a relatively simple animal. However, this strategy presents an overwhelming task for vertebrate development and instead attention has typically been focused on crucial stages of development or the generation of specific structures. In the apparently indeterminate embryos of higher vertebrates it may not be necessary or even realistic to give the same detailed attention to the development of every constituent body part, because the same generative processes will be involved repeatedly and will be mediated by cells that have a rather limited repertoire of behaviour.

The developmental specification of the vertebrate skull

The initial form of the embryonic bony skull is determined in two ways; cranially, by the relative growth of the developing brain, and facially, by the chondrocranium. Both are essentially acting as structural templates around which the bony components of the skull are assembled. Assuming, therefore, that the specification of form and pattern in the facial skeleton occurs at the formation of the chondrocranium, this paper will focus on precisely how the chondrocranium forms. Any acceptable explanation of chondrocranial morphogenesis must satisfy at least two prerequisites. First, given the constancy of chondrocranial form in vertebrates, any model proposed should be equally applicable to all vertebrates. Second, it should enable us to answer questions of homology concerning the skull and, in particular, provide explanation for those instances where ‘homologous’ structures have a different (lineage) composition.

From studies limited to a small number of amphibian, avian and mammalian species, it is apparent that chondrogenesis in the vertebrate skull is largely, if not entirely, elicited by epitheliomesenchymal tissue interactions. Analysis of such interactions (and of those promoting osteogenesis) reveals that these are matrixmediated and, recently, the expression of certain ‘relevant’ matrix components has been shown to be developmentally regulated in a fashion that correlates with the location and timing of these interactions. From these, and related, observations a morphogenetic model, the so-called ‘Flypaper Model’, has been proposed to explain the specification of chondrocranial form. A number of predictions arising from that model are currently being tested experimentally and the current status of the model is reviewed. Finally, the ability of this model to satisfy the prerequisites defined above is assessed.

An ultrastructural and morphometric analysis of an in vivo contact guidance system

The pectoral fin bud of the developing teleost embryo contains a highly ordered extracellular matrix of collagenous fibrils, called ‘actinotrichia’. During invasion of the fin fold, mesenchymal cells, migrating distally from the base of the fin, become contact aligned by the actinotrichial fibrils. Behavioural aspects of this response have previously been studied using Nomarski differential interference contrast microscopy and time-lapse video recording (Wood & Thorogood, 1984). Here we present an ultrastructural description of these cells and their matrix associations and a computer- based morphometric analysis of selected parameters within the migration substratum, relevant to this in vivo ‘contact guidance’ phenomenon. The study shows that a differentiated and aligned matrix of actinotrichial fibrils can be detected before invasion of the fin fold, at levels up to 40μm distal to the advancing mesenchymal cell margin. Subsequently, during invasion of the fin fold, aligned mesenchymal cells and processes are almost exclusively associated with actinotrichia and not the intervening surface of the epithelial basal lamina. However, aligned cell processes appear to avoid the smaller actinotrichia and at late stages of development 87á0% of actinotrichia without cell process contacts are distributed at the lower end of the size range. Study of cell ultrastructure revealed a complete absence of cytoskeletal organization within this mesenchymal cell population, although cytoskeletal components are clearly visible in adjacent epithelia. The computer-based morphometric survey of the migration substratum has shown a gradual but progressive increase in the mean diameter of actinotrichia at a level at which distal cell processes are first detectable in sections of fins. However, at similar levels over the same period the mean value for interactinotrichial spacings remained virtually constant. These results suggest that the spacing between actinotrichia is not significant in contributing to progressive changes in mesenchymal cell phenotype, but that the actinotrichia themselves are strongly implicated in providing the guidance cues to direct cell migration within the developing fin and the initiation of cell migration. These findings are discussed in the general context of cell movement and contact guidance both in vivo and in vitro.

Autonomy of differentiation in avian branchial somites and the influence of adjacent tissues

This study investigates the differentiative abilities of avian brachial somites at stages of development before, during and after the migration of somitic cells into the wing primordium. These somites are the source of cells that migrate into the forelimbs and there give rise exclusively, and totally, to the skeletal muscle lineage of the wing and yet show no morphological evidence of commitment to that fate when they leave the somites. The aim of the study was to see if the brachial somitic cells are committed to particular developmental pathways at these stages. The brachial somites were isolated from HH stage-12, -15 and -18 chick embryos, either by microdissection or enzymatic dissociation, and grown in organ culture, in explant culture on different substrata or on the chorioallantoic membrane (CAM) of host chicks, either alone or in combination with adjacent tissues. Myogenesis and chondrogenesis occurred in all stage-18 enzymatically separated somites, regardless of the growth environment. Myogenesis was reduced in stage-15 somites and unobservable in stage-12 somites; however, recombination of stage-12 somites with epithelium or neural tube increased the incidence of myogenesis at this stage. The incidence of chondrogenesis was also less in the younger explants. Unlike its effect on myogenic expression, recombination with epithelium resulted in a dramatic decrease in chondrogenesis in both stage-12 and -15 somites. The recombination experiments suggest that conditions that maintain the normal spatial relationships within isolated somites permit expression of a preexisting specification to a particular fate. They also show that the overlying epithelium can inhibit chondrogenesis in these somites. Overall, the results suggest that by the time migration of somitic cells into wing regions is finishing, brachial somitic cells have become stabilized in their ability to undergo both myogenesis and chondrogenesis for they will do so under a variety of growth conditions and independently of adjacent tissues. However, immediately before (stage 12) and shortly after (stage 15) the onset of migration, both myogenic and chondrogenic expression by brachial somitic cells are still under the influence of interactions with adjacent tissues.

Analysis of in vivo cell movement using transparent tissue systems

The embryos of certain teleost species are transparent and cell behaviour within the intact embryo can be observed and recorded using Nomarski microscopy coupled with time-lapse video recording or time-lapse cine filming. In this report we review some of our recent analyses of cell behaviour patterns underlying key morphogenetic events. (1) Contact-guided cell migration through a structurally ordered extracellular matrix during fin development; (2) movement of tissue layers during epibolic overgrowth; and (3) cell 'social' behaviour during the establishment of the body axis (i.e. notochord formation and somitogenesis). These results, on cell behaviour correlated with normal morphogenesis, provide a baseline for further work in which hypotheses concerning subcellular and molecular controls of cell behaviour can be tested by experimental perturbation in vivo.

Transient expression of collagen type II at epitheliomesenchymal interfaces during morphogenesis of the cartilaginous neurocranium

In the avian embryo a matrix-mediated tissue interaction between retinal pigmented epithelium and neural crest-derived periocular mesenchyme leads to the differentiation of (scleral) cartilage. The composition of the extracellular matrix at the interface between these two tissues has been examined immunohistochemically, both during and after the interaction has taken place. Of the matrix components studied (fibronectin, laminin, and collagen types I, II, IV, and V) only collagen type II displayed a dramatic change in distribution between the two stages. During the interaction, at stage 15, type II was present in the extracellular compartment basal to the epithelium. After completion of the interaction, collagen type II was no longer detectable at the interface even though it was readily detectable in the vitreous humor, cornea, and perinotochordal sheath, and subsequently will be expressed by the chondrogenic tissue itself as overt differentiation commences. These results suggest that collagen type II might be causally involved in this particular epitheliomesenchymal interaction. Examination of the spatial and temporal patterns of collagen type II expression elsewhere in the developing craniofacial complex revealed a hitherto unreported pattern of distribution. In addition to its predictable locations (i.e., cornea, vitreous, and perinotochordal sheath) it was found to be present at certain other sites, for example, at the basal surfaces of some neuroepithelia. These additional locations are all known to be sites of chondrogenesis-promoting tissue interactions which result in the formation of the elements of the cartilaginous neurocranium (e.g., otic vesicle). Furthermore this spatial distribution exhibits a changing temporal pattern in that it is detectable at the time that the interactions are known to be taking place, but subsequently is no longer detectable by the immunohistochemical means employed. This definable pattern of transient collagen type II expression, occurring at very early stages of craniofacial development, is interpreted as reflecting one level of morphogenetic specification of chondrocranial/skull form in the developing vertebrate head.

Specific thromboxane synthetase inhibition and retinopathy in insulin-dependent diabetics

We have previously reported that specific thromboxane synthetase inhibition may be associated with a reduction in albumin excretion rate in insulin-dependent diabetics (IDD). We now report studies of retinal morphology assessed by serial fluorescein angiography in 25 insulin-dependent diabetics during a 16-week double-blind, randomized, placebo-controlled study of the specific thromboxane synthetase inhibitor UK-38,485. Assessment of angiograms was by, computerized image analysis indicating the percentage area of vascularization and, panel scoring by 4 ophthalmologists who scored the posterior pole in each of the 4 angiograms "blind" according to a prearranged system. There was no significant change in percentage area of vascularization in either placebo or "active" group during the study. Panel scoring, however, suggested that some patients deteriorated whilst others remained unchanged or improved. There was, however, no significant difference in these parameters between those subjects on UK-38,485 and those on placebo. We conclude that specific thromboxane synthetase inhibition is not associated with any significant change in diabetic retinopathy over a 4-month period of study.

Contact behaviour exhibited by migrating neural crest cells in confrontation culture with somitic cells

When grown in confrontation culture on a planar substratum, avian neural crest cells and somite cells display both homotypic and heterotypic contact inhibition of movement as judged by analysis of time-lapse video recordings of locomotory and contact behaviour, and by use of a nuclear overlap assay. It is therefore unlikely that migration of neural crest cells within the embryo, and within embryonic tissues, can be explained on the basis of a lack of contact inhibition. The results are discussed in the general context of cell invasiveness.

An analysis of in vivo cell migration during teleost fin morphogenesis

In the teleost embryo the pectoral fin bud initially displays an apical ectodermal ridge along its entire distal margin. The ridge subsequently becomes transformed into an apical fold as the distal ectodermal epithelium grows and folds to enclose an extracellular space between the apposed basal surfaces of the epithelium. Collagen fibrils up to 2 micron in diameter, termed ‘actinotrichia’, are deposited along the proximo-distal axis in two (dorsal and ventral) arrays. The actinotrichia are aligned parallel to one another with a regular spacing along the greater part of their length. Mesenchymal cells migrating distally from the base of the fin bud encounter the dorsal and ventral arrays of actinotrichia and move between them apparently using the fibrils as a substratum. The entire structure is transparent and, using the killifish Aphyosemion scheeli, we have investigated the migration of the mesenchymal cells between 135 and 220 h of development, using Nomarski interference contrast microscopy and time-lapse video recording. The number of cellular processes per cell increased significantly during the period of observation. These processes could be graded according to their diameters. Processes of diameter greater than 2 micron were not usually aligned along actinotrichia and arose at any aspect of the cell body. In contrast, processes with diameters less than 2 micron appeared to be confined to the distal aspects of the migrating cells and showed an increasing tendency to become aligned as development progressed. Time-lapse video recordings revealed that such aligned processes move faster (mean speed 17.98 (+/− 2.25) micron/h) than non-aligned processes (mean speed 4.66 (+/− 0.67) micron/h). Whole cell translocation was generally slower than rates of process movement: the lowest mean value (1.52(+/− 0.36) micron/h) was recorded between 135 and 160 h of development rising to a maximum mean rate (4.72(+/− 0.42) micron/h) between 195 and 220 h; the period of the fastest rate of cell translocation correlated with maximum process alignment along actinotrichia. Thin 1 micron plastic sections revealed that, generally, aligned processes were in close association with the surface of the actinotrichial fibrils and not the spaces between them.

Transfilter studies on the mechanism of epitheliomesenchymal interaction leading to chondrogenic differentiation of neural crest cells

Interaction with an epithelium is a prerequisite for avian cranial neural crest (NC) cells to differentiate into cartilage and bone (Bee & Thorogood, 1980). In order to investigate the causal mechanism we have selected one such interaction - that between mesencephalic NC and retinal pigmented epithelium (RPE) for further study. Premigratory NC cells were grown transfilter to RPE explants of different developmental ages and on Nuclepore filters of different pore size which either allowed or prevented penetration by cell processes. Initial scanning electron microscopy (SEM) observations established that pores of 0.8 microns allowed the passage of cell processes through the filter whereas 0.2 microns pores did not. The transfilter experiments demonstrated that chondrogenic differentiation of NC cells will occur only if the Nuclepore filters have a pore size large enough to permit the passage of cell processes. Furthermore SEM observations established that cell processes do traverse the Nuclepore filter when NC and RPE are grown in transfilter combination. The results indicate that the mechanism is not mediated by diffusable factors but rather is mediated either by direct contact between NC cells and non-diffusable matrix closely associated with RPE or by direct plasmalemmal contact between RPE and NC cells through discontinuities in the basement membrane. The results of these experiments also demonstrate that younger (stage 17) RPE is more effective at eliciting chondrogenesis from premigratory NC cells than older (stage 24) RPE and that the interaction between RPE and NC cells is a prolonged one, taking place over days rather than within hours. Both of these in vitro observations are compatible with the timing of events leading to scleral cartilage formation in vivo.

Effects of vitamin A on the behaviour of migratory neural crest cells in vitro

It has been proposed elsewhere that the teratogenic effects of retinoids on craniofacial morphogenesis are caused by a disturbance of the migration of cranial neural crest cells. The effects of 3.5 X 10(−5) M and 3.5 X 10(−6) M-retinol on the migration of avian neural crest cells in vitro have been investigated by monitoring cell morphology, locomotory behaviour, fibronectin distribution and actin-microfilament organization. Retinol retards migration by affecting cell-to-substratum adhesiveness. Cells exposed to medium containing retinol are less adherent to the substratum, and although the cell surface is very mobile, are unable to extend or maintain lamellipodia. As a consequence the cells do not actively translocate. Fibronectin distribution at the cell surface is sparse, possibly as a result of shedding, and actin distribution remains diffuse. At the retinol molarities used all these effects are reversible. Thus cells allowed to recover in normal medium flatten out, display lamellipodia and commence active translocation. Fibronectin becomes organized into a fibrillar array and actin microfilaments become organized into cables. The period needed for this recovery is directly related to the molarity of retinol during the initial exposure; after recovery the retinol-treated cells are virtually indistinguishable from control cells. We propose that in vivo the effects of retinoids might be to impair cell-extracellular matrix interaction, thus impeding a cell's ability to migrate through that matrix. Contrary to previous suggestions, the in vivo effects are probably not in any way ‘specific’ to neural crest cells but are more accurately considered as ‘selective’, in that any cell undergoing migration would be similarly affected.

In vitro studies on skeletogenic potential of membrane bone periosteal cells

In the avian embryo ectomesenchyme cells, derived from the mesencephalic level of the cranial neural crest, migrate into the presumptive maxillary region and subsequently differentiate into the membrane bones and associated secondary cartilage of the upper jaw skeleton. The cartilage arises secondarily within the periosteum at points of articulation between membrane bones and provides an embryonic articulating surface. The stimulus for the differentiation of secondary cartilage is believed to be intermittent pressure and shear created at the developing embryonic movement. The development of one such system--the quadratojugal, has been analysed using organ and explant culture techniques and studied with particular reference to the differentiation of periosteal cells into secondary cartilage. A number of conclusions were reached. (1) Normally only cells at discrete loci express a chondrogenic potential in vivo: the periosteal cells at these sites of future articulation become committed to chondrogenesis during stage 35, more than 24 h before cartilage is identifiable in vivo. (2) However, cells with a 'latent' chondrogenic potential are widespread in membrane bone periosteum and occur over most, if not all, of the surface area of the bone. This potential is expressed in the 'permissive' environment created by submersion of the tissue in explant culture or in submerged organ culture. (3) This chondrogenic potential exists long before the time at which commitment of cartilage-forming cells occurs and even presumptive maxillary ectomesenchyme at stage 29 has a limited ability to form cartilage in vitro. It is suggested that spatial position is a principal factor controlling the differentiation of secondary cartilage. Ectomesenchyme cells with the potential to form secondary cartilage are widespread but it is only those cells whose migration from the neural crest positions them and their progeny at the site of a presumptive joint which subsequently express this potential. This epigenetic interpretation is discussed in the general context of development mechanisms underlying the spatial and temporal patterns in which neural crest-derived cells differentiate to produce bone and cartilage during the formation of the head skeleton.

Lactate dehydrogenase isoenzyme transitions during skeletal differentiation in the mouse embryo

(1) In the mouse embryo there are changes in lactate dehydrogenase activity and isoenzyme pattern during the differentiation of cartilage and bone. (2) The specific activity of lactate dehydrogenase rises during chondrogenesis and falls during osteogenesis. (3) Identical isoenzyme transitions occur in parallel in both tissues: undifferentiated limb bud mesenchyme contains isoenzymes 1-5 whereas in both the cartilaginous and bony portions of a long bone developing from the mesenchyme, there is a progressive shift towards a predominance of the 'anaerobic' isoenzymes 4 and 5.

Imidazole: a selective inhibitor of thromboxane synthetase

Imidazole inhibits the enzymic conversion of the endoperoxides (PGG2 and PGH2) to thromboxane A2 by platelet microsomes (IC50: 22 MICRONG/ML; DETERMINED BY BIOASSAY). The inhibitor is selective, for prostaglandin cyclo-oxygenase is only affected at high doses. Radiochemical data confirms that imidazole blocks the formation of 14C-thromboxane B2 from 14C-PGH2. Several imidazole analogues and other substances were tested but only 1-methyl-imidazole was more potent than imidazole itself. The use of imidazole to inhibit thromboxane formation could help to elucidate the role of thromboxanes in physiology or pathophysiology.