Radiation-induced apoptosis and limb teratogenesis in embryonic mice

In utero irradiation of the fetus during the period of organogenesis induces a dramatic increase in malformation. However, the mechanisms underlying the teratogenesis remain to be elucidated. In the present study, the correlation between radiation-induced apoptosis and limb malformation was examined in mice. The mice were exposed to X rays in utero on day 11 of gestation during the period of organogenesis of limb buds. A marked increase in the number of apoptotic cells in the predigital regions in the forelimb buds was detected 4 h after irradiation. The preinterdigital regions of the forelimb buds did not show such an increase at the same time. Aphlangy and ectrodactyly were the main types of anomalies observed on day 19 in the limbs of the fetuses irradiated with 5 Gy. The increases in prenatal death and teratogenesis in limb digits in living fetuses were dependent on dose. The possible mechanisms involved are discussed.

Expression patterns of Notch1, Serrate1, Serrate2 and Delta1 in tissues of the developing chick limb

Signalling via the receptor Notch, delivered by the ligands Delta and Serrate, plays a key role in many cell fate decisions in both Drosophila and vertebrate development (for review seeArtavanis-Tsakonas, S., Matsuno, K. and Fortini, M.E., 1995. Notch signalling. Science 268, 225-232; Lewis, J., 1996. Neurogenic genes and vertebrate neurogenesis. Curr. Opin. Neurobiol. 6, 3-10; Blair, S.S., 1997. Limb development: marginal fringe benefits. Curr Biol. 7, 686-690; Irvine, K.D. and Vogt, T.F., 1997. Dorsal-ventral signaling in limb development. Curr. Opin. Cell Biol. 9, 867-876). Recently vertebrate homologues of Notch (Notch1; Myat, A., Henrique, D., Ish-Horowicz, D. and Lewis, J., 1996. A chick homologue of Serrate and its relationship with Notch and Delta homologues during central neurogeneis. Dev. Biol. 174, 233-247) and Serrate (Serrate1 and 2; Myat, A., Henrique, D., Ish-Horowicz, D. and Lewis, J., 1996. A chick homologue of Serrate and its relationship with Notch and Delta homologues during central neurogeneis. Dev. Biol. 174, 233-247; Hayashi, H., Mochii, M., Kodama, R., Hamada, Y., Mizuno, N., Eguchi, G. and Tachi, C., 1996. Isolation of a novel chick homolog of Serrate and its coexpression with Notch-1 in chick development. Int. J. Dev. Biol. 40, 1089-96; Laufer, E., Dahn, R., Orozco, O.E., Yeo, C.Y., Pisenti, J., Henrique, D., Abbott, U., Fallon, J.F. and Tabin, C., 1996. Expression of Radical fringe in limb-bud ectoderm regulates apical ectodermal ridge formation. Nature 386, 366-373; Rodriguez-Esteban, C., Schwabe, J.W., De La Pena, J., Foys, B., Eshelman, B. and Izpisua-Belmonte, J.C., 1997. Radical fringe positions the apical ectodermal ridge at the dorsoventral boundary of the vertebrate limb. Nature 386, 360-366) were shown to be expressed in early chick limb mesenchyme and apical ridge. However, later expression patterns of these genes and of Delta 1 (Henrique, D. , Adam, J., Myat, A., Chitnis, A., Lewis, J. and Ish-Horowicz, D., 1995. Expression of a Delta homologue in prospective neurons in the chick. Nature 375, 787-790) in vertebrate limbs have not been documented. We have used whole mount in-situ hybridization to document expression patterns of Notch1, Serrate1, Serrate2 and Delta1 within the mesenchyme of the developing chick limb up to stage 31 of development. We show these genes are expressed, in different combinations, in the vasculature, the musculature and the tissues of the handplate.

Differential in situ expression of alpha2(XI) collagen mRNA isoforms in the developing mouse

Type XI collagen is an essential structural component of the extracellular matrix of cartilage and plays a role in collagen fibril formation and skeletal morphogenesis. The expression of all three type XI collagen genes is not restricted to cartilage. In addition, alternative exon usage seems to increase the structural diversity and functional potential of type XI collagen during development. In order to investigate type XI collagen expression during development, we have examined alpha2(XI) and alpha1(XI) collagen genes by in situ hybridization in mice. Transcripts of the alpha2(XI) collagen gene were first detected in the notochord of mouse embryos after 11.5 days of gestation. Subsequently, alpha2(XI) mRNA was mainly found in the cartilaginous tissues of the developing limbs and axial skeleton together with transcripts of the alpha1(XI) gene. The alpha2(XI) transcripts seemed to be alternatively spliced isoforms lacking exons 6-8, which code for an acidic domain. Expression of alpha2(XI) outside the cartilage was relatively restricted, whereas expression of the alpha1(XI) gene was widespread. However, expression of alpha2(XI) transcripts containing exons 6-8 was found in non-chondrogenic tissues, including the calvarium and periosteum where intramembranous ossification occurs. These results indicate that alpha2(XI) mRNA isoforms are differentially expressed in various tissues during development. In addition, alpha2(XI) mRNA isoforms containing alternative exons are present in osteogenic cells, and their expression may be closely related to the formation of bone or cartilage.

Limb development and evolution: a frog embryo with no apical ectodermal ridge (AER)

The treefrog Eleutherodactylus coqui is a direct developer--it has no tadpole stage. The limb buds develop earlier than in metamorphosing species (indirect developers, such as Xenopus laevis). Previous molecular studies suggest that at least some mechanisms of limb development in E. coqui are similar to those of other vertebrates and we wished to see how limb morphogenesis in this species compares with that in other vertebrates. We found that the hind limb buds are larger and more advanced than the forelimbs at all stages examined, thus differing from the typical amniote pattern. The limb buds were also small compared to those in the chick. Scanning and transmission electron microscopy showed that although the apical ectoderm is thickened, there was no apical ectodermal ridge (AER). In addition, the limb buds lacked the dorsoventral flattening seen in many amniotes. These findings could suggest a mechanical function for the AER in maintaining dorsoventral flattening, although not all data are consistent with this view. Removal of distal ectoderm from E. coqui hindlimb buds does not stop outgrowth, although it does produce anterior defects in the skeletal pattern. The defects are less severe when the excisions are performed earlier. These results contrast with the chick, in which AER excision leads to loss of distal structures. We suggest that an AER was present in the common ancestor of anurans and amniotes and has been lost in at least some direct developers including E. coqui.

Hoxb-8 has a role in establishing early anterior-posterior polarity in chick forelimb but not hindlimb

The distribution of Hoxb-8 transcripts through the chick flank and early forelimb mirrors the distribution of polarizing activity in the flank at these early stages. Polarizing activity displayed by Hoxb-8-expressing tissue is only realised when placed adjacent to the AER and appears to be mediated through Shh induction, suggesting that Hoxb-8 may lie genetically upstream of Shh. Accordingly, Hoxb-8 expression is rapidly induced by retinoic acid (RA) treatment in the anterior of the forelimb in a spatial and temporal manner that is consistent with the induction of Shh and formation of the ZPA. Furthermore, inhibition of RA synthesis in the flank downregulates the expression of endogenous Hoxb-8 and results in the loss of Shh expression. However, once the ZPA has become established the posterior limb mesoderm displays resistance to the induction of Hoxb-8 expression. Grafting of ZPA cells to the anterior of a host limb renders the host anterior tissue resistant to RA-induced Hoxb-8 expression. These results indicate that Hoxb-8 expression may be regulated by the established ZPA through a negative feedback loop. The anterior AER also secretes an inhibitory factor, preventing RA-induced or already established Hoxb-8 expression in the cells immediately underneath the AER. Consistent with a role for Hoxb-8 in positioning of the forelimb ZPA, Hoxb-8 expression is not seen in RA-induced duplications at the anterior of the hindlimb. However, grafting of Hoxb-8-expressing tissue to the hindlimb can lead to Shh expression and similar duplications, suggesting that factors mediating ZPA formation are very similar in both wing and leg.

Recent molecular advances in understanding vertebrate limb development

Considerable recent advances have been made in understanding the mechanisms of vertebrate limb development. New information about molecules governing cell interactions in embryonic limbs begins to bridge the gap between the experimental analysis and genetics of congenital limb defects. There are four main stages in vertebrate limb development: initiation, specification of limb pattern, tissue formation accompanied by limb morphogenesis, and growth. Although classical embryology focused on chick embryos and recent molecular analysis centres on limbs of both chickens and mice, most of the fundamental mechanisms that have been uncovered appear to be conserved between vertebrates and are likely to be directly applicable to human limb development.

Abnormal cutaneous nerve outgrowth: a secondary effect of methyl triazene teratology in developing mouse limb buds

The cutaneous branches originating from the superficial distal division of the ulnar nerve exhibit abnormal developmental features in 70.2% of the forelimb buds from embryos submitted 2 or 3 days earlier to methyl triazene administered to their pregnant mother. Similar abnormalities characterize the preaxial ventral cutaneous nerve of the thumb in 17.8% of forelimb primordia. The affected nerves undergo anticipated growth with respect to the normal schedule, follow abnormal pathways through areas of extensive cell death, and finally reach the apical ectoderm where they run in close contact with the basement membrane without forming a plexus. Histological observations gathered in pyronin-methyl green stained serial sections as well as in whole limb buds after cholinesterase method suggest that three factors probably contribute to modify nerve outgrowth: (1) a discrepancy between the rate of nerve progression and that of mesodermal growth in the prospective zeugopod territory which is preferentially affected by the teratogen; (2) facilitated nerve pathfinding into areas strongly hit by triazene-induced mesodermal cell death; and (3) alteration or abolition of some unknown ectodermal influence necessary to stimulate selective guidance of terminal sensory afferents and to maintain them transiently at some distance from the epidermis.

Murine Wnt10a and Wnt10b: cloning and expression in developing limbs, face and skin of embryos and in adults

Wnt genes encode a family of secreted proteins having oncogenic potential and important roles in developmental processes. We report the isolation of cDNAs for a novel murine Wnt gene, Wnt10a, and for two alternatively spliced transcripts of Wnt10b, as well as analyses of the expression patterns of these genes in adult and embryonic tissues. Wnt10a and full length Wnt10b encode relatively closely related (62% identity) Wnt proteins containing the 24 cysteines characteristic of most Wnt proteins. The deduced protein encoded by the alternatively spliced Wnt10b transcript lacks a central region encompassing 25% of the full length isoform. Both Wnt genes were widely expressed in the adult, although the levels of expression varied significantly. Wnt10a RNA was most abundant in adult brain with a high concentration in the pituitary gland, and expression of Wnt10b was highest in adult lung and uterus. RNAs of both genes were detected in thymus and spleen, tissues not previously reported as sites for Wnt gene expression. Wnt10a and Wnt10b expression broadly peaked at approximately 13.5 and 15.5 days of gestation, respectively, and Northern blot analyses of RNAs from dissected embryos revealed that expression of both genes was strongest in the face, limbs and skin and that Wnt10a was also expressed in the liver, the major site of hematopoiesis in the embryo. These expression patterns suggest that Wnt10a and Wnt10b may function in various tissues of the adult and embryo with notable expression in tissues responsible for the formation and maturation of blood cells, in the pituitary gland (Wnt10a), and in the face, limbs and skin of developing embryos.

Hoxa-13 and Hoxd-13 play a crucial role in the patterning of the limb autopod

Members of the Abdominal-B-related Hox gene subfamily (belonging to homology groups 9 to 13) are coordinately expressed during limb bud development. Only two genes from homology group 13 (Hoxa-13 and Hoxd-13) are specifically expressed in the developing distal region (the autopod), which displays the most complex and evolutionarily flexible pattern among limb ‘segments’. We report here that targeted disruption of the Hoxa-13 gene leads to a specific forelimb and hindlimb autopodal phenotype, distinct from that of the Hoxd-13 paralogous gene inactivation. In both limbs, Hoxa-13 loss of function results in the lack of formation of the most anterior digit and to altered morphogenesis of some ‘preaxial’ carpal/tarsal elements. We have generated mice with all possible combinations of disrupted Hoxa-13 and/or Hoxd-13 alleles, which allowed us to investigate the degree of functional specificity versus redundancy of the corresponding gene products in the developing limb autopod. The phenotype of any double mutant was much more severe than the sum of the phenotypes seen in the corresponding single mutants, indicating that these genes act in a partially redundant manner. Our major findings were: (1) an abnormal autopodal phenotype in Hoxa-13+/−/Hoxd-13+/− double heterozygous mutants, which mostly consists of subsets of the alterations seen in each individual homozygous mutant, and therefore appears to result from quantitative, rather than qualitative, homeoprotein deficiency; (2) partly distinct alterations in mutants harboring a single non-disrupted allele of Hoxa-13 or Hoxd-13, indicating that the remaining reduced protein amounts are not functionally equivalent; (3) a polydactyly in the forelimbs of Hoxa-13+/−/Hoxd-13−/−double mutants, consisting of seven symmetrically arranged, truncated and mostly non-segmented digits; (4) an almost complete lack of chondrified condensations in the autopods of double homozygous mutants, showing that the activity of group 13 Hox gene products is essential for autopodal patterning in tetrapod limbs.

Evidence of a role for T-box genes in the evolution of limb morphogenesis and the specification of forelimb/hindlimb identity

Tetrapod fore-and hindlimbs have evolved from the pectoral and pelvic fins of an ancient vertebrate ancestor. In this ancestor, the pectoral fin appears to have arisen following the rostral homeotic recapitulation of an existing pelvic appendage (Tabin and Laufer (1993), Nature 361, 692-693). Thus the basic appendage outgrowth program is reiterated in both tetrapod fore- and hindlimbs and the pectoral and pelvic fins of extant teleost fishes (Sordino et al. (1995) Nature 375, 678-681). Recently a novel family of putative transcription factors, which includes the T (Brachyury) locus, has been identified and dubbed the "T-box' family. In mice, all of these genes have expression patterns indicative of involvement in embryonic induction (Chapman et al. (1996) Dev. Dyn., in press), and four (Tbx2-Tbx5) are represented as two cognate, linked gene pairs (Agulnik et al., (1996), Genetics, in press). We now report that, whereas Tbx2 and Tbx3 are expressed in similar spatiotemporal patterns in both limbs, Tbx5 and Tbx4 expression is primarily restricted to the developing fore- and hindlimb buds, respectively. These observations suggest that T-box genes have played a role in the evolution of fin and limb morphogenesis, and that Tbx5 and Tbx4 may have been divergently selected to play a role in the differential specification of fore- (pectoral) versus hind- (pelvic) limb (fin) identity.

Specific and redundant functions of the paralogous Hoxa-9 and Hoxd-9 genes in forelimb and axial skeleton patterning

Using gene targeting, we have produced mice with a disruption of Hoxa-9 or Hoxd-9, two paralogous Abdominal B-related genes. During embryogenesis, these genes are expressed in limb buds and along the vertebral axis with anterior expression boundaries at the level of prevertebra #20 for Hoxa-9 and #23 for Hoxd-9. Skeletal analysis revealed homeotic transformations corresponding to anteriorisations of vertebrae #21 to #25 (L1 to L5) in the lumbar region of Hoxa-9−/− mutants; vertebrae #23 to #25 (L3 to L5) in the lumbar region together with vertebrae #28, #30 and #31 (S2, S4 and Ca1) in the sacrum and tail were anteriorized in Hoxd-9−/− mutants. Thus, anteriorisation of vertebrae #23 to #25 were common to both phenotypes. Subtle forelimb (but not hindlimb) defects, corresponding to a reduction of the humerus length and malformation of its deltoid crest, were also observed in Hoxd-9−/−, but not in Hoxa-9−/−, mutant mice. By intercrosses between these two lines of mutant mice, we have produced Hoxa-9/Hoxd-9 double mutants which exhibit synergistic limb and axial malformations consisting of: (i) an increase of penetrance and expressivity of abnormalities present in the single mutants, and (ii) novel limb alterations at the level of the forelimb stylopod and additional axial skeleton transformations. These observations demonstrate that the two paralogous genes Hoxa-9 and Hoxd-9 have both specific and redundant functions in lumbosacral axial skeleton patterning and in limb morphogenesis at the stylopodal level. Taken all together, the present and previously reported results show that disruption of different Hox genes can produce similar vertebral transformations, thus supporting a combinatorial code model for specification of vertebral identity by Hox genes.

Effects of all-trans-retinoic acid on skeletal pattern, 5'HoxD gene expression, and RAR beta 2/beta 4 promoter activity in embryonic mouse limbs

Mouse embryos were exposed to all-trans-retinoic acid on day 11 or day 12 of development and the resulting skeletal pattern alterations compared with early effects on Hoxd-11 and Hoxd-13 expression domains and RAR-beta 2/beta 4 promoter activity. The effects on skeletal pattern showed a clear correlation between the timing of retinoic acid exposure and the sequence of mesenchymal condensation. Ectopic RAR-beta 2/beta 4 promoter activity was detected within 2 hr of exposure to retinoic acid, and was present throughout the limb bud after 5 hr; it remained high in the apical ectodermal ridge and proximal mesenchyme after 12 hr, by which time the abnormal digital pattern could be seen. HoxD gene expression domains in the distal handplate were narrowed by 5 hr after maternal retinoic acid administration on day 11. Following retinoic acid treatment on both day 11 and day 12, the normal downregulation of Hoxd-11 and Hoxd-13 in the digital mesenchymal condensations was retarded. There was no evidence to suggest that RAR-beta 2/beta 4 promoter activity mediates the effects of RA on HoxD gene expression, but ectopic promoter activity is a useful indicator of at least some of the sites in which RA levels are raised. We suggest (1) that the apical ectodermal ridge is the most functionally significant of these sites, (2) that raised retinoic acid levels in the ridge result in altered gene expression and/or altered cell proliferation within this epithelium, (3) that both altered HoxD gene expression domains and altered skeletal pattern formation are secondary to this effect. There was a good correlation between the effects of retinoic acid on Hoxd-11 and Hoxd-13 expression and delay of skeletal differentiation, suggesting that this may be a direct effect.

Different roles for fibronectin in the generation of fore and hind limb precartilage condensations

Fibronectin expression and spatiotemporal distribution were examined in relation to the distinctive patterns of mesenchymal condensation and chondrogenesis seen in high-density serum-free cultures of chicken wing and leg bud precartilage cells. More fibronectin protein was produced on a per cell basis by leg than by wing mesenchyme, both in freshly isolated tissue and during the prechondrogenic condensation period in culture, where the difference was twofold. The quantitative difference in fibronectin expression in freshly isolated wing and leg mesenchyme was also seen at the level of total and poly (A)+ RNA. During the condensation phase, fibronectin was distributed in the wing and leg mesenchymal cultures in a way that prefigured the eventual distribution of cartilage in these cultures: in wing cultures condensations were broad and flat, and rich in diffusely organized fibronectin; in leg cultures, condensations were compact and spheroidal, and contained abundant deposits of fibronectin. In addition, the leg condensations were connected by long fibronectin-rich fibers. Transient treatment with TGF-beta early during the culture period led to increase in fibronectin production and expansion of condensations in both wing and leg cultures. Leg mesenchyme was more responsive to transforming growth factor-beta than wing mesenchyme with respect to fibronectin production, and this was reflected in a greater enhancement of cartilage formation in later cultures. Treatment of cultures with monoclonal antibody 304 directed against the amino-terminal heparin-binding domain of fibronectin inhibited condensation formation and reduced chondrogenesis in wing mesenchyme, but left these two processes unchanged in leg mesenchyme, despite disruption by the antibody of the leg-specific fibronectin fibers. These studies indicate that for both wing and leg mesenchyme the morphology, extent, and spatiotemporal regulation of precartilage condensation and subsequent chondrogenesis closely parallels the deposition of fibronectin. But whereas the interaction between cells and fibronectin in wing bud mesenchyme is mediated in part by the protein's amino-terminal domain, this domain does not appear to be involved in analogous interactions in leg bud mesenchyme.

Down-regulation of adhesion receptors on cells of primate embryos as a probable mechanism of the teratogenic action of thalidomide

In spite of ongoing speculation, there has been no evidence that adhesion receptors are expressed on the cells of mammalian embryos. In this report, we provide the first proof that a variety of such receptor (beta 1-, beta 2-, and beta 3-integrins and selectin) are indeed expressed on cells of essentially all primordia of marmoset embryos at early organogenesis (developmental stages 11 to 13, or even earlier). Treatment with low doses (20 or as little as 1 mg/kg body weight) of a highly teratogenic derivative (EM12) of thalidomide, the most notorious human teratogen, triggers a dramatic and statistically highly significant down-regulation of several surface adhesion receptors (e.g. CD11a/CD18, CD49d/CD29, CD61, etc.) on early limb bud cells and on cells of some other primordia during early organogenesis of embryos of a primate (marmoset, Callithrix jacchus). Some of these receptors almost disappear, or they are expressed at a lower epitope density in the exposed embryos. These down-regulations of surface adhesion receptors may be expected to alter cell-cell- and cell-extracellular matrix interactions, and they are suggested to be a long-sought primary mechanism of the teratogenic action of thalidomide-type substances.

Calcitonin gene-related peptide (CGRP) in the developing mouse limb

The mitogenic effects of neuropeptides and their localization to developing tissues suggest an important role for these peptides during gestation. We examined the expression and action of the neuropeptide calcitonin gene-related peptide (CGRP) in the developing mouse limb bud, an excellent model system for studying musculoskeletal development. CGRP immunoreactivity (CGRP-ir) was detected in the developing limb at day 16.5 of gestation (E 16.5) and was limited to nerve fibers surrounding blood vessels, within the developing muscle or in close proximity to the developing cartilaginous skeleton. Although CGRP-ir was not observed until E 16.5, limb buds were responsive to CGRP as early as E 11.5. Within 5 min of exposure to CGRP (10(-8) to 10(-7) M) a 2--3-fold increase in cyclic AMP (cAMP) levels was observed. This CGRP-induced increase in cAMP was abolished by the addition of human CGRP8-37, a CGRP receptor antagonist. This result suggests that the effect on cAMP was mediated by the interaction of CGRP with CGRP receptors. Our findings indicate that mouse limbs are responsive to CGRP when they are composed of primarily undifferentiated mesenchyme and that CGRP-ir appears at a later stage of development in association with cartilage and muscle differentiation.

In vitro model of syndactyly replicates the morphologic features observed in vivo

Syndactyly is a common congenital hand anomaly that may occur after exposure to teratogens. We have developed an in vitro model of syndactyly to investigate the molecular mechanisms underlying this malformation of digit development. Retinoic acid, which regulates pattern formation in vertebrate limb development and is associated with teratogenic malformations, was used in the development of this syndactyly model system. Pregnant Swiss-Webster mice were given retinoic acid by oral gavage on days 10 and 11 of embryonic development (E10 and E11, respectively). The mice were sacrificed on gestational days 13 and 17 (E13, E17) and immediately postnatally (PN). The fetuses were removed and the forelimbs dissected under the operating microscope. The E13 limbs were cultured for 4 days (E13+4) in an organ culture system using a serumless, chemically defined medium. The E17, PN, and E13+4 forelimbs were critically examined for malformations of digit separation and digit development. Retinoic acid-induced fetal mouse forelimb syndactyly was observed in all the groups; 81 percent of E17 limbs, 75 percent of PN limbs, and 77 percent of E13+4 limbs had syndactyly. The morphology of the digital malformations was similar in the E17, PN, and E13+4 limbs. This in vitro model permits further studies to characterize the molecular changes that occur during the development of a congenital hand anomaly.

Hst-1 (FGF-4) antisense oligonucleotides block murine limb development

The initiation of limb development depends on the site specific proliferation of the mesenchyme by the signals from the apical ectodermal ridge (AER) in embryonic mouse. We have previously reported that the local expression of Hst-1/Fgf-4 transcripts in AER of the mouse limb bud is developmentally regulated, expressed at 11 and 12 days post coitus (p.c.) embryo. In an effort to further understand the role of Hst-1/FGF-4 in mouse limb development, an antisense oligodeoxynucleotides (ODNs) study was performed. We first established a novel organ culture system to study mouse limb development in vitro. This system allows mouse limb bud at 9.5-10-d p.c. embryo, when placed on a sheet of extracellular matrix in a defined medium, to differentiate into a limb at 12.5-d p.c. embryo within 4.5 d. Using this organ culture system, we have shown that exposure of 9.5-10-d p.c. embryonal limb bud explants to antisense ODNs of Hst-1/FGF-4 blocks limb development. In contrast, sense and scrambled ODNs have no inhibitory effect on limb outgrowth, suggesting that Hst-1/FGF-4 may work as a potent inducing factor for mouse limb development.

Polydactylous limbs in Strong's Luxoid mice result from ectopic polarizing activity

Strong's Luxoid (1stD) is a semidominant mouse mutation in which heterozygotes show preaxial hindlimb polydactyly, and homozygotes show fore- and hindlimb polydactyly. The digit patterns of these polydactylous limbs resemble those caused by polarizing grafts, since additional digits with posterior character are present at the anterior side of the limb. Such observations suggest that 1stD limb buds might contain a genetically determined ectopic region of polarizing activity. Accordingly, we show that mutant embryos ectopically express the pattern-determining genes fibroblast growth factor 4 (fgf-4), sonic hedgehog (shh), and Hoxd-12 in the anterior region of the limb. Further, we show that anterior mesoderm from mutant limbs exhibits polarizing activity when grafted into host chicken limbs. In contrast to an experimentally derived polydactylous transgenic mouse, forelimbs of homozygotes show a normal pattern of Hoxb-8 expression, indicating that the duplication of polarizing tissue here occurs downstream or independently of Hoxb-8. We suggest that the 1st gene product is involved in anteroposterior axis formation during normal limb development.

Clusterin expression during programmed and teratogen-induced cell death in the postimplantation rat embryo

Clusterin appears to play a role in multiple cellular processes including reproductive cell function, lipid transport, complement regulation, and endocrine secretion. In addition, clusterin has been shown to be associated with both developmental and induced cell death. We have used immunohistochemistry and in situ hybridization to study the relationship between clusterin expression, normal programmed cell death (PCD) in the developing rat limb bud, and abnormal cell death induced by hyperthermia in day 11 rat embryos. Immunohistochemical localization of clusterin in day 14-16 limb buds showed that the most intense immunostaining was associated with the condensing mesenchyme of the developing digit, a tissue exhibiting low levels of PCD. Moreover, areas of digital cell death, confined to future interphalangeal spaces, were devoid of clusterin immunostaining. Clusterin immunostaining was also observed in the interdigital mesenchyme and partially overlapped the cell death that occurs in this tissue during the early development of the digits. Although clusterin immunostaining overlaps areas of interdigital cell death, most apoptotic cells in the interdigital mesenchyme and underlying the surface ectoderm were not associated with clusterin immunostaining. We also examined the expression of clusterin in day 11 rat embryos exposed to 43 degrees C, an exposure that induces extensive cell death primarily in the developing neuroepithelium. In control embryos cultured at 37 degrees C, clusterin mRNA and protein were expressed at high levels in the heart, a tissue that is completely resistant to the cytotoxic effects of hyperthermia. Within 2.5 hr after an exposure of 43 degrees C, clusterin mRNA showed a dramatic induction in the prosencephalic mesenchyme and only a modest induction in the prosencephalic neuroepithelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of apoptosis and cathepsin D in limbs exposed in vitro to an activated analog of cyclophosphamide

Apoptosis, a form of active cell death, plays a role during normal limb development. The present study was done to test the hypothesis that the teratogen cyclophosphamide, an alkylating agent and commonly used anticancer drug, produces malformations by disturbing the regulation of apoptosis in the limb. The effects of a preactivated analog of cyclophosphamide, 4-hydroperoxycyclophosphamide, on limb development and on apoptosis in the limb were determined in vitro. Cathepsin D is a lysosomal protease which is induced in tissues undergoing destruction by apoptosis. To further examine the process of apoptosis in the limb, the effects of 4-hydroperoxycyclophosphamide exposure on cathepsin D protein concentration and on the immunolocalization of cathepsin D in limb buds were assessed. Limb buds from gestational day 12 mice were excised and cultured in roller bottles in a chemically defined medium for up to 6 days. The addition of 4-hydroperoxycyclophosphamide (1 or 10 micrograms/ml) to the culture medium produced time- and concentration-dependent limb malformations. Electrophoresis of the DNA extracted from both control and treated limbs revealed a DNA fragmentation pattern characteristic of apoptosis. Limbs cultured in the control medium showed a "DNA ladder" only after 72 hours in vitro; however, those in the drug-treated groups showed fragmentation within 12 hours of drug exposure. Acridine orange staining and examination of cell ultrastructure with the electron microscope further confirmed that apoptotic cell death in the interdigital areas was accelerated in drug-exposed limbs. The relative abundance of cathepsin D in limbs exposed to 4-hydroperoxycyclophosphamide for 24 hours was increased compared to control limbs.(ABSTRACT TRUNCATED AT 250 WORDS)

Hox gene expression in teleost fins and the origin of vertebrate digits

Hox genes are essential for growth and patterning of the tetrapod limb skeleton. Mice mutant for the Hoxd-13 gene have an important delay in morphogenesis owing to reduced proliferation. Based on the appearance of atavisms in such mice, we suggested that modifications of Hox gene regulation may have been a source of morphological variation during the evolution of tetrapod limbs. Pectoral and pelvic fins are homologous to fore- and hindlimbs, respectively. To compare the relative importance of Hox genes during fin versus limb morphogenesis, we cloned zebrafish (Danio rerio) HoxD and HoxA complex genes and analysed their expression during fin development. The results suggest a scheme for the fin-limb transition in which the distal autopods (digits) are neomorphic structures produced by unequal proliferation of the posterior part of an ancestral appendix.

Toxicity of four alkylating agents on in vitro rat embryo differentiation and development

The relative developmental toxicity of four direct acting, alkylating agents was determined in primary cultures of differentiating rat embryo midbrain (CNS) and limb bud (LB) cells and compared with that observed in the rat whole embryo postimplantation culture system. The alkylating agents tested include methylnitrosourea (MNU), ethylnitrosourea (ENU), methyl methanesulfonate (MMS), and ethyl methanesulfonate (EMS). These alkylating agents have been shown to produce developmental toxicity following either in vitro or in vivo exposure. Viability for both CNS and LB was assessed by a neutral red dye assay. Differentiation of CNS cells was assessed by hematoxylin staining of neurons; differentiation of LB cells was assessed by Alcian blue staining of extracellular proteoglycans. Relative potencies of these compounds in the cell culture system were not the same as those observed in the embryo culture system. Whereas rank order of potency in the cell culture system, for viability and differentiation, was MMS > MNU > ENU > EMS, rank order in the embryo culture system, for embryo lethality and malformations, was MNU > ENU > MMS > EMS. Effective concentrations for cell culture viability and differentiation by MNU and ENU in cell culture were about three to nine times higher than comparable values previously reported for embryos, while effective concentrations for MMS and EMS were two to seven times lower than those observed in the embryos. Differences in potency between the two culture systems may be related to differences in formation and repair of DNA adducts, as well as differences in culture conditions.

Outgrowing nerves in the foreleg of a mouse embryo as viewed by three-dimensional reconstruction from electron micrographs

Electron micrographs from serial cross-sections of 12-day-old mouse forelegs were digitized and three-dimensional reconstruction of the data was carried out on an Apple Macintosh Quadra 700 computer using a program especially designed for this purpose. Two nerve endings of the palmar net of the median nerve were visualized together with their accompanying Schwann cells and the surrounding processes of fibroblasts. Naked axons invade straightly into the embryonic connective tissue and serve as contact guidance for the Schwann cells to follow. Fibroblasts with long processes extend around the axons with a parallel orientation. Contacts between axons and fibroblasts are occasionally observed. It is unclear whether the parallel orientation of nerve endings and fibroblast extensions have any biological significance.

The timing of ossification of the limb bones, and growth rates of various long bones of the fore and hind limbs of the prenatal and early postnatal laboratory mouse

In order to study the pattern of ossification of the skeletal components of the fore and hind limb of the mouse, intact embryos were isolated between days (d) 15 and 19 of pregnancy (the morning of finding a vaginal plug is termed d 1 of pregnancy), and postnatal animals isolated on d 1 (newborns), 7 and 14 after birth. The total number of fore and hind limbs studied for each day of pregnancy or postnatal day for the bone growth study is given in parentheses: d 15 (2), d 16, 17, 18 and 19 of pregnancy (5 specimens for each of these days), d 1 (newborn), wk 1 and 2, postnatal (4 specimens analysed at each of these times), since only the right limbs were studied. For the study involving the time of first appearance of ossification centres, either the right or the left limb of each of these prenatal and postnatal specimens was analysed. All specimens were fixed in 80% ethanol, bulk-stained using alizarin and Alcian blue, in order to stain ossification centres and cartilage, respectively, and cleared. The limbs were then disarticulated from the axial skeleton at the sternoclavicular and sacroiliac joints to facilitate (1) the determination of the sequential pattern of ossification in the various cartilage primordia analysed, and (2) the analysis of the pattern of growth of the humerus, ulna, femur and tibia. The latter values were plotted graphically, and the individual growth rate of each of the long bones studied was then deduced and also plotted graphically. The findings demonstrated that, with the exception of the femur and ulna, all of the long bones studied had significantly different growth patterns. The time of appearance of the various centres of ossification in the skeletal elements studied proceeded in a similar order to that described by previous authors, though there was some discrepancy in the exact time of first appearance of certain ossification centres. Of particular interest was the somewhat unusual pattern of ossification of the first digits of both the fore and hind limb compared with that of the other digits. The data presented here provide useful baseline information on the normal sequential pattern of ossification in the fore and hind limb, and the characteristic growth pattern of the individual long bones of the limbs in this species.

Ultrastructural characterization of normal and abnormal chondrogenesis in micromass rat embryo limb bud cell cultures

Inhibition of chondrogenesis in limb bud cell micromass cultures has been proposed as a short-term teratogen detection test. Validation studies were performed by testing large series of reference compounds and comparing their teratogenic potential with their ability to inhibit chondrogenesis; however, there are few reports describing the histological and ultrastructural changes associated with inhibition of chondrogenesis in vitro. The objective of this study was to provide a qualitative description of the histological and ultrastructural alterations induced by three chondrogenesis inhibitors: retinoic acid (RA) and 6-aminonicotinamide (6AN), two teratogens, and doxylamine succinate (DS), a nonteratogen compound. In addition, in order to have a basis for the interpretation of the morphological alterations induced by the test compounds, the histological and ultrastructural changes which occur during the time course of chondrogenesis in control cultures were described and compared with those in rat embryo limb buds. We found that RA at 0.5 micrograms/ml led to a marked decrease in the number and size of cartilaginous foci; most cells lacked morphological signs of differentiation but their ability to proliferate was unaffected. At concentrations of 2 micrograms/ml and more, 6AN delayed cell proliferation, reduced staining of the extracellular matrix, and induced the formation of endoplasmic cisternae. DS at 50 micrograms/ml affected both differentiation and proliferation; pigment deposits were observed in chondrocytes, suggesting phospholipid metabolism disorders. In conclusion, this study showed that inhibition of chondrogenesis in this simple cell culture system can be associated with different types of histological and ultrastructural alterations. Examination of these alterations can provide useful information about the teratogenic potential of tested compounds and their mechanism of action.