Initiation of dorso-ventral axis during chick limb development

Genetics of the patella

We review genetic diseases with identified molecular bases that include abnormal, reduced (hypoplasia), or absent (aplasia) patellae as a significant aspect of the phenotype. The known causal genes can be broadly organized according to three major developmental and cellular processes, although some genes may act in more than one of these: limb specification and pattern formation; DNA replication and chromatin structure; bone development and differentiation. There are also several genes whose phenotypes in mice indicate relevance to patellar development, for which human equivalent syndromes have not been reported. Developmental studies in mouse and chick embryos, as well as patellar involvement in human diseases with decreased mobility, document the additional importance of local environmental factors in patellar ontogenesis. Patellar anomalies found in humans can be an important clue to a clinical genetic diagnosis, and a better knowledge of the genetics of patellar anomalies will improve our understanding of limb development.

The apical ectodermal ridge of the mouse model of ectrodactyly Dlx5;Dlx6-/- shows altered stratification and cell polarity, which are restored by exogenous Wnt5a ligand

The congenital malformation split hand/foot (SHFM) is characterized by missing central fingers and dysmorphology or fusion of the remaining ones. Type-1 SHFM is linked to deletions/rearrangements of the DLX5–DLX6 locus and point mutations in the DLX5 gene. The ectrodactyly phenotype is reproduced in mice by the double knockout (DKO) of Dlx5 and Dlx6. During limb development, the apical ectodermal ridge (AER) is a key-signaling center responsible for early proximal–distal growth and patterning. In Dlx5;6 DKO hindlimbs, the central wedge of the AER loses multilayered organization and shows down-regulation of FGF8 and Dlx2. In search for the mechanism, we examined the non-canonical Wnt signaling, considering that Dwnt-5 is a target of distalless in Drosophila and the knockout of Wnt5, Ryk, Ror2 and Vangl2 in the mouse causes severe limb malformations. We found that in Dlx5;6 DKO limbs, the AER expresses lower levels of Wnt5a, shows scattered β-catenin responsive cells and altered basolateral and planar cell polarity (PCP). The addition of Wnt5a to cultured embryonic limbs restored the expression of AER markers and its stratification. Conversely, the inhibition of the PCP molecule c-jun N-terminal kinase caused a loss of AER marker expression. In vitro, the addition of Wnt5a on mixed primary cultures of embryonic ectoderm and mesenchyme was able to confer re-polarization. We conclude that the Dlx-related ectrodactyly defect is associated with the loss of basoapical and PCP, due to reduced Wnt5a expression and that the restoration of the Wnt5a level is sufficient to partially reverts AER misorganization and dysmorphology.

Anisotropic stress orients remodelling of mammalian limb bud ectoderm

The physical forces that drive morphogenesis are not well characterized in vivo, especially among vertebrates. In the early limb bud, dorsal and ventral ectoderm converge to form the apical ectodermal ridge (AER), although the underlying mechanisms are unclear. By live imaging mouse embryos, we show that prospective AER progenitors intercalate at the dorsoventral boundary and that ectoderm remodels by concomitant cell division and neighbour exchange. Mesodermal expansion and ectodermal tension together generate a dorsoventrally biased stress pattern that orients ectodermal remodelling. Polarized distribution of cortical actin reflects this stress pattern in a β-catenin- and Fgfr2-dependent manner. Intercalation of AER progenitors generates a tensile gradient that reorients resolution of multicellular rosettes on adjacent surfaces, a process facilitated by β-catenin-dependent attachment of cortex to membrane. Therefore, feedback between tissue stress pattern and cell intercalations remodels mammalian ectoderm.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.

Cell tracing reveals a dorsoventral lineage restriction plane in the mouse limb bud mesenchyme

Regionalization of embryonic fields into independent units of growth and patterning is a widespread strategy during metazoan development. Compartments represent a particular instance of this regionalization, in which unit coherence is maintained by cell lineage restriction between adjacent regions. Lineage compartments have been described during insect and vertebrate development. Two common characteristics of the compartments described so far are their occurrence in epithelial structures and the presence of signaling regions at compartment borders. Whereas Drosophila compartmental organization represents a background subdivision of embryonic fields that is not necessarily related to anatomical structures, vertebrate compartment borders described thus far coincide with, or anticipate, anatomical or cell-type discontinuities. Here, we describe a general method for clonal analysis in the mouse and use it to determine the topology of clone distribution along the three limb axes. We identify a lineage restriction boundary at the limb mesenchyme dorsoventral border that is unrelated to any anatomical discontinuity, and whose lineage restriction border is not obviously associated with any signaling center. This restriction is the first example in vertebrates of a mechanism of primordium subdivision unrelated to anatomical boundaries. Furthermore, this is the first lineage compartment described within a mesenchymal structure in any organism, suggesting that lineage restrictions are fundamental not only for epithelial structures, but also for mesenchymal field patterning. No lineage compartmentalization was found along the proximodistal or anteroposterior axes, indicating that patterning along these axes does not involve restriction of cell dispersion at specific axial positions.

Adherens junction breakdown in the periderm following cadmium administration in the chick embryo: distribution of cadherins and associated molecules

BACKGROUND

The teratogenic metal cadmium (Cd) has been found to cause ventral body wall defects similar to human omphalocele when administered to post-gastrulation chick embryos prior to body wall folding. From 4h after Cd, affected embryos demonstrate varying degrees of cell junction breakdown and desquamation in the periderm. We examined the effect of Cd on tissue and cell distribution of cadherins and their intracellular associates.

METHODS

Chicks were explanted and given 50microl of 50microM Cd solution at 60h incubation (Hamburger-Hamilton stage 16-17). To examine peridermal junctions, embryos were processed into resin and ultra-thin sections examined by transmission electron microscopy (TEM). Tissue was processed into paraffin and 6microm sections treated according to standard protocols for immunohistochemical detection of L-CAM, pan-cadherin, beta-catenin, alpha-1 and alpha-2 catenin. To examine actin distribution, frozen sections were cut at 10-20microm, stained with oragon green phalloidin and nuclei counter-stained with propidium iodide.

RESULTS

The overall tissue distribution of L-CAM, pan-cadherin and the alpha-catenins did not appear to be altered following Cd. However, beta-catenin changed from its normal sub-membranous location to a more general cytoplasmic distribution, with translocation to the nucleus in both peridermal and ectodermal cells. Similarly, actin distribution in the periderm in embryos demonstrating cell junction breakdown was markedly altered, with clumping and disorganization after 4h.

CONCLUSIONS

Although L-CAM is distributed normally after Cd, post-translational modification may occur causing breakdown of its normal association with the catenins and actin, and allowing beta-catenin to translocate to the nucleus in peri-ectodermal tissue, mimicking the canonical Wnt pathway.

Defect in dorso-ventral patterning, asplenia, and conotruncus in a spontaneously aborted fetus

We describe a very unusual combination, and previously unreported, of malformations in an 18-week, spontaneously aborted male fetus. The fetus had a reversed dorsoventral positioning of the head and upper limbs relative to the body axis with the head and both upper limbs directed dorsally, and an abrupt rotation of the vertebral bones at the level of CZ The fetus also had asplenia, single ventricle, and conotruncus. The fetus also had flexion deformities at the wrist, reduction deformity of the left second digit, anomalies in ossification of the bones of the left hand, and bilateral talipes calcaneovalgus. The major malformations in this fetus were all blastogenetic in origin and consisted of dorsoventral patterning defect involving structures cranial to C7, a laterality, and a septation defect of the ventricle and outflow tract of the heart. The defects are interpreted as being the result of abnormal coordination of the molecular signaling involved in dorsoventral axis formation and laterality of the limbs and trunk, and possibly also in cardiac septation.

Tbx18 and boundary formation in chick somite and wing development

The chicken Tbx gene, Tbx18, is expressed in lateral plate mesoderm, limb, and developing somites. Here we show that Tbx18 is expressed transiently in axial mesenchyme during somite segmentation. We present evidence from overexpression and transplantation experiments that Tbx18 controls fissure formation in the late stages of somite maturation. In presumptive wing lateral plate mesoderm, ectopic Tbx18 expression leads to anterior extension of the wing bud. These results suggest that Tbx18 is involved in producing mesodermal boundaries, generating in paraxial mesoderm morphological boundaries between somites and in lateral plate mesoderm a wing- or non-wing-forming boundary.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.

Novel roles of Fgfr2 in AER differentiation and positioning of the dorsoventral limb interface

The epithelial b variant of Fgfr2 is active in the entire surface ectoderm of the early embryo, and later in the limb ectoderm and AER,where it is required for limb outgrowth. As limb buds do not form in the absence of Fgfr2, we used chimera analysis to investigate the mechanism of action of this receptor in limb development. ES cells homozygous for a loss-of-function mutation of Fgfr2 that carry aβ-galactosidase reporter were aggregated with normal pre-implantation embryos. Chimeras with a high proportion of mutant cells did not form limbs,whereas those with a moderate proportion formed limb buds with a lobular structure and a discontinuous AER. Where present, the AER did not contain mutant cells, although mutant cells did localize to the adjacent surface ectoderm and limb mesenchyme. In the underlying mesenchyme of AER-free areas,cell proliferation was reduced, and transcription of Shh and Msx1 was diminished. En1 expression in the ventral ectoderm was discontinuous and exhibited ectopic dorsal localization, whereas Wnt7a expression was diminished in the dorsal ectoderm but remained confined to that site. En1 and Wnt7a were not expressed in non-chimeric Fgfr2-null mutant embryos, revealing that they are downstream of Fgfr2. In late gestation chimeras, defects presented in all three limb segments as bone duplications, bone loss or ectopic outgrowths. We suggest that Fgfr2 is required for AER differentiation, as well as for En1 and Wnt7a expression. This receptor also mediates signals from the limb mesenchyme to the limb ectoderm throughout limb development, affecting the position and morphogenesis of precursor cells in the dorsal and ventral limb ectoderm, and AER.

Patterning systems--from one end of the limb to the other

A combination of embryology and gene identification has led us to the current view of vertebrate limb development, in which a series of three interlocking patterning systems operate sequentially over time. This review describes current understanding of these regulatory mechanisms and how they form a framework for future analysis of limb patterning.