Retinoic acid in development and regeneration

Retinoids are low molecular weight, lipophilic derivatives of vitamin A which have profound effects upon the development of various embryonic systems. Here I review the effects on developing and regenerating limbs, regenerating amphibian tails and the developing central nervous system (CNS). In the regenerating amphibian limb, retinoids can proximalize, posteriorize and ventralize the axes of the blastema. In the chick limb bud retinoids can only posteriorize the tissue. In the regenerating amphibian tail retinoids can homeotically transform tail tissue into hindlimb tissue. In the developing and regenerating limb retinoic acid has been detected endogenously, confirming that this molecule plays a role in the generation of pattern and we have shown that limbs cannot develop in the absence of retinoic acid. In the developing CNS retinoic acid specifically affects the hindbrain where it causes a transformation of anterior rhombomeres into more posterior ones. Again, endogenous retinoic acid has been detected in the CNS and in the absence of retinoids the posterior hindbrain has been found to be affected. The effects of retinoids on the CNS are most likely to be mediated via theHox genes acting in the mesoderm after gastrulation. It has also been proposed that the establishment of the head-to-tail axis in the mesoderm is established by retinoic acid. These data show that retinoids play an important role in both the development and regeneration of various systems in the embryo and post-embryonically     

Role of nitric oxide in chick embryonic organogenesis and dysmorphogenesis

BACKGROUND: Nitric oxide (NO), produced by the nitric oxide synthase family of enzymes, mediates multiple signaling functions, and when unchecked, NO causes pathological damage. Exposure of embryos to a variety of teratogens, including carbon monoxide (CO), has been shown to increase reactive intermediates, such as NO, and recent work showed that either the excess or absence of NO caused morphological defects. While endogenous NO is known to regulate many adult tissues, its role during embryonic organogenesis and/or in mediating responses to teratogen exposure has not been explored. METHODS: We have examined here the presence of NO during normal chick embryonic organogenesis, and investigated the teratogenicity of NO through the application of sodium nitroprusside (SNP), which mimics NO overproduction, and NG-monomethyl-L-arginine (L-NMMA), which inhibits endogenous NOS activity. RESULTS: Topical treatment with SNP or L-NMMA for 18 h resulted in morphological defects, specifically in the neural tube and somites, which corresponded to sites of altered apoptosis. The location of NO was histochemically correlated with the observed morphological defects. Coadministration of SNP or L-NMMA with CO showed functional coregulation and interaction between NO and CO in chick embryonic development. CONCLUSIONS: Our results showed that regulation of NO is essential for normal axial development, that sites of altered NO expression correlate to those of altered apoptosis and dysmorphogenesis, and that CO coadministration resulted in a rectification of normal NO expression. Collectively, these results suggest that alteration in endogenous NO/CO signaling is responsible, at least in part, for the observed NO-induced teratogenesis.     

Fetal cardiac effects of maternal hyperglycemia during pregnancy

Maternal diabetes mellitus is associated with increased teratogenesis, which can occur in pregestational type 1 and type 2 diabetes. Cardiac defects and with neural tube defects are the most common malformations observed in fetuses of pregestational diabetic mothers. The exact mechanism by which diabetes exerts its teratogenic effects and induces embryonic malformations is unclear. Whereas the sequelae of maternal pregestational diabetes, such as modulating insulin levels, altered fat levels, and increased reactive oxygen species, may play a role in fetal damage during diabetic pregnancy, hyperglycemia is thought to be the primary teratogen, causing particularly adverse effects on cardiovascular development. Fetal cardiac defects are associated with raised maternal glycosylated hemoglobin levels and are up to five times more likely in infants of mothers with pregestational diabetes compared with those without diabetes. The resulting anomalies are varied and include transposition of the great arteries, mitral and pulmonary atresia, double outlet of the right ventricle, tetralogy of Fallot, and fetal cardiomyopathy. A wide variety of rodent models have been used to study diabetic teratogenesis. Both genetic and chemically induced models of type 1 and 2 diabetes have been used to examine the effects of hyperglycemia on fetal development. Factors such as genetic background as well as confounding variables such as obesity appear to influence the severity of fetal abnormalities in mice. In this review, we will summarize recent data on fetal cardiac effects from human pregestational diabetic mothers, as well as the most relevant findings in rodent models of diabetic cardiac teratogenesis. Birth Defects Research (Part A), 2009. © 2009 Wiley‐Liss, Inc.     

Amphibian teeth: current knowledge, unanswered questions, and some directions for future research

Elucidation of the mechanisms controlling early development and organogenesis is currently progressing in several model species and a new field of research, evolutionary developmental biology, which integrates developmental and comparative approaches, has emerged. Although the expression pattern of many genes during tooth development in mammals is known, data on other lineages are virtually non-existent. Comparison of tooth development, and particularly of gene expression (and function) during tooth morphogenesis and differentiation, in representative species of various vertebrate lineages is a prerequisite to understand what makes one tooth different from another. Amphibians appear to be good candidates for such research for several reasons: tooth structure is similar to that in mammals, teeth are renewed continuously during life (=polyphyodonty), some species are easy to breed in the laboratory, and a large amount of morphological data are already available on diverse aspects of tooth biology in various species. The aim of this review is to evaluate current knowledge on amphibian teeth, principally concerning tooth development and replacement (including resorption), and changes in morphology and structure during ontogeny and metamorphosis. Throughout this review we highlight important questions which remain to be answered and that could be addressed using comparative morphological studies and molecular techniques. We illustrate several aspects of amphibian tooth biology using data obtained for the caudate Pleurodeles waltl. This salamander has been used extensively in experimental embryology research during the past century and appears to be one of the most favourable amphibian species to use as a model in studies of tooth development.     

The natural variability of morphogenesis: a tool for exploring the mechanics of gastrulation movements in amphibian embryos

The investigation of natural variability of metric morphological characters in frog gastrulation revealed that in genetically and environmentally homogeneous samples of embryos their variability is of a higher order of magnitude than that known for quantitative metric characters in adult organisms. Matching the coefficients of variation of characters under consideration to the specific rates of their changes in normal development revealed a strong positive correlation between the rates of morphological change and the amount of morphological variance. The increase in the variance is mainly in characters concerned with shaping of moving embryonic areas and arises as a result of a positive feedback between the movement of a given area and recruitment of cells from surrounding areas into the movement. The account of natural variation suggests a new model of amphibian gastrulation whose essential feature is the intimate connection between the movement and shaping of the dorsal blastopore lip of the gastrula.     

Molecular changes associated with teratogen-induced cyclopia

BACKGROUND: Exposure of zebrafish embryos to a number of teratogens results in cyclopia, but little is known about the underlying molecular changes. METHODS: Using zebrafish embryos, we compare the effects cyclopamine, forskolin, and ethanol delivered starting just before gastrulation, on gene expression in early axial tissues and forebrain development. RESULTS: Although all three teratogens suppress gli1 expression, they do so with variable kinetics, suggesting that while suppression of Shh signaling is a common outcome of these three teratogens, it is not a common cause of the cyclopia. Instead, all teratogens studied produce a series of changes in the expression of gsc and six3b present in early axial development, as well as a later suppression of neural crest cell marker dlx3b. Ethanol and forskolin, but not cyclopamine, exposure reduced anterior markers, which most likely contributes to the cyclopic phenotype. CONCLUSIONS: These data suggest that each teratogen exposure leads to a unique set of molecular changes that underlie the single phenotype of cyclopia.     

Retinoic acid causes abnormal development and segmental patterning of the anterior hindbrain in Xenopus embryos.

Retinoic acid is a very potent teratogen and has also been implicated as an endogenous developmental signalling molecule in vertebrate embryos. One of the regions of the embryo reliably affected by exogenously applied RA is the hindbrain. In this paper, we describe in detail the hindbrain of Xenopus laevis embryos briefly treated with various levels of RA at gastrula stages. Such treatments lead to development of embryos with loss of anterior structures. In addition, RA has a general effect on rhombomere morphology and specific effects on the development of the anterior rhombomeres. This effect is demonstrated using neurofilament antibodies, HRP staining and in situ hybridisation using a probe for expression of the Xenopus Krox-20 gene. Anatomically it is evident that the development of the hindbrain normally anterior to the otocyst (rhombomeres 1-4) is abnormal following RA treatment. Sensory and motor axons of cranial nerves V and VII form a single root and the peripheral paths of V and VII and IX and X are also abnormal, as is the more anterior location of the otocyst. These anatomical changes are accompanied by changes in the pattern of expression for the gene XKrox-20, which normally expresses in rhombomeres 3 and 5, but is found in a single band in the anterior hindbrain of treated embryos which standardly fail to generate the normal external segmental appearance. The results are discussed in terms of both the teratogenic and possible endogenous roles of RA during normal development of the central nervous system. We conclude that low doses of RA applied during gastrulation have specific effects on the anterior Xenopus hindbrain which appear to be evolutionarily conserved in the light of similar recent findings in zebrafish.