Differential inducibility of specific mRNA corresponding to five CYP3A isoforms in female rat liver by RU486 and food deprivation: comparison with protein abundance and enzymic activities

Cranial neural fold fusion in the chick embryo is known to commence in the midbrain region before progressing cranially and caudally to involve the fore- and hindbrain regions, respectively. The two epithelial layers at the tips of the neural folds that participate in fusion are the surface ectoderm and the neuroepithelium. We have examined and compared cranial neural fold fusion in both layers, and our results show that fusion of the neuroepithelial component of the neural folds, unlike that of the surface ectoderm, starts in the caudal portion of the forebrain. Second, contrary to the widely accepted opinion, we have demonstrated that in the hindbrain region, fusion of the neuroepithelial component of the neural folds does not occur. Soon after neural fold apposition, a neuroepithelial eminence appears in rhombomeres 1 and 2, and this, together with other neuroepithelial cells in the dorsal midline of the hindbrain, undergoes massive apoptosis. The absence of neuroepithelial fusion in the hindbrain may be due to the presence of massive apoptosis among neuroepithelial cells that should have participated in the fusion process. The events described above may predispose the hindbrain to the development of neural tube defects. The appearance of cranial neural crest cells in the midline during their migration may enhance the fusion of the surface ectodermal portion of the neural folds.     

Ectopic expression of Hoxa-1 in the zebrafish alters the fate of the mandibular arch neural crest and phenocopies a retinoic acid-induced phenotype

Considerable evidence has demonstrated that retinoic acid influences the formation of the primary body axis in vertebrates and that this may occur through the regulation of Hox gene expression. In this study, we show that the phenotype induced by exogenous retinoic acid in the zebrafish can also be generated by the overexpression of Hoxa-1 following injection of synthetic RNA into the fertilised egg. The isolation, sequence and expression pattern of the zebrafish Hoxa-1 gene is described. We show that exogenously applied retinoic acid causes the ectopic accumulation of Hoxa-1 message during gastrulation in the hypoblast in the head region. Overexpression of Hoxa-1 following injection of RNA causes abnormal growth of the anterior hindbrain, duplication of Mauthner neurons in rhombomere (r) 2 and fate changes of r2 mesenchymal and neurogenic neural crest. These results are discussed in terms of the role of Hoxa-1 in controlling anterior hindbrain patterning and the relationship between expression of Hoxa-1 and retinoic acid.     

Segmental identity can change independently in the hindbrain and rhombencephalic neural crest

In this study we tested whether the segmental identities of the hindbrain and its derived neural crest are necessarily linked or, instead, if they can be altered independently. Using morphological criteria, we show that the hindbrains of Hoxa-2 mutant mice, in which the second arch skeletal derivatives assume first arch characteristics (Gendron-Maguire et al. [1993] Cell 75:1317-1331; Rijli et al. [1993] Cell 75:1333-1349), retain normal segmental identities. Also, by phenotypic analysis, we show that, with retinoic acid, changes can be induced in the identity of the preotic hindbrain without effects in its derived neural crest. Our data thus indicate that identity changes in the hindbrain and branchial arch neural crest can occur independently. Moreover, if Hoxa-2 is concomitantly induced by retinoic acid in the first branchial arch, the proximal derivatives of this arch are also affected. We propose a model for the patterning of the branchial region, according to which the segmental identity in this area is provided mainly by the branchial arches.     

A new source of cells contributing to the developing gastrointestinal tract demonstrated in chick embryos

BACKGROUND & AIMS: Smooth muscle cells in the walls of the gastrointestinal tract are thought to derive solely from mesoderm surrounding the primitive gut. A population of neuroepithelial cells has recently been shown to migrate from the ventral part of the neural tube in the region joined by the vagus nerve. We sought to determine if these cells contributed to the development of the stomach and intestine. METHODS: Cells of the ventral hindbrain of chick embryos were tagged by replication-deficient retroviral vectors containing the lacZ gene, providing a permanent label that is transmitted without dilution as the cells divide. Embryos were processed for detection of labeled cells. Specific markers were used to determine differentiation of progeny in the gastrointestinal tract. RESULTS: Cells labeled in the ventral neural tube migrate in association with the vagus nerve. Labeled cells are found in the intestine and stomach after time for further migration and differentiation. Using a specific marker, they were clearly identified as smooth muscle cells. CONCLUSIONS: Some of the smooth muscle cells of the gastrointestinal tract are derived from precursor cells that originate in the ventral part of the hindbrain neural tube. Their developmental importance and functional significance remain to be determined.     

HOXD4 and regulation of the group 4 paralog genes

From an evolutionary perspective, it is important to understand the degree of conservation of cis-regulatory mechanisms between paralogous Hox genes. In this study, we have used transgenic analysis of the human HOXD4 locus to identify one neural and two mesodermal 3' enhancers that are capable of mediating the proper anterior limits of expression in the hindbrain and paraxial mesoderm (somites), respectively. In addition to directing expression in the central nervous system (CNS) up to the correct rhombomere 6/7 boundary in the hindbrain, the neural enhancer also mediates a three rhombomere anterior shift from this boundary in response to retinoic acid (RA), mimicking the endogenous Hoxd4 response. We have extended the transgenic analysis to Hoxa4 identifying mesodermal, neural and retinoid responsive components in the 3' flanking region of that gene, which reflect aspects of endogenous Hoxa4 expression. Comparative analysis of the retinoid responses of Hoxd4, Hoxa4 and Hoxb4 reveals that, while they can be rapidly induced by RA, there is a window of competence for this response, which is different to that of more 3' Hox genes. Mesodermal regulation involves multiple regions with overlapping or related activity and is complex, but with respect to neural regulation and response to RA, Hoxb4 and Hoxd4 appear to be more closely related to each other than Hoxa4. These results illustrate that much of the general positioning of 5' and 3' flanking regulatory regions has been conserved between three of the group 4 paralogs during vertebrate evolution, which most likely reflects the original positioning of regulatory regions in the ancestral Hox complex.     

Expression of the human antigen SPAG2 in the testis and localization to the outer dense fibers in spermatozoa

There is increasing evidence to suggest that opioid peptides may have widespread effects as regulators of growth. To evaluate the hypothesis that endogenous opioids control cellular proliferation during neural development, we have used in situ hybridization to examine opioid peptide and receptor mRNA expression in neuroepithelial zones of fetal rat brain and spinal cord. Our data show that proenkephalin mRNA is widely expressed in forebrain germinal zones and choroid plexus during the second half of gestation. In contrast, prodynorphin mRNA expression is restricted to the periventricular region of the ventral spinal cord. Little mu or delta receptor mRNA expression was detected in any regions of neuronal proliferation prior to birth. However, kappa receptor mRNA is widely expressed in hindbrain germinal zones during the 3rd week of gestation. Our present findings support the hypothesis that endogenous opioids may regulate proliferation of both neuronal and non-neuronal cells during central nervous system development. Given the segregated expression of proenkephalin mRNA in forebrain neuroepithelium and kappa receptor mRNA within hindbrain, different opioid mechanisms may regulate cell division in rostral and caudal brain regions.     

Angiotensinogen genotype, sodium reduction, weight loss, and prevention of hypertension: trials of hypertension prevention, phase II

Hox genes are segmentally expressed in the developing vertebrate hindbrain, neural crest cells and pharyngeal arches suggesting an important role in patterning these structures. Here we discuss the cellular and molecular mechanisms controlling segmentation and specification in the branchial region of the head. In addition, based on the recent phenotypical and molecular analysis of loss-of-function mutants in the mouse, we speculate that Hox genes may act like Drosophila selector genes in this system.     

Effects of 13-cis-retinoic acid on hindbrain and craniofacial morphogenesis in long-tailed macaques (Macaca fascicularis)

Hindbrain and craniofacial development during early organogenesis was studied in normal and retinoic acid-exposed Macaca fascicularis embryos. 13-cis-retinoic acid impaired hindbrain segmentation as evidenced by compression of rhombomeres 1 to 5. Immunolocalization with the Hoxb-1 gene product along with quantitative measurements demonstrated that rhombomere 4 was particularly vulnerable to size reduction. Accompanying malformations of cranial neural crest cell migration patterns involved reduction and/or delay in pre- and post-otic placode crest cell populations that contribute to the pharyngeal arches and provide the developmental framework for the craniofacial region. The first and second pharyngeal arches were partially fused and the second arch was markedly reduced in size. The otocyst was delayed in development and shifted rostrolaterally relative to the hindbrain. These combined changes in the hindbrain, neural crest, and pharyngeal arches contribute to the craniofacial malformations observed in the retinoic acid malformation syndrome manifested in the macaque fetus.     

Ectopic expression of hoxb2 after retinoic acid treatment or mRNA injection: disruption of hindbrain and craniofacial morphogenesis in zebrafish embryos

To investigate pattern formation in the vertebrate hindbrain, we isolated a full length hoxb2 cDNA clone from zebrafish. In a gene phylogeny, zebrafish hoxb2 clusters with human HOXB2, and it maps on linkage group 3 along with several other loci whose orthologues are syntenic with human HOXB2. In the hindbrain, hoxb2 is expressed at high levels in rhombomere 3 (r3), lower levels in r4, still lower in r5, and at undetectable levels in r6. In r7, r8, and the rostral spinal cord, hoxb2 is expressed at a lower level than in r5. Lateral cells appearing to emanate from r4 express both hoxb2 and dlx2, suggesting that they are neural crest. Overexpression of hoxb2 by mRNA injections into early cleavage stage embryos resulted in abnormal morphogenesis of the midbrain and rostral hindbrain, abnormal patterning in r4, fusion of cartilage elements arising from pharyngeal arches 1 and 2, and ectopic expression of krx20 and valentino (but not pax2, rtk1, or hoxb1) in the rostral hindbrain, midbrain, and, surprisingly, the eye. Treatments with retinoic acid produced a phenotype similar to that of ectopic hoxb2 expression, including ectopic krx20 (but not valentino) expression in the eye, and fusion of cartilages from pharyngeal arches 1 and 2. The results suggest that hoxb2 plays an important role in the patterning of hindbrain and pharyngeal arches in the zebrafish.