The cortical distributions were radially symmetric. The mechanisms that generate these distributions are not known, but because the patterns of bcd RNA and protein are so similar, we suggest that the mechanisms are probably related.
Although these mechanisms cannot be deduced from the geometries of the distributions, the complex and well-defined shapes and the rapidity with which the distributions form and change would seem to be incompatible with passive diffusion.
We assume that the bcd RNA and Bcd protein of the early, first gradient contribute to the second, and therefore call this a two-step model. This model contrasts with the models that have been proposed previously in which the dispersion of protein and RNA from the anterior pole is continuous during both the pre-syncytial blastoderm and syncytial blastoderm stages Figure 9.
Although the two-step model is presumably generated by motor-driven directed movement, the high yolk content of the embryo cytoplasm has impeded analysis of its cytoskeletal elements. However, several studies have reported microtubule networks in the pre-syncytial embryo Fahmy et al. These networks offer a possible mechanism that might transport particles of bcd RNA Fahmy et al.
Two contrasting models of Bcd gradient formation are depicted as viewed from a sagittal section in the middle of pre-cellular embryos, oriented anterior left.
A model that assumes a continuous redistribution of bcd RNA and Bcd protein red from the anterior pole upper row contrasts with the two-step model lower row in which the bcd products generate a plume in the middle of the embryo during the first four nuclear cycles and then generate a second gradient at the cortex in the syncytial blastoderm stages nc Our analysis of Kr expression revealed that transcription initiates as early as nc7 Figure 1.
These results show that Bcd regulates Kr in nuclei that are far from the cortex, and importantly, that the internal plume of Bcd protein is a functional distribution. Although these findings do not imply what the role of the subsequent cortical Bcd gradient in the syncytial blastoderm stages Bcd gradient might be, it is most likely that the cortical gradient derives from the earlier distribution and therefore that it reflects the outputs of the pre-syncytial blastoderm, internal gradient.
Morphogens such as Hedgehog, Wingless, Decapentaplegic, and Fibroblast growth factor distribute in concentration gradients across fields of cells in the tissues of developing animals.
The generation of the Bcd concentration gradients in the pre-cellular embryo would appear to have little in common with the gradients that form by cytoneme-mediated dispersion across fields of cells, but we pose the question whether they do.
Neither appears to be dependent on passive diffusion and both appear to involve dispersion along cytoskeletal cables. The critical attribute that these mechanisms share is that they provide ways to regulate movement in space and time. Embryos 0. Embryos were incubated for 1. For secondary antibody, embryos were incubated with secondary antibody in 0.
Stage 14a and 14b oocytes were not extruded from the distal tip of the ovary; 14c and 14d oocytes were wholly or partially in a lateral branch of the oviduct. The stage 14c and 14d oocytes were sensitive to hypochlorite treatment and had not completed meiosis I, and were therefore designated as not activated Sartain and Wolfner, Oocytes isolated either without or with incubation were subdivided and either stained with DAPI or processed for in situ hybridization or antibody staining.
Results of in situ hybridization and antibody staining with oocytes obtained from freshly dissected ovaries and from ovaries incubated ex vivo were indistinguishable. Fixed oocytes in MeOH were transferred to a glass cavity slide and viewed under a dissecting microscope. Fine forceps were used to hold the dorsal appendages and the dorso-lateral side of the chorion and vitelline membrane were sliced with a fine needle.
Oocytes were incubated with secondary antibody in 0. Embryos were mounted with slight pressure under coverslips and orientation was determined by analysis of serial optical sections.
R and quantified by absorbance with a nanodrop spectrophotometer. In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.
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The Drosophila egg and early embryo provide much of our current insight into how maternal factors contribute to early developmental patterning. The formation of the bicoid protein gradient by the regulated translation of maternally localized bcd mRNA represents a key aspect of this process, one that has become the textbook example of a "morphogen gradient.
The consensus opinion among the reviewers is that the work presented here adds to our understanding of Drosophila embryo patterning and Bcd gradient formation. Several important findings are reported. This proves that the process of gradient formation must be more complex than previously thought. This demonstrates that the translational block of bcd mRNA is relieved prior to fertilization and that Bcd protein is present precisely where bcd mRNA is sequestered. The authors describe a different shape and demonstrate that zygotic gene expression starts earlier than the pre-cellular blastoderm stage by forcing a revision in the previously accepted timing and spatial distribution of bcd RNA and protein, and by contradicting some aspects of the leading model for how the gradient is produced.
In the Discussion, the authors propose a two-step model of Bcd gradient formation, which differs from an earlier model mainly by the initial gradient formation of the medial plume during nc This gradient is followed by a cortical gradient, consistent with a model previously proposed Spirov et al. This latter model and the model proposed by the authors are consistent with the gradient being formed by the bcd mRNA that, by translation, produces the Bcd protein gradient although the authors are more conservative.
The most important conclusion is that mechanisms of morphogen gradient formation in general are not dependent on passive diffusion, overthrowing a tenet maintained over many decades, but "appear to involve dispersion along cytoskeletal cables.
While the authors document expression earlier than previously reported, an achievement that likely required improved sensitivity of DIG in situ hybridization as well as of antibody staining of early embryos to achieve a much better signal-to-noise ratio.
However, the manuscript does not include experiments directly comparing different in situ hybridization methods, including in particular smFISH, which is generally accepted to have greater sensitivity in many applications. It remains unclear why the low level expression described here was not detected in previous studies. Consequently, statements about the relative sensitivity of various methods should be removed from the manuscript unless data from such tests are added.
A recent study shows that stage 14 oocytes about to be ovulated can be recognized by the loss of posterior follicle cells Deady et al.
The authors may wish to investigate whether the onset of bcd translation takes place before or after ovulation, in vivo. A small number of relatively minor revisions are now requested.
The text in the Results now reads:. The earlier literature showing foci of in situ hybridization as sites of nascent transcript has been cited as follows:. The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication. We thank: Drs. This article is distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use and redistribution provided that the original author and source are credited.
Article citation count generated by polling the highest count across the following sources: Crossref , Scopus , PubMed Central. Retinal progenitor cells RPCs divide in limited numbers to generate the cells comprising vertebrate retina. The molecular mechanism that leads RPC to the division limit, however, remains elusive.
We further show the hyperproliferation of Tsc1 -deficient RPCs and the degeneration of MG in the mouse retina disappear by concomitant deletion of hypoxia-induced factor 1-alpha Hif1a , which induces glycolytic gene expression to support mTORC1-induced RPC proliferation. Collectively, our results suggest that, by having mTORC1 constitutively active, an RPC divides and exhausts mitotic capacity faster than neighboring RPCs, and thus produces retinal cells that degenerate with aging-related changes.
This article has been corrected. Read the correction notice. Cited 21 Views 6, Annotations Open annotations. The current annotation count on this page is being calculated. Cite this article as: eLife ;5:e doi: Figure 1 with 2 supplements see all. Download asset Open asset. Figure 2.
Figure 2—source data 1 Source data for 2B. Download elifefig2-data1-v3. Ptx1 is the only nuclear factor of this DNA-binding specificity that is detected in AtT cells, and it is expressed at high levels in a subset of adult anterior pituitary cells that express POMC.
However, Ptx1 is expressed in most cells of Rathke's pouch at an early time during pituitary development and before final differentiation of hormone-producing cells. Thus, Ptx1 may have a role in differentiation of pituitary cells; its early expression pattern suggests that it may have a role in pituitary formation.
In the adult pituitary gland, Ptx1 appears to be recruited for cell-specific transcription of the POMC gene Lamonerie, Ptx1, a homeo box gene related to bicoid and orthodenticle and Otx-1 and Otx-2 in mammals, activates transcription upon binding a sequence related to the target sites bicoid and orthodenticle in Drosophila Ptx1 is a member of the small bicoid family of homeobox-containing genes; it was isolated as a tissue-restricted transcription factor of the pro-opiomelanocortin gene.
The homeodomain of Ptx1 contains a lysine at position 9 of the recognition helix position 60 of the homeodomain. This residue is strategically placed in the major groove of DNA and it is a major determinant of DNA-binding specificity recognizing the CC doublet of the target site. This lysine residue defines the bicoid subfamily of homeoboxes, including Otx1 and 2 and Goosecoid. Ptx1 expression during mouse and chick embryogenesis was determined by in situ hybridization in order to delineate its putative role in development.
In the head, Ptx1 expression is first detected in the ectoderm-derived stomodeal epithelium at E8. Initially, expression is only present in the stomodeum and in a few cells of the rostroventral foregut endoderm. A day later, Ptx1 mRNA is detected in the epithelium and in a streak of mesenchyme of the first branchial arch, but not in other arches.
Ptx1 expression is maintained in all derivatives of these structures, including the epithelia of the tongue, palate, teeth and olfactory system, and in Rathke's pouch. Expression of Ptx1 in craniofacial structures is strikingly complementary to the pattern of goosecoid See Drosophila Goosecoid expression.
Gsc labelling in the mandibular component is confined to a central stripe of mesenchyme whereas Ptx1 labelling is observed more laterally. Similarly, the epithelium of the first arch, a site of strong Ptx1 expression, is not labelled by the Gsc probe. Ptx1 is expressed early E6. The restriction of expression to the posterior lateral plate is later evidenced by exclusive labelling of the hindlimb but not forelimb mesenchyme. In the anterior domain of expression, the stomodeum is shown by fate mapping to derive from the anterior neural ridge ANR which represents the most anterior domain of the embryo.
The concordance between these fate maps and the stomodeal pattern of Ptx1 expression supports the hypothesis that Ptx1 defines a stomodeal ectomere that lies anterior to the neuromeres that have been suggested to constitute units of a segmented plan directing head formation. Drosophila Gsc is expressed in the stomodeal invagination, while vertebrate Gsc is not. Based on these gene expression patterns, it is thought that the vertebrate stomodeum is an evolutionary innovation, assuring the ventral placement of the mouth Lanctot, Pituitary homeobox 1 Ptx1 is a homeodomain-containing transcription factor acting on transcription of all pituitary hormone genes.
Its expression is first detected in the stomodeal ectoderm and is maintained in all derivatives of this structure, including Rathke's pouch. Ptx1 is expressed in all pituitary cells but it is differentially expressed in different lineages at both the messenger RNA and protein levels. On day Coimmunolocalization studies reveal that alpha-glycoprotein subunit-positive cells express the highest levels of Ptx1 throughout development and in the adult gland.
Ptx1 belongs to an expanding family of bicoid-related vertebrate homeobox genes. These genes, like their Drosophila homolog, seem to play a role in the development of anterior structures and, in particular, the brain and facies.
The chromosomal localization of mouse Ptx1 is reported, and the cloning, sequencing, and chromosomal localization of the human homolog PTX1. Murine Ptx1 was localized, by interspecific backcrossing, to Chromosome Chr 13 within 2.
The gene resides centrally on Chr 13 in a region syntenic with human Chr 5q. Subsequent analysis by fluorescent in situ hybridization places the human gene, PTX1, on 5q31, a region associated with Treacher Collins Franceschetti Syndrome. Taken together with the craniofacial expression pattern of Ptx1 during early development, the localization of the gene in this chromosomal area is consistent with an involvement in Treacher Collins Franceschetti Syndrome Crawford, The Ptx1 pituitary homeobox 1 homeobox transcription factor was isolated as a transcription factor of the pituitary POMC gene.
In corticotrope cells that express POMC, cell-specific transcription is conferred in part by the synergistic action of Ptx1 with the basic helix-loop-helix factor NeuroD1.
Since Ptx1 expression precedes pituitary development and differentiation, its expression and function in other pituitary lineages was investigated. Ptx1 is expressed in most pituitary-derived cell lines, as is the related Ptx2 gene.
However, Ptx1 appears to be the only Ptx protein in corticotropes and the predominant one in gonadotrope cells. Most pituitary hormone-coding gene promoters are activated by Ptx1. Thus, Ptx1 appears to be a general regulator of pituitary-specific transcription. In addition, Ptx1 action is synergized by cell-restricted transcription factors to confer promoter-specific expression. Indeed, in the somatolactotrope lineage, synergism between Ptx1 and Pit1 is observed on the PRL promoter, and strong synergism between Ptx1 and SF-1 is observed in gonadotrope cells on the betaLH promoter but not on the alphaGSU glycoprotein hormone alpha-subunit gene and betaFSH promoters.
Synergism between these two classes of factors is reminiscent of the interaction between Drosophila Fushi tarazu and Ftz-F1. Antisense RNA experiments performed in alphaT cells that express the alphaGSU gene show that expression of endogenous alphaGSU is highly dependent on Ptx1, whereas many other genes are not affected.
Human Ptx3 is a member of the Bicoid-related subgroup of transcription factors that includes Drosophila Bicoid, Orthodenticle and Goosecoid. The mesencephalic dopaminergic mesDA system regulates behavior and movement control and has been implicated in psychiatric and affective disorders.
Ptx3 is uniquely expressed in the neurons of this system. Its expression starting at E The number of Ptx3-expressing neurons is reduced in Parkinson patients, and these neurons are absent from 6-hydroxydopamine-lesioned rats, an animal model for this disease.
Thus, Ptx3 is a unique transcription factor marking the mesDA neurons at the exclusion of other dopaminergic neurons, and it may be involved in developmental determination of this neuronal lineage Smidt, Genetic analysis of mouse mutants has demonstrated the importance of the homeobox genes Rpx, Lhx3 and Pit1 for anterior pituitary gland development.
Pit1 mutations have also been identified in several human families with multiple pituitary hormone deficiencies. To identify additional homeobox regulators of pituitary development, an adult pituitary gland cDNA library was screened for homeobox sequences. The identification of a novel bicoid-related homeodomain gene is reported, expressing two alternatively spliced mRNA products that encode proteins of and amino acids, respectively. Ptx2 is expressed in both developing and adult pituitary gland, eye and brain tissues, suggesting an important role in development and maintenance of anterior structures.
Ptx2 was mapped close to Egf on mouse chromosome 3, in a region having extensive synteny homology with HSA 4q. These data make the human Ptx2 homolog a candidate gene for Rieger syndrome, an autosomal-dominant disorder with variable craniofacial, dental, eye and pituitary anomalies Gage, Signaling molecules such as Activin, Sonic hedgehog, Nodal, Lefty, and Vg1 have been found to be involved in determination of left-right L-R asymmetry in the chick, mouse, or frog.
However, a common signaling pathway has not yet been identified in vertebrates. Pitx2, a bicoid-type homeobox gene expressed asymmetrically in the left lateral plate mesoderm, may be involved in determination of L-R asymmetry in both mouse and chick. Since Pitx2 appears to be downstream of lefty-1 in the mouse pathway, whether mouse Lefty proteins could affect the expression of Pitx2 in the chick was examined.
The results indicate that a common pathway from lefty-1 to Pitx2 likely exists for determination of L-R asymmetry in vertebrates Yoshioka, Left-right asymmetry in vertebrates is controlled by activities emanating from the left lateral plate. How these signals get transmitted to the forming organs is not known. A candidate mediator in mouse, frog and zebrafish embryos is the homeobox gene Pitx2. It is asymmetrically expressed in the left lateral plate mesoderm, tubular heart and early gut tube.
Localized Pitx2 expression continues when these organs undergo asymmetric looping morphogenesis. Ectopic expression of Xnr1 in the right lateral plate induces Pitx2 transcription in Xenopus.
Misexpression of Pitx2 affects situs and the morphology of organs. These experiments suggest a role for Pitx2 in promoting looping of the linear heart and gut Campione, The cloning and temporal and spatial expression patterns are reported for a novel homeobox gene Backfoot BFT for the human gene; Bft for the mouse gene whose expression reveals an early molecular distinction between forelimb and hind limb.
In addition to the homeodomain, it shares a carboxyl-terminal peptide motif with other paired-like homeodomain proteins. Northern hybridization analysis of RNAs from human tissues reveals that human BFT is highly expressed in adult skeletal muscle and bladder.
During midgestation embryogenesis, mouse Bft is expressed in the developing hind limb buds, mandibular arches, and Rathke's pouch. The expression of Bft begins prior to the appearance of hind limb buds in mesenchyme but is never observed in forelimbs. At later stages of limb development, the expression is progressively restricted to perichondrial regions, most likely in tendons and ligaments.
Delete Content. Delete Cancel. A fresh Drosophila egg shows a concentration of bicoid bcd expression in its anterior end. This image is linked to the following Scitable pages:. You may take for granted the fact that your body isn't the same from head to toe.
Have you stopped and wondered why? Controlling gene expression to turn on and off at specific times is no simple feat. Comments Close. The Comment you have entered exceeds the maximum length.
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