Biomedical subjects

M S Saha

Publications and source records attributed to M S Saha.

10 recordsLinked to original sources

PubMed1997

Dorsal-ventral patterning during neural induction in Xenopus: assessment of spinal cord regionalization with xHB9, a marker for the motor neuron region.

While the role of the notochord and floor plate in patterning the dorsal-ventral (D/V) axis of the neural tube is clearly established, relatively little is known about the earliest stages of D/V regionalization. In an effort to examine more closely the initial, preneural plate stages of regionalization along the prospective D/V neural axis, we have performed a series of explant experiments employing xHB9, a novel marker of the motor neuron region in Xenopus. Using tissue recombinants and Keller explants we show that direct mesodermal contact is both necessary and sufficient for the initial induction of xHB9 in the motor neuron region. We also show that presumptive neural plate explants removed as early as midgastrulation and cultured in isolation are already specified to express xHB9 but do so in an inappropriate spatial pattern while identical explants are specified to express the floor plate marker vhh-1 with correct spatial patterning. Our data suggest that, in addition to floor plate signaling, continued interactions with the underlying mesoderm through neural tube stages are essential for proper spatial patterning of the motor neuron region.

Amino Acid Sequence↗

PubMed1997

Neovascularization of the Xenopus embryo.

The receptor tyrosine kinase, Flk-1 or VEGFR-2, and its ligand, vascular endothelial growth factor (VEGF) are required for the development of the embryonic vasculature. Targeted disruption of either gene in mice results in the failure of vascular system formation. The Xenopus homologues of flk-1 and VEGF have been cloned and their expression has been examined throughout early embryonic development. These studies indicate that flk-1 is expressed in groups of endothelial precursor cells which will form the major blood vessels of the embryo, including the posterior cardinal veins, the dorsal aorta, the vitelline veins, and the endocardium. VEGF expression is found in tissues adjacent to the mesenchyme containing the flk-1-expressing endothelial precursors. Expression of both flk-1 and VEGF is transient, appearing as the primary vascular plexus is forming and declining steadily after the onset of functional embryonic circulation. After establishment of the primary vascular structures, flk-1 expression is also observed in the intersegmental veins which form by an angiogenic mechanism. Overall, these results support a role for VEGF/flk-1 signaling in both vasculogenesis and angiogenesis in the Xenopus embryo. When VEGF is expressed ectopically in Xenopus embryos by microinjection of either plasmid DNA or synthetic mRNA, large, disorganized vascular structures are produced. This result indicates that ectopic VEGF is capable of altering the architecture of the developing vascular network.

Amino Acid Sequence↗

PubMed1994

Xenopus gamma-crystallin gene expression: evidence that the gamma-crystallin gene family is transcribed in lens and nonlens tissues.

Crystallins, the major gene products of the lens, accumulate to high levels during the differentiation of the vertebrate lens. Although crystallins were traditionally thought to be lens specific, it has recently been shown that some are also expressed at very low levels in nonlens tissues. We have examined the embryonic expression pattern of gamma-crystallins, the most abundant crystallins of the embryonic lens in Xenopus laevis. The expression profile of five Xenopus gamma-crystallin genes mirrors the pattern of lens differentiation in X. laevis, exhibiting on average a 100-fold increase between tailbud and tadpole stages. Four of these genes are also ubiquitously expressed outside the lens at a very low level, the first demonstration of nonlens expression of any gamma-crystallin gene; expression of the remaining gene was not detected outside the head region, thus suggesting that there may be two classes of gamma-crystallin genes in X. laevis. Predictions regarding control mechanisms responsible for this dual mode of expression are discussed. This study raises the question of whether any crystallin, on stringent examination, will be found exclusively in the lens.

Animals↗

PubMed1993

Characterization of Xenopus laevis gamma-crystallin-encoding genes.

In order to gain insight into crystallin (Cry)-encoding gene (cry) evolution and developmental function, we have determined the gene structure and sequence of several Xenopus laevis gamma-cry. These encode the most abundant Cry in the embryonic lens. Four of the X. laevis gamma-cry, which are part of a multigene family, were isolated from a X. laevis genomic library and demonstrated to have the same gene structure as gamma-cry from other vertebrates, thereby providing further evidence that the split between beta and gamma members of the beta gamma cry family occurred relatively early in evolution. Sequence comparisons indicate that these X. laevis genes share 88-90% nucleotide sequence identity in the protein coding regions, which is slightly higher than the identity observed between gamma-cry of other species. The 5' upstream regions of X. laevis gamma-cry contain a few short stretches of homology and one putative promoter element conserved among all cry genes but lack other regions common to gamma-cry promoters from other organisms. The deduced amino acid sequences of all four genes and one cDNA suggest that the structure of X. laevis gamma-Cry is highly conserved with that of other vertebrate gamma-Cry, as deduced from the known three-dimensional structure of bovine gamma B Cry.

Amino Acid Sequence↗

PubMed1993

Early opsin expression in Xenopus embryos precedes photoreceptor differentiation.

The visual pigment which serves as the first step in the phototransduction cycle in vertebrate rod cells consists of a retinal chromophore which is linked to the transmembrane protein, opsin. Opsin genes have been isolated from a number of different organisms and studies have shown opsin to be developmentally regulated with both mRNA and protein expression associated with the morphological differentiation of photoreceptor cells. Due to its potential utility as a marker for rod photoreceptor determination in studies of retinal tissue interactions, and because no amphibian opsin genes have as yet been cloned, we isolated cDNA clones of the Xenopus laevis opsin gene. Sequence analysis shows that within the coding region Xenopus opsin shares a high degree of identity with other rod opsin genes, except at the C-terminal where it more closely resembles the mammalian color opsins. A developmental analysis, on the other hand, reveals that Xenopus opsin transcripts are detectable in a retina-specific fashion early in retinal development. Using in situ hybridization we find that Xenopus opsin mRNA is initially restricted to a few isolated cells in the presumptive photoreceptor layer which express the gene at relatively high levels. This suggests that rod photoreceptor determination occurs in single cells, and that the mechanisms controlling opsin expression in Xenopus are initiated well before any evidence of morphological differentiation.

Amino Acid Sequence↗

PubMed1993

A Xenopus homebox gene defines dorsal-ventral domains in the developing brain.

One of the distinguishing features of vertebrate development is the elaboration of the anterior neural plate into forebrain and midbrain, yet little is known about the early tissue interactions that regulate pattern formation in this region or the genes that mediate these interactions. As an initial step toward analyzing the process of regionalization in the anterior-most region of the brain, we have screened an anterior neural cDNA library for homeobox clones and have identified one which we have called XeNK-2 (Xenopus NK-2) because of its homology to the NK-2 family of homeobox genes. From neurula stages, when XeNK-2 is first detectable, through hatching stages, XeNK-2 mRNA is expressed primarily in the anterior region of the brain. By swimming tadpole stages, XeNK-2 expression resolves into a set of bands positioned at the forebrain-midbrain and the midbrain-hindbrain boundaries, after which XeNK-2 transcripts are no longer detectable. In addition to localized expression along the anterior-posterior axis, XeNK-2 may also play a role in the process of regionalization along the dorsal-ventral axis of the developing brain. At all stages examined, XeNK-2 mRNA is restricted to a pair of stripes that are bilaterally symmetrical in the ventral-lateral region of the brain. To begin to identify the tissue interactions that are required for the proper spatial and temporal localization of XeNK-2, we have performed a series of explant experiments. Consistent with earlier work showing that the A/P axis is not fixed at mid-gastrula stages, we show that XeNK-2 expression is activated when assayed in gastrula stage explants taken from any region along the entire A/P axis and that the tissue interactions necessary to localize XeNK-2 along the A/P axis are not completed until later neurula stages.

Amino Acid Sequence↗

PubMed1992

A labile period in the determination of the anterior-posterior axis during early neural development in Xenopus.

The process by which the vertebrate central nervous system acquires its regional properties remains a central problem in developmental biology. It is generally argued that at early gastrula stages the dorsal mesoderm possesses precise anterior-posterior positional information, which is subsequently imparted to the overlying ectoderm. However, using regionally specific gene probes to monitor regional responses in Xenopus embryos, we find that anterior-posterior properties are not fixed until early neurula stages. During gastrulation the regional inducing capacities of the dorsal mesoderm as well as the regional responses of the presumptive neural ectoderm are activated along the entire anterior-posterior axis when these properties are assayed in recombinant and explant experiments, respectively. Restriction of regional inducing capacity in the mesoderm and responsiveness in the neural ectoderm occur only at neural plate stages.

Animals↗

PubMed1992

Vertebrate eye development.

Vertebrate eye determination is mediated by a series of inductive interactions that have now been more precisely defined with the use of regional markers. Analyses of the genes responsible for eye mutations and the cloning of genes delimiting spatial domains within the developing eye have begun to elucidate the molecular basis of this process.

Animals↗

PubMed1992

Recent progress on the mechanisms of embryonic lens formation.

Formation of the lens during embryonic development depends on tissue interactions as shown clearly both from teratological data and from extensive experimental analysis. Recent work has, however, altered our view of the importance of particular tissue interactions for lens formation. While earlier work emphasises the role of the optic vesicle in lens induction, more recent studies argue that lens-inducing signals important for determination act before optic vesicle formation. Evidence is given for a four stage model in which ectoderm first becomes competent to respond to lens inducers. It then receives inductive signals, at least in part emanating from the anterior neural plate, so that it gains a lens-forming bias and subsequently becomes specified for lens formation. Complete lens differentiation does require signals from the optic vesicle, and in addition an inhibitory signal from head neural crest may suppress any residual lens-forming bias in head ectoderm adjacent to the lens.

Animals↗

PubMed1989

Embryonic lens induction: more than meets the optic vesicle.

The classic model of lens induction stipulated that the optic vesicle is both a necessary and sufficient inductor of the lens in amphibian development. Although this view has subsequently been modified to encompass the contributions of earlier inductors, such as the involuting endo-mesoderm, it is still widely reported that the optic vesicle alone is able to elicit lens formation from ectoderm. Recent work, which has employed a host- and donor-marking scheme, has shown the optic vesicle to be a weak inductor of the lens, involved only in the final rather than the initial phases of determination. In addition, a review of the literature substantiates this conclusion since many of the transplantation experiments arguing for the sufficiency of the optic vesicle are characterized by the lack of adequate criteria for judging the authenticity of the resulting lens responses, particularly the absence of a host- and donor-marking strategy. This analysis of the literature, together with our own results, lead us to propose a new model of lens determination in which tissue interactions during gastrulation are required to confer a lens-forming bias upon a large area of head ectoderm allowing the optic vesicle to induce lens formation in a defined area of this primed ectoderm. Data from studies on mesoderm and neural induction are also beginning to suggest a multistep model involving the initial establishment of bias and subsequent interactions resulting in determination, and we propose that this framework will serve as a general paradigm for embryonic induction.

Amphibians↗