A crucial resource for studying vertebrate embryology is the genus of African frogs known as Xenopus, also referred to as the "African clawed frogs." It makes it possible to understand how networks of regulatory and interactional systems control embryonic development. Early vertebrate development's morphogenetic processes are controlled by the combined activity of a large number of genes and a population of cells (Schoenwolf). The shroom family of proteins are the primary regulators of the major developmental steps of apical constriction, hinge point creation, and neural tube closure. The alternative experiment to test the validity of this alternative explanation involved an analysis by whole amount of spatial and temporal expression of shroom family proteins in situ hybridization in a series of diverse stages of embryo from mid-gastrula to late tail bud stages.
During gastrulation in both invertebrates and vertebrates, apical constriction leads to blastopore formation. The contraction of the apical side of a cell causes the cell to take a wedged shape. This shape change is coordinated across the many cells of the epithelial layer. This generates the forces that can bend or fold the cell sheet. Shroom, an actin binding protein plays a key role in inducing apical constriction. Shroom-induced apical constriction is associated with actin accumulation (Haigo et al). The expression of shroom in the Xenopus blastula results in a dramatic accumulation of actin coincident with apical constriction. This has been observed in apically constricting cells often displaying a concentration of apically localized actin.
Shroom is required for hinge point formation and epithelial sheet bending during neurulation- the transformation of the neural plate into the neural tube. During this process, apical constriction does not occur uniformly throughout the neural plate. The cell shape change is instead pronounced at discrete regions in the hinge points (Haigo et al). These forms distinct lines of bending that facilitate medial movements of the neural folds. In Xenopus development, this occurs predominantly in two paired hinge points.
The closure of the neural tube normally does not take place simultaneously. It closes as the paired neural folds are brought together at the dorsal midline. Shroom is sufficient to bring on apical constriction in epithelia. One of the most intriguing studies is that epithelial sheet bending may be patterned in embryos simply by regulating expression of a single gene. Identifying the regulators of shroom is thus crucial.
Shroom is adequate for inducing apical constriction of the native epithelial cells. In this regard, to evaluate the Shroom’s activities, there is a need to express the protein within the population of the epithelial cells (Haigo et al). The blastomeres' outer layer within the early Xenopus blastula offers the best model of epithelium. Just like in most animals, the outer cells of cleavage-stage Xenopus embryo make the epithelium to possess a strong apicobasal polarity shown by the apically contained epithelial junctions, as well as by the apicobasal differences within the membrane adhesive properties and the targeting secretory vesicles and membrane proteins.
In the course of the early development, such epithelial cells are found to be undifferentiated and remain to be transcriptionally quiescent for several hours; hence, ectopic protection on the behavior of cells serves to represent a direct impact of the protein on the cellular mechanism that is already in place within the native epithelial cells. Fundamentally, to test the Shroom’s function, an injection of the mRNA into the animal pole of some two blastomeres in four cell Xenopus blastula is carried out. After that, their effects are examined before blastomeres differentiation and before the beginning of the zygotic transcription (Schoenwolf). The expression of the wild kind mouse Shroom induces a dramatic pigment concentration within undifferentiated blastomeres.
Since the pigment granules are localized apically, the cells’ apical constriction brings about an increased pigments granules density. Moreover, to carry out the test on the likelihood that the concentration of the Shroom-induced pigment results from the apical constriction, examination of the external shroom-expressing blastulae, in comparison with the cuboidal control cells was carried out (Haigo et al). The pigment concentrations, as well as apical constriction, are observed after Shroom expression in any external blastomere, irrespective of presumptive fate or position.
When observed from the surface, the cells, which are darkly pigmented, appear to have some smaller surface compared to the neighboring cells that are normally pigmented in a similar pattern like the apical constriction (Schoenwolf). To authenticate this finding, there is a need to quantify the apical surface are of the Shroom-expressing cells. In this regard, the average surface of the darkly pigmented outer blastomeres in the Shroom-expressing blastulae is found not to be different from the one of the uninjected blastulae. Shroom is a master regulator in Xenopus development study, a model tool used by Biologist to study vertebrate embryology. This is shown in its key role in controlling apical constriction, hinge point formation and neural tube closure, important stages in morphogenesis.
Works Cited
Haigo, Saori et al. “Shroom Induces Apical Constriction and Is Required for Hingepoint
Formation during Neural Tube Closure.” Current Biology, vol. 13, no. 14, 2003, pp. 2125-2137.
Schoenwolf, Charles. “Microsurgical analyses of avian neurulation: separation of medial and
lateral tissues.” J. Comp. Neurol, vol. 276, 1988, pp. 498-507.
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