Readings for TT2 from Gilbert's Dev Biol

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Cell and Systems Biology
Maurice Ringuette

P. 250-251 FN and the Pathways for Mesodermal Migration  How are the involuting cellsinformed where to go once they enter the inside of the embryo?  Amphibians – involuting mesodermal precursors migrate toward the AP on a FN lattice secreted by the cells of the blastocoels roof o Shortly before gastrulation, the presumptive ectoderm of the blastocoels roof secretes an ECM that contains fibrils of FN which the involuting mesoderm travel along  FN-containing ECM appears to provide both a substrate for adhesion as well as cues for the direction of cell migration  In xenopus, FN is similarly secreted by the cells lining the blastocoels roof -> band of FN lining the rood  Convergent extension pushes the migrating cells upward toward the AP  FN fibrils are necessary for the head mesodermal cells to flatten and to extend broad (lamelliform) processes in the direction of migration  Studies using inhibitors of FN formation have shown that FN fibrils are necessary for the direction of mesoderm migration, the maintenance of intercalation of AC cells, and the initiation of radial intercalation in the marginal zone  Mesodermal cells are thought to adhere to a5b1  Mesodermal migration can also be arrested by the microinjection of Ab against either FN or a5 subunit  A5 appears just prior to gastrulation and it persists on the surfaces throughout gastrulation and disappears when gastrulation ends  The integrin coordinated the interaction of the FFN on the blastocoels roof with actin filaments within the migrating mesodermal cells -> this allows for increasedtraction and determines the speed of migration  The coordinated synthesis of FN and its receptor signals the times for the mesoderm to begin, continue, and stop migration P. 333-340 The Emergence of the Ectoderm: CNS and epidermis  A portion of the dorsal ectoderm is specified to become neural ectoderm, and its cells become distinguishable by their columnar appearance  This region of the embryo is called the neural plate  The process by which the neural plate tissue forms a neural tube – the rudiment of the CNS – is called neurulation, and an embryo undergoing such changed is called a neurula  The neural tube forms the brain anteriorly and the spinal cord posteriorly Establishing the Neural Cells  Neural cells become specified through their interactions with other cells  At least 4 stages through which the pluripotent cells of the epiblast or blastula become neural precursor cells, or neuroblasts: 1. Competence: multipotent cells can become neuroblasts if they are exposed to the appropriate combination of signals 2. Specification: cells have received the appropriate signals to become neuroblasts, but progression along the neural differentiation pathway can still be repressed by other signals 3. Commitment (determination): neuroblasts enter the neural differentiation pathway and will become neurons even in the presence of inhibitory signals 4. Differentiation: the neuroblasts leave the mitotic cycle and express those genes characteristic of neurons Formation of the Neural tube  2 major ways of converting the neural plate into a neural tube  Primary neurulation: the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube  Secondary neurulation: the neural tube arises from the coalescence of mesenchyme cells into a solid cord that subsequently forms cavities that coalesce to create a hollow tube  In general, ant. portion is made by primary and post. is made by secondary neurulation  Complete neural tube forms by joining the tubes together Primary neurulation  The events of primary neurulation divide the original ectoderm into 3 sets: 1. The internally positioned neural tube -> brain and spinal cord 2. Externally positioned epidermis 3. Neural crest cells  NC cells formin the region that connects the neural tube and epidermis, but they migrate to new locations where they will generate the peripheral neurons and glia, the pigment cells of the skin, and several other cell types  The process of primary neurulation appears to be similar in all vertebrates: o Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds, while a U-shaped neural groove appears in the center of the plate, dividing the future right and left sides of the embryo o The neural folds migrate toward the midline of the embryo, eventually fusing to form the neural tube beneath the overlying ectoderm o The cells at the dorsal most portion of the tube become NC cells  Primary neurulation can be divided into 4 distinct but spatially and temporally overlapping stages: 1. Formation and folding of the neural plate 2. Shaping and elevation of the neural plate 3. Convergence of the neural folds, creating a neural groove 4. Closure of the neural groove to form the neural tube Formation and Shaping of the Neural Plate  The process of neurulation begins when the underlying dorsal mesoderm signals the ectodermal cells aboveit to elongate into columnar neural plate cells  Their elongated shape distinguishes them from the flatter pre-epidermal cells surrounding them  As much as half the ectoderm is in the plate  The neural plate lengthens along the ant-post axis and narrows by convergent extension, intercalating several layers of cells into a few layers  Divisions of the NP cells are preferentially in the ant-post direction  If the neural plate is isolated, its cells converge and extend to make a thinner plate, but fail to roll up into a neural tube  If the “border region” containing both the presumptive epidermis and NP tissue is isolated, it will form small neural folds in culture -> epidermis isimportant in shaping the plate Bending and Convergence of the Neural Plate  The bending of the neural plate involves the formation of hinge regions where the neural plate contacts surrounding tissues o In birds and mammals, the cells at the midline of the plate form the medial hinge point o The cells of the MHP are derived from the portion of the neural plate just anterior to Hensen’s node o MHP cells become anchored to the notochord beneath them and form a hinge, which forms a furrow at the dorsal midline o The notochord induces the MHP cells to decrease in height and to become wedge-shaped; cells lateral to the MHP do not undergo such a change o Shortly thereafter, 2 other hinge regions form furrows near the connection of the neural plate with the remainder of the ectoderm o These regions are the dorsolateral hinge points and they are anchored to the surface ectoderm and also increase their height and become wedge-shaped  Cell wedging is intimatelylinked to changes in cell shape o In the DLHPs, microtubules and microfilaments are both involved in these changes  Surface ectoderm of the chick embryo pushes toward the midline of the embryo, providing another motive force for bending the neural plate Closure of the Neural Tube  The neural tube closes as the paired neural folds are brought together at the dorsal midline o The folds adhere to each other and the cells from the two folds merge o In birds, the NC cells do not migrate from the dorsal region of the neural tube until after the it has closed at that site o In mammals, the cranial NC cells migrated while the neural folds are still being elevated, whereas in the spinal cord region, the NC cells do not migrate until closure has occurred  In chicks, neural tube closure is initiated at the level of the future midbrain and “zips up” in both directions  In mammals, it is initiated at several places along the ant-post axis  Failure of closure in different regions of the neural tube causes different neural tube defects o Failure to close the posterior -> spina bifida: the severity depends on how much of the spinal cord remains exposed o Failure to close sites 2 and 3 in the rostral neural tube keeps the ant. Neuropore open, resulting in a lethal condition -> anencephaly in which the forebrain remains in contact with the amniotic fluid and subsequently degenerates; the fetal forebrain ceases development and the vault of the skull fails to form  The neural tube eventually forms a closed cylinder that separat
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