Analysis of Biological Development (K. Kalthoff)

Updates to Topic 13: Ectodermal Organs

Answers to Questions in Text

Patterning Effect of Notochord on Neural Tube (pp.308-311)

  1. Morphogen models imply that the concentration of a diffusible molecule decreases with increasing distance from its source. What does this imply about the turnover of the morpphogen? Answer: The morphogen must broken down relatively quickly at one or more sites away from the source. If the morphogen is degraded anywhere its concentration decreases exponentially with increasing distance from the sink. If the morphogen is broken down in specialized “sink” cells, then the morphogen concentration decreases linearly between source and sink.
  2. Morphogen models also imply that morphogens ultimately control differential gene expression. Since Shh is a secreted glycoprotein it will not cross the plasma membrane of its target cells. How then can it control the target cells' gene activity? Answer: By binding to a matching receptor in the plasma membrane of target cells. The receptor is then activated and starts an intracellular signal chain that eventually changes the target cells' gene activities.
  3. While notochord and floor plate are closely attached during neurulation, the notochord subsequently separates from the neural tube. Does this separation affect the usefulness of the notochord as a source of a morphogen for the ventral portion of the neural tube? Answer: Most likely it does. However, by the time the notochord separates, the floor plate has already been induced and serves a new source of morphogen.

Determination of Neural Crest Cells (p. 326)

  1. According to the experiments described here, NC cells that detach from the neural tube late and enter the dorsolateral pathway are at least biased to become pigment cells. However, the data do not show whether these NC cells are actually determined to form pigment cells. Alternatively, these cells are could still be pluripotent, and their beginning differentiation as pigment cells might still be reversible. Ideally, which experimental strategy should be used to decide this question? Answer: NC cells that have detached late from the neural tube, or have entered the dorsolateral pathway, should be labeled, transferred to the ventral pathway, and monitored for the development of neurons or other NC derivatives. Conversely, cells that have detached early from the neural tube, or have entered the ventral pathway, should be labeled, transferred to the dorsolateral pathway, and monitored for pigment cell formation. An experiment with NC cells from salamander embryos that comes close to using this strategy is diagrammed in Fig. 13.32. The results show that NC cells removed from neural tube at a given time can give rise to neurons or pigment cells, depending on whether they develop in contact with ECM from the ventral or dorsolateral pathway, respectively. This means that NC cells just leaving the neural tube are still pluripotent, at least in salamander embryos.
  2. What kind of changes might occur in NC cells that make the dorsolateral pathway attractive to them or at least allows them to utilize it? Answer: The loss or masking of cell surface molecules that are repelled by other molecules present in the dorsolateral pathway.


Clarifications and Corrections

p. 305, Fig. 13.2, the label "Epidermal Placodes" should be "Ectodermal Placodes".

New Review Articles

Altmann C.R. and Brivanlou A.H. (2001) Neural patterning in the vertebrate embryo. Int Rev Cytol. 203: 447-482

Baker C.V.H. and Bronner-Fraser M. (2001) Vertebrate Cranial Placodes. I. Embryonic Induction. Devel.Biol. 232: 1-61

Christiansen JH, Coles EG, Wilkinson DG. (2000) Molecular control of neural crest formation, migration and differentiation. Curr Opin Cell Biol. 12: 719-724.

McMahon A.P. (2000) Neural patterning: The role of Nkx genees in the ventral spinal cord. Genes & Devel. 14: 2261-2264

Moens C.B. and Prince V.E. (2002) Constructing the Hindbrain: Insights from the Zebrafish. Devel.Dynam. 224: 1-17

Sieber-Blum M. (2000) Factors controlling lineage specification in the neural crest. (2000) Int Rev Cytol. 197: 1-33

Thisse C. and Zon L.I. (2002) Organogenesis--heart and blood formation from the zebrafish point of view. Science 295: 457-462

New Research Articles

Odenthal J., van Eden F.J.M., Haffter P., Ingham P.W. and Nüsslein-Volhard C. (2000) Two distinct cell populations in the floor plate of the zebrafish are induced by different pathways. Devel. Biol. 219: 350-363

The zebrafish floor plate is about three cells wide, consisting of a medial floor plate (MFP) row of cells expressing both sonic hedgehog+ and fkd4+, and two lateral floor plate (LFP) rows expressing fkd4+ but not shh+. Mutagenesis screen have revealed several mutants in which either MFP or LFP cells are absent. The mutant phenotypes, and the observation that injection of shh mRNA increases the number of LFP but not MFP cells, indicate that shh synthesized in notochord and/or MFP cells induces LFP cell formation.

Lee K.J., Dietrich P. and Jessell T.M. (2000) Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification. Nature 403: 734-740

In order to investigate the role of the dorsal roof plate in neuronal patterning, the researchers devised a genetic method for killing roof plate cells. They accomplished this by expressing an introduced gene for a diphteria toxin selectively in roof plate cells. They found that mouse embryos without a roof plate were also lacking all the interneuron subtypes normally generated in the dorsal third of the neural tube. By lineage analysis, they showed that the lacking interneurons are not descendants of roof plate cells, so they are not missing simply because their progenitors have been killed. Rather, the normal development of the dorsal interneurons must depend on one or more signals released from the roof plate.

Web Sites

Great SEMs and drawings of mouse and human embryonic development may be found on this web site, which is slated for additional illustrations of pre-implantation and fetal development as well as the genesis of birth defects.

The Multidimensional Human Embryo is funded by the National Institute of Child Health and Human Development (NICHD) and provides a three-dimensional image reference of the Human Embryo based on magnetic resonance imaging.

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