Q J Med 2003; 96: 459-460
© 2003 Association of Physicians
Biologic |
The new mapping
The new techniques of experimental biology have greatly increased the powers of resolution of classical embryological science. Some are very sophisticated, some direct—it is perhaps surprising (or a mammalian conceit) that it took so long to realise that the Zebra fish was an experimental animal you could see through as things developed. What is gained from increasing sophistication is sometime surprising and sometimes counter-intuitive, but it is fair to say that it gradually moves us to a unified, if complex, view of how tissues and organs are developed. The neural crest is a good exemplar.
The neural crest is unique to vertebrates—it has been called the fourth germ layer, and ’ordinary’ anatomy has shown that the cells of the neural crest, like the other germ layers, form a transient population that exists only during development. By straightforward morphology, it has been shown that the crest cells originate from the lateral tips of the dorsal neural tube at the junction of the neural plate and the surface ectoderm, and that they migrate shortly after the closure of the tube, in a space between the dorsal tube and the dorsal somite. The divergent progeny of these cells include neurons, glia, Schwann cells, pigment cells, the cells of the adrenal medulla and much of the bone and cartilage of the head and neck.
These patterns of migration have been defined using a number of anatomical/pathological techniques. These include both autoradiography and the use of the condensed heterochromatin of the quail nucleolus as a tracer (quail cells are introduced into chicken embryos, where they are tolerated and can be spotted by their distinctive nuclei as they move). Lysinated rhodamine dextran (which fluoresces) has been injected into individual dorsal neural tube cells as a further type of tracer, and specific antibodies for neural crest cells can identify migrating cells.
Data from all these types of study show that the migration is influenced by locally produced proteins and carbohydrates, and that these influences are significant. It is clear that rat neural crest cells can change type as a result of external factors acting after the start of migration and differentiation. Bronner-Fraser and Fraser1 have shown that individual crest cells give rise to clones consisting of multiple cell types (assessed by location and morphology). Sensory neurons, presumptive pigment cells, ganglion supporting cells, cells that form the adrenal medulla and neural tube cells were all found within individual clones, suggesting that some, at least, of the cells of the neural crest are multipotent before their departure from the neural tube. The apparent existence of a neural crest stem-cell population was later confirmed by Nicole Le Dourain’s group.2
We know that the patterns of spread of these discrete end-populations are distinct. Vagal neural crest cells (arising at the level of somites 1–7) provide a cell population that invades the gut, migrates caudally and populates the enteric ganglia. Trunk neural crest cells (somites 8–28) migrate through the somites and underneath the ectoderm, and produce sensory and sympathetic ganglia, adrenal chromaffin cells and melanocytes. Sacral neural crest cells (somites 28 and lower) give rise to enteric ganglia and some sympathetic and sensory ganglion cells. Cranial crest cells follow a different migratory route to give rise to elements of the cranial ganglia, connective tissues, cartilage and the bones of the face. A further specialized population migrates from the cranial neural crest to join those specialized epithelial cells, which have undergone an epithelial-mesenchymal transition to form the thickened ectodermal epithelium of the sensory placodes. These structures, by invagination, migration and condensation, form sense organs after differentiation into neurons, receptors and support cells.
Although a great deal of important data has been gathered by the classical techniques, the new biology can be married to these manipulations to increase our understanding of what is happening. At its simplest, by looking for gene expression (by hybridization) or for gene products, it was soon shown that the migration of neural crest cells is triggered by expression of BMP-4 and negatively regulated by noggin; where noggin expression is high, neural crest cells do not migrate.
But it is possible to do things from a completely different approach. If you take a tissue and extract all its RNA, you can tell which proteins are being expressed in the anlage at the stages investigated. It is then possible to make complementary DNA (cDNA) by reverse transcription-polymerase chain reaction, and identify gene expression in a temporal sequence (quantitative determinations are more difficult). Using hybridization techniques and a library of 150 000 clones Gammil and Bronner-Fraser were able to show that 83 genes were expressed strongly in the neural folds and/or in migrating neural crest cells.3
The largest group of genes expressed were those related to the cytoskeletal proteins; these were expressed in phases. When the cells were stationary, there was expression of the intermediate filament genes (these are involved with cell form and the siting of organelles) but when they got on the move, expression of the genes for the actin cytoskeleton was much in evidence. Palladin, a gene that is expressed in growth cones and is necessary for neurite outgrowth, was expressed in the migrating cells. As it is clearly necessary for cells that are to move to lose some of their attachments, the genes of the rho family of GTPases are manifest (they play a major role in the formation of intercellular contacts). Genes for cell cycle progression, for intercellular signalling (important in axon guidance) and for the maintenance of cells (trophic effects and direct protection from apoptosis) were also expressed. Genes having an effect on the matrix were also in action and degeneration of the matrix by plasminogen activators is part of the synchronized set of activities. A new gene, with no defined function, was also found.
There are a number of general conclusions that may be drawn from this type of study. In a dividing and migrating cell populations the controls of the cell cycle must permit proliferation and an increase in size of the anlage, and concurrent apoptosis must be regulated (around 50% of cells in the CNS are eliminated by apoptosis in the development of most mammals). The cytoskeleton must permit or be active in generating movement, and cell junctions will form, disappear or change in type as differentiation begins. It is safe to assume that genetic activity related to these functions will be found in a number of systems. Indeed the whole subset of genes active in this example shows a marked similarity to that which drives the development of the endothelial cells of the embryo, another migrating population with regional specialization where separation into lymphatics and capillaries, and in selective differentiation of venous versus arterial endothelial cell form, is vital.
There are gaps in the information provided by a programme even as comprehensive as this. It is not clear what decides which cells stay and which migrate. It is not really possible to say from this type of work how much of a part post-transcriptional regulation plays in the later stages of differentiation. It is probable that commitment takes place at different stages in different lineages; Reissmann et al.4 have shown that the bone morphogenetic proteins BMP-4 and BMP-7 produced by the dorsal aorta (itself a product of neural crest mesenchyme) direct sympathetic neuron differentiation. What is clear is that closely comparable sets and sequences of classes of genetic activity permit the formation of an appropriate cell mass, control the morphogenetic movements of its descendants and the later differentiation of many apparently disparate developing systems.
But there is more drama. In a paper in Science, Schnieder and Helms5 show that it is possible to produce a duck bill (long and flat) on a quail embryo or a quail bill (shorter and narrower) on a duck ’simply’ by transferring neural crest cells. The neural crest donor produces its own form from host cells and major morphological changes (repositioning of external nasal openings, in the form of the egg tooth) take place. The neural crest clearly drives the process, using host epidermis and mesoderm. It is in the temporal expression of the crest genes that the production of these differences lies. The gene set can be scrutinized to determine what matters when.
References
1. Bronner-Fraser M, Fraser SE. Cell lineage analysis reveals multipotency of some avian neural crest cells. Nature 1988; 335:161–4.[CrossRef][Medline]
2. Baroffio A, Dupin E, et al. Common precursors for neural and mesectodermal derivatives in the cephalic neural crest. Development 1991; 112:301–5.[Abstract]
3. Gammil LS, Bronner-Fraser M. Genomic analysis of neural crest induction. Development 2002; 129:5731–65.
4. Reissmann E, Ernsberger U, et al. Involvement of bone morphogenetic protein-4 and bone morphogenetic protein-7 in the differentiation of the adrenergic phenotype in developing sympathetic neurons. Development 1996; 122:2079–88.[Abstract]
5. Schneider RA, Helms JA. The cellular and molecular origins of beak morphology. Science 2003; 299:565–8.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||