icakeov said:
I guess these would be all the floating around the body cells? Or the nerve cells that are sent off to climb up the spine to find its place?
Most cells don't do a lot of moving during embryogenesis. Exceptions are the neural crest cells and the blood cells (which of course circulate). There may be others. Many embryonic movements involve masses of cells moving as either a linked unit or individual cells.
Nerve cells don't climb up the spine. The vertebrate central nervous system (CNS) forms from a defined area of dorsal (top) ectoderm (outside layer embryonic, like skin) that after being instructed to form skin by signals from underlying tissues, rolls up into a tube which sinks into the dorsal body along the head to tail axis of the developing embryo. This forms the neural tube which is the precursor of the nervous system. The neural crest forms from cells at the border of the induced neural tube and the ectoderm it used to be continuous with. The cells go all over the place and make lots of interesting tissues including much of the peripheral (non-CNS) nervous system. Eventually some of the cells in the neural tube become neurons in the structure of the tube (made of other cells). Some migrate around a bit, but not like the neural crest and not up the spinal cord. Their axons in the other hand act like little migrating cells when they grow out, but maintain a connection with the cell body via the axon that strings out behind them as they move out to their final location. These growing axons can go great distances. Motorneurons (which innervate muscles) residing in your spinal cord project axons down to muscles in you feet among other areas. Corresponding distances in other animals, like a giraffe, can be even longer.
Master Regulatory Genes:
There are two classical lines of evidence for master regulatory genes. They were originally conceived of as a single gene, at the top of a regulatory hierarchy, turning on, producing a protein that then binds control sequences for other genes. These in turn may turn on other genes, etc., thereby turning on a whole set of genes for a particular cell type.
One line of evidence for master regulatory genes are mutations found in animals that result in changed fates of developing cells in certain areas of the embryo.
For example, homeotic mutations like bithorax can change whole body segments from one fate (such as second thoracic segment to first thoracic segment). These
pictures show the wild type condition (normal) and mutants similar to bithorax. the normal have two large wings (one pair on the first segment) and two small "balancers" (on the second segment). Fruitflies evolved the second pair into the balancers, while dragonflies, which did not evolve this, have 2 pair of large wings. Its really shocking to see these mutants if you are used to seeing the normal flies.
There many mutations like this, so master regulatory genes are an easy explanation, but it is not necessarily the only explanation. Undirected (meaning produced randomly) mutations like these basically cause a malfunction in the machinery underlying the developmental process. Its been compared to throwing a monkey wrench into a veery complex machine resulting in a problem. It does not rule out a requirement of a coordinated action of many genes for proper development. Developmental regulation is often complex. Taking one gene out may cause some effect, but the same or a different effect may result from taking out a different gene.
Another line of evidence in support of master regulatory genes in forcing gene expression in some cells which results in those cells taking a particular cell fate.
Now, the expression of many or maybe all genes expressed in a cell (or cell type; a cell's transcriptome as described above) can be followed and recombinant organisms can be made. Transcriptomes of different cell types vary. These new tools provide many ways to test/manipulate the cells and hypotheses, but I am no longer current on them.
