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Somites and the Timing of Development

http://www.ehd.org/images/gasserbook/Gasser_Fig4-1gs.jpg

We know that some sort of timing mechanism must be in place for an embryo to develop properly. This is clearly displayed in a chick embryo. During development, the embryo is developing on top of the yolk sac. A slit forms, known as the primitive streak, where there is an inward movement of cells. At one side of the streak a node (Hensons's node) forms that moves back down the streak, anteriorly to posteriorly (head to tail). In the wake of the node, structures form, such as the notochord, and on either side of the notochord, the somites (somites eventually give rise to the body and limb muscle, vertebral column, and dermis). Somite formation occurs in a temporal pattern, with the somites forming one after another at a distal site from Henson's node. Between the node and the most recently formed somite, there is an unsegmented region - the pre-somitic mesoderm. The changes in cell shape and intercellular contacts in this region are what eventually give rise to the somites. See the second figure on this page for the region of the pre-somitic mesoderm. At the caudal side is where Henson's node is, and on the rostral side, somites are beginning to form.In a chick, the cells that give rise to the somites start out in the epiblast, and then move into the primitive streak to form a population of somitogenic stem cells around Henson's node. The stem cells divide, with some cells staying in the node, and staying stem cells, and some that leave the region as the node regresses toward the posterior side of the embryo. So as new cells are entering the node at the posterior end, somites are forming at the anterior end. The interesting thing about the pattern of formation is that if you cut out a region of the pre-somitic mesoderm, flip it 180 degrees, and reinsert it into the embryo, the somites retain their pattern of development. Imagine this schematic as an embryo's somites (A=anterior, Sn = somite #, P = posterior, H=henson's node):

A-S1-S2-S3-S4-S5-------------H----------------------P

Normally, the region between S5 and H would continue the pattern of somites (eg. S6-S7-S8...). If, however, you cut out the region between S5 and S8, and flipped it around, the somites would still retain their pattern of formation, but reversed:

A-S1-S2-S3-S4-S5-S10-S9-S8-S7-S6-S11-S12-S13-S14---H---P

This suggests that somite formation is an autonomous process and that at this time, no extracellular signal specifying position or timing is involved. So, before somite formation even begins, a molecular pattern is layed down that specifies the time of formation of each somite. You also need to keep in mind, however, that each somite forms in sequence, so there is some kind of clock like mechanism working also.

This clock like mechanism will be the topic of the next post.

Fate, Determination, and Differentiation

The fate of a cell can be determined by marking cells of the developing embryo, and observing what they normally develop into, without intervention. With this a fate map of various embryonic regions can be constructed. Cells can be labeled with dyes that fluoresce or alternatively, a cell transplant can be performed where similar cells with different observable features are introduced that allows differentiation in the adult organism. It should be noted however that a particular fate does not imply that a cell could not develop differently if placed in a different environment.

A cell is considered determined when a cell at a particular level of development, if transplanted to a new site, will retain the fate from its original site when transplanted to a new site. This implies a stable change in the internal state of a cell such that its fate is now fixed.

Finally, cell differentiation involves the gradual emergence of cell types that have a clear cut identity in the adult, such as skin, nerve or muscle cells. They are not able to revert and develop into a different cell type. Differentiation involves the expression of so called 'luxury genes'. Genes that are not required for the maintenance of normal cell function, but instead give them characteristics of their particular cell type.

General Transcription Factors Vs. Gene Specific Factors

In eukaryotic cells, a general set of transcription factors is needed to bind RNA polymerase onto the correct region of the DNA to start transcription. These proteins are known as general transcription factors, which form an a complex with the polymerase at the promoter site.

This initial binding must occur at all genes, and occurs at the TATA box, located near the start point of transcription. This binding, however, is not sufficient to start transcription of the gene. Gene expression is highly regulated, especially during development. So, additional binding of gene specific regulatory proteins are needed elsewhere in the cis regulatory regions.

For any gene, its activation is due to a combiniation of SPECIFIC gene regulatory proteins binding to individual sites in the control regions. These regulatory sites can be within the promotor region, adjacent to the TATA box, or at sites outside of the immediate promotor region, and may in fact be thousands of base pairs away from the start point of transcription. This is possible because it is thought that DNA can form loops, bringng the sites closer to the proximity of the promoter region, and hence RNA polymerase. This is yet another way that the regulation of gene expression can occur.

The Role of Cis-Regulatory Sequences and Gene-Regulatory Proteins in Gene Expression

A major question in the development of vertebrates is how do genes know when to turn on during development in order to produce the right protein products that differentiate one cell type from another? Gene expression is regulated by a variety of mechanisms, from the control of transcription factors to the actual structure of the DNA itself.

The cis-regulatory control region of a gene is comprised of the sequences that flank he gene, and contain sites that can be bound by activators or repressors that can control the expression of that gene.

These control regions, in turn, can contain a variety of different cis-regulatory modules. These are short regions containing multiple binding sites for various transcription factors (e.g. activators and repressors). The combination of these factors that are bound are what determines whether a gene is switched on or off.

These activators or repressors are gnown as gene regulatory proteins, or transcription factors, that bind to control regions in DNA and help to switch genes on or off.

http://www.ucl.ac.uk/~ucbzwdr/teaching/b250-99/transcription.jpg

In addition to regulation of gene expression by transcription factors, a genes expression can also be controlled by the structure of the chromosome itself. It has been observed that if DNA is methylated (that is, has a -CH3 group bound) within a gene, transcription is downregulated. The packaging of DNA into histones also can affect gene expression, and can be modulated by transcription factors.

Histones are what DNA is wrapped around that packages it into chromatin. If a histone is acetylated by an enzyme known as histone acetyltransferases (HATs), which are usually part of large multiprotein complexes known generally as chromatin-remodeling comlexes, it tends to be more accessible to transcription. Histones can also become de-acytelated and this may play a part in gene repression.

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