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.