DEPARTMENT OF BIOLOGY
The Basics of Evolution
Vertebrate Limb Development--an Example of the Development of Body Parts
At a relatively early stage in development (in the figure above, the last image) vertebrate embryos develop a series of bumps along their backs, and two small nubbins on each side. The two nubbins are the limb buds. There are two limb buds on the right (forelimb and hindlimb), and the corresponding two on the left. These begin to form when the embryo reaches a developmental stage at which each body segment begins to differentiate from the others.
The limb buds are very small -- small enough that cell contact and diffusible molecules can serve as cell-cell communication mechanisms to establish cellular identities. A variety of genes are activated in limb buds. Some produce diffusible short-range hormones, some produce gene-control proteins. In early stages of limb bud development, an abbreviated pattern of gene expression is something like that shown in the diagram shown above (see Tickle, 2000, www.ijdb.ehu.es/fullaccess/fulltext.feb00/Tickle.pdf). These different genes establish cellular identities in the limb bud, and consequently establish the "pattern" of limb development.
If we use a human arm to provide terminology, we can describe it this way: The expression of Shh (red in the diagram), determines the "thumb-to-pinkie" pattern. The pinkie forms on the side closest to the cells that activate the Shh gene. The different Hox genes ensure that the digit next to the pinkie is the 4th finger, and the one next to that is the 3rd finger, and the one next to that is the index finger, and the farthest is the thumb.
As the limb grows outward from the flank of the embryo, additional genes and diffusible molecules (such as retinoic acid, a form of Vitamin A) establish the positions of shoulder, upper arm, forearm, wrist, and fingers. This same basic pattern has been seen in the developing limbs of the vertebrates that have been studied.
Because these developmental events all occur in a very small piece of tissue, encompassing a relatively small number of cells, any changes in the cell-cell communication systems or the diffusible molecules can have dramatic effects on the overall pattern that eventually forms. A chicken wing differs from a human hand only in being smaller overall, having the outer digits fail to develop, and having the cells between the digits fail to die. All of this is controlled by the relative positions of cells that activate the genes for certain gene-control proteins, under the influence of diffusible small molecules, and cell-cell contact.
As we said above, we can use the limb as an example of the kinds of controls that govern the development of body parts. Whether limbs, eyes, livers, on pancreases, all body parts start out as very small groups of cells in which cell-cell contact and diffusible molecules can set up a pattern of which cells activate which gene-control proteins. As the gene-control proteins activate additional genes, the developing organ acquires the various protein micromachines that build the structures.
last updated:Jan. 15, 2009