Question

Explain how programmed cell death helps to shape the vertebrate limb. Please include all citations (in-text as well).

Answer 1

Cell death plays a role in sculpting the limb. It is essential if our joints are to form and if our fingers are to become separate. The death of specific cells in the vertebrate limb is genetically programmed and has been selected for during evolution. Between the time when the cell's death is determined and when death actually takes place levels of DNA, RNA ane protein synthesis in the cell decrease dramatically.

In addition to the interdigital necrotic zone, there are three other regions that are sculpted by "cell death ". The ulna and radius are separated from each other by an interior necrotic zone and two other regions the anterior and posterior necrotic zones further shape the end of the limb. Although, these zones are said to be "necrotic", this term is a holdover from the days when no distinction was made between necrotic cell death and apoptotic cell death. These cells die by apoptosis and death of interdigital tissue is associated with the fragmentation of their DNA.

The signal for apoptosis is probably provided by the BMP proteins. BMP2, BMP4 and BMP7 are each expressed in the interdigital mesenchyme and blocking BMP signaling ( by infecting progress zone cells with retorviruses carrying dominant negative BMP receptors) prevents interdigital apoptosis. Since these BMPs are expressed throughout the progress zone mesenchyme, it is thought that cell death would be the default state unless there were active suppression of the BMPs. This suppression comes from the noggin protein, which is made is the developing digits and in the perichondrial cells surrounding them. If noggin is expressed throughout the limb bud, no apoptosis is seen.

Use the book development biology by Scott F. Gilbert.