The role of cell death in the developing chick limb

Cell death is a normal occurrence in limb development although its role is unclear in most instances. Several questions were addressed in this study. 1) Are malformations induced by the drug Methotrexate caused in part by changes in the normal pattern of cell death? 2) Is cell death in the posterior necrotic zone (PNZ) "programmed" as suggested by the literature or is it a response to changes in the local environment? What is its role in normal development? 3) Is the signal to die mediated via a calcium or calmodulin-dependent pathway? These questions were investigated in the chick embryo model system using light and electron microscopic level histology, microsurgery, tissue culture and isotopic labelling techniques.

Methotrexate is highly embryotoxic as well as teratogenic in the chick. The malformations induced by the drug do not seem to be mediated by cell death. The types of limb reductions seen support the suggestion that inhibition of cell divisions may play a role in producing malformations.

Cell death in the PNZ is not irreversibly "programmed." Instead, the cells are responding to changes in their microenvironment. If the overlying ectodermal ridge is maintained in a thickened state, death will not occur. Likewise, if the ridge is stripped off early enough, death can be inhibited. The process of ridge flattening may be important in controlling cell death in the PNZ as well as in other regions of the limb.

The signal to die does not seem to be mediated by a calmodulin-dependent pathway. However, it was found that exposing dying tissues to phenothiazines blocks the pathway necessary for recycling internalized membrane in macrophages.