Role of cell death
As development of the limb proceeds waves of death or necrosis of large masses of mesodermal cells occur in certain regions at different stages. This has been observed in lizard, (calotes), chick, duck and mouse, but has not been observed in amphibians. Cell death occurs in four zones:
(1) Anterior necrotic zone. ANZ and
(2) Posterior necrotic zone. PNZ, along the anterior and posterior margins,
(3) Opaque patch, OP, in deep mesoderm and
(4) Interdigital necrotic zones, INZ .
The death of cells in these is preprogrammed to occur at certain well defined stages. It plays a major role in the separation of radius and ulna in the forelimb, tibia and fibula in the hindlimb as well as in separation of digits from each other at the distal end of both limb types. The tissue between the digits of chick feet and both limbs of calotes is completely destroyed separating all digits from each other. In the duck only some cells of inter-digital tissue die, others remain to form the webs. The morphogenetic significance of cell death in ANZ and PNZ is not clear. It may be helping in sculpturing the contours of the limbs.