Talpid2 mutant chick limb has anteroposterior polarity and altered patterns of programmed cell death

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Abstract
The talpid2(ta2) chick mutant is of interest for the study of limb pattern formation. Talpid2 is a simple Mendelian recessive lethal mutation which affects the mesoderm and results in short, spade-like, polydactylous wings and legs. Here, we describe ta2 limb development with particular attention to those aspects of ta2 which may illuminate the process of normal limb development. From the onset of budding, ta2 limb buds are significantly wider than normal buds along the anteroposterior axis. They lack the normal anterior and posterior necrotic zones and have variable development of the central opaque patch. Interdigital programmed cell death is variable and may result in development of distal phalanges without more proximal ones. Talpid2 wing vasculature is similar to that of normal wings, but ta2 legs are supplied by four large blood vessels. Feathers form regular, parallel rows, similar to normal feathers, but ta2 embryos lack the loose mesenchyme which separates the feather buds. Finally, and most significantly, ta2 wings and legs display anteroposterior polarity. Anterior and posterior limb skeletal elements can be clearly distinguished from one another within the ta2 phenotype. Our observations suggest that the ta2 mutant may be useful in analyzing programmed embryonic cell death and anteroposterior limb pattern formation.