Double anterior chick limb buds and models for cartilage rudiment specification.

Most models for the specification of the skeletal elements in the developing limb bud are based on a chemical specification well before overt cartilage differentiation. By contrast, a physico-mechanical model proposes that the process of condensation--an early feature of cartilage differentiation--is itself the basis for patterning the elements. The models thus make quite different predictions as to when the rudiments are specified. Double anterior limb buds have been constructed at stages earlier than condensation, with the expectation that, if specification of the humerus occurs before cartilage condensation, then limbs containing two humeri should develop, since the presumptive humerus lies largely in the anterior region. The development of anterior and posterior parts, on their own, was in general, consistent with the fate map; both developed a humerus that was thinner than normal. Double anterior limbs developed two humeri in 28% of cases and a much thicker humerus in 39%. These results strongly support models based on an early specification of limb rudiments and cannot be accounted for by the physical model. Double anterior limbs in which the two parts were from different stages, developed such that a digit 3 could lie adjacent to the radius, giving further striking evidence for early specification and local autonomy of development.