Limb deformity proteins during avian neurulation and sense organ development.

The nuclear Limb deformity (Ld) proteins (formins) are expressed during the avian primitive streak stages. Initially, they are detected predominantly in cells of the forming notochord, scattered mesodermal precursors and the induced neural plate. No expression is detected in endodermal cells. The subsequent graded distribution of Ld positive cells along the anterior-posterior axis of the neural tube follows the antero-posterior progression of its differentiation. The Ld proteins are also differentially expressed during induction and development of both the inner ear and eye. An unequal distribution of Ld proteins along the dorso-ventral axis of the otic vesicle is observed during its initial patterning. In the eye, the Ld proteins are expressed by the optic vesicle during secondary induction of the lens placode. Following induction, the proteins are also expressed by the newly formed lens placode, a process which is reminiscent of homeogenetic induction. During differentiation of the retina and lens, the Ld domains seem to demarcate territories, giving rise to specific eye structures. A comparative analysis of the Ld distribution and BrdU incorporation in the two sense organs indicates that the proteins are predominantly expressed by committed and/or differentiating (post-mitotic) cells. In general, expression of Ld proteins is induced during determination and remains during differentiation of particular cell-types. This study implies that the nuclear Ld proteins are involved in morphogenesis of both neuro-ectodermal and mesodermal structures.