Abstract
An often overlooked aspect of digit development is the special nature of the terminal phalanx, a specialized structure with characteristics distinct from other phalanges, for example the presence of ectodermal derivatives such as nails and claws. Here, we describe the unique ossification pattern of distal phalanges and characteristic gene expression in the digit tips of chick and duck embryos. Our results show that the distal phalanx of chick wing digit 1 is a genuine tip with a characteristic ossification pattern and expression of Bambi and Sp8; however, the terminal phalanx of digits 2* and 3 is not a genuine tip, and these are therefore truncated digits. Bambi and Sp8 expression in the chick wing provides a direct molecular assessment of digit identity changes after experimental manipulations of digit primordia. In contrast, digits 1 and 2 of the duck wing both possess true tips. Although chick wing-tip development was not rescued by application of Fgf8, this treatment induced the development of extra phalanges. Grafting experiments show that competence for tip formation, including nails, is latent in the interdigital tissue. Our results deepen understanding of the mechanisms of digit tip formation, highlighting its developmental autonomy and modular nature, with implications for digit reduction or loss during evolution. * Numbering of wing digits is 1, 2, 3 from anterior to posterior.
Highlights
The vertebrate limb is a classic model for the study of pattern formation and morphogenesis during embryonic development
Outgrowth of the limb bud in the proximodistal axis is directed by a signalling centre, the apical ectodermal ridge (AER), a thickened specialized epithelium at the dorsoventral boundary of the distal limb bud
Taken together with previous data, these features allow a true digit tip to be defined as a distal phalanx with a pointed shape, a distal ossification centre, expression of Bambi, Sp8 and other genes, and containing ectodermal derivatives such as nails or claws
Summary
The vertebrate limb is a classic model for the study of pattern formation and morphogenesis during embryonic development (reviewed in [1]). Limbs arise from the lateral plate mesoderm as small buds composed of undifferentiated mesenchyme encased in an ectodermal jacket. Outgrowth of the limb bud in the proximodistal axis is directed by a signalling centre, the apical ectodermal ridge (AER), a thickened specialized epithelium at the dorsoventral boundary of the distal limb bud (reviewed in [2]). Initial mesenchymal condensations give rise to a cartilage template that is later replaced by bone through endochondral ossification [3]. The sequence of cartilage condensation follows a proximo-distal order, with elements near to the body (humerus/femur) appearing first, intermediate elements (ulna-radius-fibula) and digits forming last
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