A growth-promoting influence from the mesonephros during limb outgrowth.

It has been suggested that the mesonephros has a role in normal limb development. This hypothesis was directly tested by removing the mesonephros adjacent to the presumptive limb region of stage 12-18 chick embryos using microsurgery or laser ablation. The experimental manipulation resulted in reduced limb outgrowth on the operated side. The poor limb outgrowth was correlated with either the lack of or the presence of a rudimentary mesonephros on the operated side. Furthermore, the presence of nephric tissue in limb bud organ culture enhanced growth and morphological differentiation of cartilage formed in culture. In vivo, the influence of the mesonephros resulted in significantly higher cell proliferation in the adjoining medial half of the limb mesoderm compared with the lateral half. The removal of the mesonephros adjoining the prospective limb region reduced the number of dividing cells in the medial mesoderm. The higher proliferation in the medial limb mesoderm is significant to limb outgrowth since grafting experiments showed that most of the cells that form the limb are derived from the medial mesoderm. The results suggest that the influence from the mesonephros may provide some signal for limb outgrowth.