The Danforth's short tail mutation acts cell autonomously in notochord cells and ventral hindgut endoderm

Danforth's short tail (Sd) is a semidominant mutation in mouse affecting the axial skeleton and urogenital system. The notochord is the first visibly abnormal structure in mutant embryos, and disintegrates beginning around embryonic day 9.5 along its entire length, suggesting an essential role for Sd in notochord development and maintenance. Here, we report on the fate of Sd/+ and Sd/Sd cells in chimeric embryos. Up to day 9-9.5, Sd cells contributed efficiently to the notochord of chimeric embryos. In advanced day 9.5 embryos, Sd cells were less abundant in the posterior-most region of the notochord and in the notochordal plate. During subsequent development, Sd cells were specifically lost from the notochord and replaced by wild-type cells. In Sd/+<-->+/+ chimeras, the notochord appeared histologically and functionally normal, leading to a rescue of the mutant phenotype. However, strong Sd/Sd<-->+/+ chimeras showed malformations of the axial skeleton and urogenital system. All Sd/Sd<-->+/+ chimeras with malformations of the axial skeleton also had kidney defects, whereas chimeras without vertebral column defects had highly chimeric kidneys that appeared normal, suggesting that the urogenital malformations arise secondarily to impaired posterior development caused by the degenerating notochord. Sd mutant cells also were specifically absent from the ventral portion of the hindgut, whereas they contributed efficiently to the dorsal region, implying the existence of distinct cell populations in the dorsal and ventral hindgut. Our findings demonstrate that the Sd mutation acts cell autonomously in cells of the notochord and ventral hind gut. Sd leads to the degeneration of notochord cells and the number or allocation of notochord precursors from the tail bud to the notochordal plate seems impaired, whereas notochord formation from the node appears to be unaffected.