APOPTOSIS OF EPAXIAL MYOTOME IN DANFORTHS SHORT-TAIL (SD) MICE IN SOMITES THAT FORM FOLLOWING NOTOCHORD DEGENERATION

Danforth's short-tail (Sd) mouse is a semidominant mutation that preve nts completion of notochord development. In homozygous mutant mice, th e notochord completely degenerates at embryonic day 9.5 (E9.5), wherea s the neural tube and somites continue to form, permitting analysis of somite development in the absence of inductive signals from the notoc hord and floor plate. In the somites formed after notochord degenerati on, Myf5 expression initiates in a normal temporal sequence, but inste ad of the normal restriction to the dorsomedial somite, its expression extends into the ventral somite. MyoD, myogenin, and myosin heavy cha in are normally expressed in the ventral myotome and there is normal d evelopment of hypaxial muscles. In contrast, subsequent to initial ll Myf5 expression, muscle gene expression was not detected in the dorsal myotome and a high level of apoptosis was observed with significantly decreased formation of epaxial muscles. The apoptosis of epaxial musc le in somites that formed after notochord degeneration is consistent w ith a role for the notochord in the survival and differentiation of th e dorsal myotome. (C) 1998 Academic Press.