Homozygous embryos are arrested in their development soon after embryonic day 9.5 (E9.5) but exhibit an overtly normal organization of neural, mesodermal, and endodermal tissues. Histological analysis of embryos reveals abnormalities in the organization of the vascular endothelium and its surrounding mesenchyme, notably a disruption in the formation of the dorsal aorta. An impairment in vascular development is therefore a possible cause of the embryonic lethality in homozygous mutants. Motor neurons are not generated without ISL1 protein, although many other aspects of cell differentiation in the neural tube occur normally. A population of interneurons that express engrailed 1 (EN1), however, also fails to differentiate in these mutant embryos. ISL1 is required for the generation of motor neurons and motor neuron generation is required for the subsequent differentiation of certain interneurons.
Homozygote embryos analyzed at E9.5 express a modified transcript in mesodermal cells, but no expression is detected in neural tissues. No ISL1 immunoreactivity is detected in any tissues in homozygous mutant embryos, indicating that the targeted mutation eliminates expression of ISL1 protein.
