Little or no Htl protein is detected in heart precursors at stage 11. It is presumed that the second phase expression of Htl in most somatic muscle precursors starts during stage 11. Expression in heart precursors begins with stage 12. The heartless expressing heart precursors are Even-skipped negative. A second round of htl expression in visceral muscle precursors for midgut and hindgut is discernible at stage 14, when dorsal pharyngeal muscles and foregut visceral muscles, derivatives of the cephalic mesoderm, are Htl positive.
Neurons provide critical signals that regulate both the number and differentiation of glia.
In addition, glia are attracted to and enwrap neuronal axonal processes. In the Drosophila FGF receptor mutant heartless , glia migrate to axons, but fail to elongate around them. This study shows that in the similar but larger grasshopper CNS, FGF signaling is likely to mediate one step in the close interaction between glia and axons. To examine the expression of htl in the grasshopper embryo, antiserum against the extracellular portion of the fly protein was used to stain gel-resolved protein extracts of grasshopper embryos as well as intact ganglia. Two large bundles of axons connect each ventral nerve cord ganglion and contain a central core of axons surrounded by glial cells.