First developmental signs of the scolopale (glial) cell and neuron comprising the chordotonal organ in the Drosophila embryo.

The chordotonal (scolopidial) organ is a segmental stretch receptor (proprioceptor) and a four-celled organ of the peripheral nervous system of Drosophila. This organ has become a model in studies of embryogenesis involving molecular genetics and developmental biology. We determined how three glial cells and a bipolar neuron (the chordotonal organ) develop and assemble to become a sensitive stretch receptor. Our focus was on the scolopale cell-neuron association which is the core of this organ. The first anatomical appearance of these developing organs appears in Stage 12 embryos: a central fissure forms in the scolopale cell and two intracellular adherence loci redirect the cleft to wall off a cylindrical sector. This portion hollows out and the dendrite of a sensory neuron enters the cavity. Cytoplasmic walls of the cylinder then regress to leave a more prominent lymph space, within which is the cilary portion of the sensory dendrite. A cap (glial) cell then covers and connects the distal portion of the scolopale-neuron pair. Chordotonal organs are formed in about two hours and assembled throughout Stages 13 to 15. We have drawn schematics of these developmental phases with the resultant four-celled organ. Lanthanum tracer in the embryonic hemocoel never gained access to the neuron housed by the scolopale cell. Thus a blood-nerve barrier forms as early as Stage 13 in the peripheral nervous system. Paracellular clefts, sealed by occlusive septate junctions between accessory (glial) cells, are the probable basis for barrier properties.