FIGURE 9.

Dendrite towing governs AM dendrite morphogenesis and apical constriction contributes to placement of sensory organ pores. (A) Alternative models for AM dendrite morphogenesis. Top: During dendrite towing, the anterior-directed migration of the epidermis generates pulling forces that lead to movement of the AM sensory organ. Pulling forces can be transduced since the amphid organ partially intercalates into the epidermal layer through junctions of the socket glia cell, to which the dendrite tips of the AM neurons form contacts. Middle: In the case of retrograde extension, it has been claimed that the posterior-directed migration of neurons would lead to dendrite extension. Bottom: During growth cone advancement, dendrites would extend actively by forming a migratory specialization at their anterior ends. (B) Working model for collective morphogenesis of AM dendrites, neuron internalization, and mouth formation. Through dendrite towing, the anteriorly migrating epidermal sheet mediates AM sensory organ morphogenesis, while apical constriction of anteriorly located cells leads to tissue bending and formation of an anterior neuropore that connects to the mouth formed through apical constriction of arcade cells.