SENSORY NEUROANATOMY OF A SKIN-PENETRATING NEMATODE PARASITE STRONGYLOIDES-STERCORALIS .2. LABIAL AND CEPHALIC NEURONS

Host recognition, contact, and skin-penetration by Strongyloides sterc oralis infective larvae are crucially import ant behavioral functions mediating transition from free-living to parasitic Life. The sensilla of the worm's anterior tip presumably play an important role in these processes, Besides the main chemosensilla, the amphids, which are of c entral importance, the larva has 16 putative mechanosensilla. There ar e six inner labial sensilla: two dorsal, two ventral, and two lateral. The two dorsal and ventral pairs are each innervated by two neurons, whereas each lateral sensillum is singly innervated. The six outer lab ial and four cephalic sensilla are all singly innervated. All of these have the characteristics of mechanoreceptors: they are closed to the external environment, and closely associated with the overlying cuticl e. Distally, their dendritic processes contain granular material and a ssociated microtubules. With two exceptions, the relevant neuronal cel l bodies Lie in lateral ganglia adjacent to the nerve ring, their posi tions remarkably similar to those of their homologues in the free-livi ng nematode, Caenorhabditis elegans. Cell bodies of two neuronal pairs , one of two dorsal inner labial neurons and one of two ventral inner labial neurons per side, are however, found far anterior to the remain ing cell bodies. All labial and cephalic sensilla are apparently mecha noreceptors, complementing the well-developed chemosensilla. Presumabl y infective larvae require touch and stretch receptors, not only to in itiate skin penetration by finding irregularities as points of access, but also to bore through tissue to reach their ultimate enteral desti nation. (C) 1997 Wiley-liss, Inc.