Epithelial System of the Male (Part II) - Sensilla: The Hook

Hook composition and structure -Hook neuron connectivity -Hook development- Back to Contents

Hook composition and structure

The sensory hook is a sclerotized cuticular structure, located immediately anterior of the cloaca and shaped like a shield (MaleEpiFIG19A,19B). The hook contains the processes of two sensory neurons, HOA and HOB, a sheath cell (HOsh), a socket cell (HOso) and 4 hypodermal cells (P10.papp, P11.ppaa, P11.ppap, P11.ppp and P12.pa/hyp12) (MaleEpiFIG20A-C; MaleEpiTABLE3). Hook cell bodies are located in the preanal ganglion (PAG).


MaleEpiTABLE3: Summary of Hook Cells

Cell type

Cell name Lineage name Syncytial No. nuclei Other noteworthy features
Sheath HOsh P10.ppppp no 1  
Socket HOso P10.ppaa no 1  
Epithelia (other) P10.papp   no 1  
  P11.ppp no 1  
P11.ppap no 1  
P11.ppaa no 1  
P12.pa no 1  
Sensory neuron HOA P10.pppa no 1 striated rootlet; ends in socket cell
HOB P10.ppap no 1 large cell body containing dilated cisternae; dendrite contains a transition zone and has dense ending that is exposed to the environment

HOA and HOB neurons share several features in common with RnA and RnB neurons of the rays: HOB has dilated cisternae in its cell body, expresses PKD-2 and LOV-1 (like RnB; MaleEpiFIG20D; Barr and Sternberg, 1999), has a distinct basal body and electron dense material in its tip; HOA (like RnA) has a striated rootlet (MaleEpiTABLE3; Sulston et al., 1980).


Hook neuron connectivity

In male mating behavior, the hook acts partially redundantly with the post-cloacal sensilla (PCS) in location of the vulva and inducing spicule prodding behavior (Liu and Sternberg, 1995; Barr and Sternberg, 1999; Garcia et al, 2001). Consistent with this, PCS neurons are major targets of HOB (Sulston et al., 1980). Stimulation of spicule prodding however, is likely to be indirect as hook neurons do not innervate spicule protractor muscles. HOB and HOA are connected to each other along their lengths both by numberous gap junctions and a few chemical synapses.

Hook development

Hook cells are derived from the 3 posterior ventral Pn.p cells P10.p-P12.p (MaleEpiFIG21). The strategy used to pattern these precursors and establish hook cell fates bears remarkable similarity to that used to pattern the hermaphrodite vulva, also Pn.p cell-derived (P3.p -P8.p cells; HERMAPHRODITE HANDBOOK - THE REPRODUCTIVE SYSTEM - PART IVa). Male P10.p and P11.p cells, along with P9.p, form a hook sensillum competence group (HCG) in which all cells have equal potential to generate hook cell lineages. This competence requires the activity of a Wnt pathway (with frizzled receptor LIN-17) and a Hox gene (MAB-5). Also as in the vulva, cell-cell signaling events establish differences among HCG cells so that each adopts a different fate: 1° , 2° or 3° (defined by the lineage pattern): P11.p adopts a 1° fate, P10.p 2° and P9.p 3° fate (fusion with hyp 7 or division followed by fusion of its daughters) (Sulston and White,1980). In contrast to vulval patterning however, Wnt, and not EGF signaling, establishes 1° fate among the HCG (Yu and Sternberg, 2004).


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