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Sensory structures that differ in the male can be divided into two types: male-specific and sexually dimorphic. Male-specific sensory organs have no homologs in the hermaphrodite and are formed by cell types generated only in the male. Sexually dimorphic sense organs contain at least some cells that have the same lineal origin in both sexes. These sensilla are considered sexually dimorphic because they express distinct differentiated characteristics in the two sexes, reflected in cell morphology, cell contacts or organization.
Four types of male-specific sensory organs are located in the copulatory apparatus of the tail: the sensory rays, the hook, the post-cloacal sensilla (PCS) and the spicules (MaleEpiFIG10A). These sensilla have been shown to function in specific steps of male mating behavior (MaleEpiFIG10B; Sulston et al., 1980; Loer and Kenyon, 1993; Liu and Sternberg, 1995; Barr and Sternberg, 1999; Garcia et al, 2001).
With the exception of the rays, male sensilla contain interfacial cells of two types: sheath cells and socket cells (MaleEpiFIG10C). In contrast to the sensilla of the head, which are common to both sexes, the number of non-neuronal cells can be between 1 and 3 (in the head there are invariably 2) (Sulston et al., 1980). Sheath cells wrap around the neuron ending forming a protective pocket and are linked to the neuron by adherens junctions (AJ). Its close proximity to the neuron, together with the the presence of numerous secretory vesicles, suggest that sheath cells are functionally analogous to glial cells in other organisms. The neurons, as in sense organs elsewhere in the animal, are ciliated. In many the axoneme of the cilia is associated with a transition zone (TZ) and some cilia also contain a striated rootlet (SR) extending proximally. The socket cell acts as a transitional cell that is linked both to the sheath and to the adjacent hyp by adherens junctions. Sheath and socket cells are often connected by gap junctions indicating that there is cell-cell communication between these cell types.
Examples of sexually dimorphic sense organs are the cephalic sensilla of the head and the phasmids of the tail. In the male each of the four cephalic sensilla contain the sensory endings of an additional male-specific CEM (D/V L/R) (cephalic companion) neuron. The phasmids of the male contain the same cells as in the hermaphrodite, however, the cells are organized differently and one of the socket cells exhibits unusual ultrastructural features (Sulston et al., 1980).
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