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The epithelial system includes cells of the hypodermis, the lateral seam and the interfacial cells that line the major openings to the outside such as the rectum and the sensory organ openings. Most differences between male and hermaphrodite epithelial systems are found in the posterior. The male tail bears a copulatory apparatus which is composed of several types of male-specific sensory organs (EPITHELIAL SYSTEM OF THE MALE - Part II: the rays, hook, the post-cloacal sensilla and spicules), a modified rectum (EPITHELIAL SYSTEM OF THE MALE - Part III: the proctodeum and cloaca) and a remodeled hypodermal tail tip (EPITHELIAL SYSTEM OF THE MALE - Part I). The posterior lateral seam in the male, instead of producing alae, makes sensory organs called rays that are embedded in a cuticular fan (MaleEpiFIG1A; Sulston and Horvitz, 1977; Sulston et al., 1980; Sulston et al., 1983;EPITHELIAL SYSTEM OF THE MALE - Part I).
Most sex-specific differences in the epithelial system are established post-embryonically. The various sense organs and copulatory structures of the male are derived from blast cells that are present in both sexes at hatching but that execute different lineages in males and hermaphrodites (MaleEpiFIG1B). These cells produce not only different cells in the male but in general more cells. For example, in males rectal epithelial cell B produces 47 progeny that will form the proctodeum (proctodeal chamber and spicules). By contrast, the hermaphrodite B cell does not divide and contributes only itself to the lining of the hermaphrodite rectum. Also during post-embryonic development, some sex-common structures, such as the phasmids and the tail tip hypodermis, become sexually dimorphic: these structures are formed from the same cells in the two sexes but in the male cell borders or cell morphologies are altered (Sulston and Horvitz, 1977; Sulston et al., 1980; Sulston et al., 1983; Nguyen et al., 1999).
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