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The male reproductive system differs from that of the hermaphrodite both in the nature of gametes it produces and the somatic tissues associated with it (ReprodMaleFig1). By analogy with the hermaphrodite reproductive system, we define the male reproductive system as including all those tissues directly involved in the production of gametes or their transfer during copulation: (1) the somatic gonad and neurons that innervate it (THE SOMATIC GONAD - REPRODUCTIVE SYSTEM OF THE MALE, PART II) (2) the germ line (THE GERM LINE - REPRODUCTIVE SYSTEM OF THE MALE, PART III) and (3) the proctodeum (male posterior gut) and associated neurons and muscles (described in detail in THE PROCTODEUM - EPITHELIAL SYSTEM OF THE MALE, PARTIII). The male germ line generates only male gametes (sperm) and does so for the entire life of the animal (Klass et al., 1976). The germ line is only partially covered by the somatic gonad; both the gonad and germ line are ensheathed in a gonadal basal lamina. Together the germ line and somatic gonad form a single J-shaped arm that connects proximally to the cloaca (rectal epithelial tube) of the proctodeum. The cloaca opens to the exterior at the ventral posterior via the anus (cloacal opening). During copulation the spicules, housed inside the cloaca, probe for the vulval opening and insert into the vulva fully so that the cloacal opening is closely opposed to the vulva. Behavior of the spicule is regulated by neurons (see SPICULES - EPITHELIAL SYSTEM OF THE MALE, PART II) and muscles associated with the proctodeum (see MUSCLES OF THE MALE, PART II). Full spicule insertion triggers sperm release from the gonad and transfer (the process of ejaculation).
As in the hermaphrodite, the various components of the male reproductive system have diverse lineal origins. They orginate from cells that are present in both sexes at hatching but that execute different lineages or have different fates in each sex (ReprodMaleFig1). The somatic gonad and germ line develop from the same 4-celled gonadal primordium (GP) as its counterparts in the hermaphrodite. Cells of the cloaca, spicule channels and spicules are generated by the rectal epithelial cells B, Y, F, U , K and neuroblast P12. In the hermaphrodite most rectal cells do not divide and instead form the hermaphrodite rectum, which is not part of the reproductive system. Most of the muscles associated with the proctodeum are derived from muscle myoblast M, which is the source of an entirely different complement of sex muscles in the hermaphrodite. Neurons that innervate the male somatic gonad and the proctodeum are derived from B and Y (Klass et al., 1976; Sulston and Horvitz, 1977; Kimble and Hirsh, 1979; Sulston et al., 1980; Hodgkin, 1988; The Male Wiring Project).
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