General Description - Lineal origin of the reproductive system - Back to Contents
The reproductive system of the hermaphrodite produces mature gametes and provides the structure and environment for fertilization and egg-laying (ReprodFIG1). It can be divided into three major parts: the somatic gonad (Part II), the germ line (Part III) and the egg-laying apparatus (Part IVa and IVb) (ReprodTABLE1 below). The somatic gonad and germ line together form two symmetrical U-shaped tubes (arms) joined by a common uterus. Somatic gonad consists of the distal tip cell (DTC), the gonadal sheath, spermatheca (sp), spermatheca-uterine (sp-ut) valve and the uterus (the uterus can also be considered part of the egg-laying apparatus). The adult germ line is organized in a distal-to-proximal manner, with proximal defined as nearest the point where embryos (in the hermaphrodite) or sperm (in the male) exit from the animal. Germ cells in the distal-most part of the gonad arm are mitotic and undifferentiated. As germ cells move proximally, they enter and pass through the stages of prophase of meiosis I, reaching pachytene in the loop region and progressing further through meiosis in the proximal gonad (PG) (ReprodFIG1). The hermaphrodite is considered a specialized female because the soma is female but the germ line first produces a fixed number of male gametes (sperm) before switching to the sole production of female gametes (oocytes) (Schedl, 1997; L'Hernault, 1997). Hermaphrodites can therefore produce approximately 300 embryos by fertilization of oocytes with self-sperm (the process of self-fertilization). Fertilization can also be achieved using male-derived sperm, transferred during copulation. In the proximal gonad, oocytes undergo maturation and are ovulated in a single file, assembly line fashion into the sperm-containing spermatheca where they are fertilized (Singson, 2001). Fertilized eggs then move into the uterus. Activity within the egg-laying apparatus (uterus, vulva, uterine and vulval muscles, HSNL/R and VC1-6 neurons), subsequently forces eggs out into the environment by passing them through a ventral opening called the vulva.
Formation of the reproductive system spans the entire post-embryonic period. It is formed by cells from several lineages (ReprodTABLE1, ReprodFIG2) including some that originate more posteriorly and must migrate considerable distances to be included in the developing system (e.g., the HSNs and uterine and vulval muscle precursors) (Sulston and Horvitz, 1977; Sulston et al., 1983). Not surprisingly, the organization of this complex system involves a hierarchy of temporally and spatially coordinated signaling events and cell-cell interactions (Sulston and White, 1980; Kimble, 1981; Sternberg and Horvitz, 1986; Sternberg 1988;Thomas et al., 1990). The developing gonad itself serves as the primary organizer, promoting development of the vulva and uterus and guiding the precise positioning of sex muscle precursors (Kimble, 1981; Sternberg and Horvitz, 1986; Thomas et al., 1990; Newman et al., 1995). The vulva, in turn, acts as a secondary organizer for assembly of the egg-laying apparatus (Thomas et al., 1990; Li and Chalfie, 1990; Garriga et al., 1993; Chang et al., 1999; Shen and Bargmann, 2003; Shen et al., 2004). Finally, within the gonad itself, interactions between somatic tissues and the germ line play a critical role in promoting germline proliferation, polarity, progression of meiosis, ovulation and gamete sexual identity (Kimble and White, 1981; Seydoux et al., 1990; McCarter et al., 1997; Pepper et al, 2003; Killian and Hubbard, 2004).
Some maturation events occur remarkably late in reproductive system development. For instance, several anatomical changes are associated with the first ovulation. Sperm mature in the spermatheca (L'Hernault, 1997). The spermathecal lumen itself and adjacent sp-ut valve also undergo structural modification with the passage of the first oocyte. The lumen of the uterus is an empty, collapsed structure until it becomes inflated by the first fertilized oocyte (J. White unpublished; Hall, unpublished).
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