THE MALE REPRODUCTIVE SYSTEM- (Part II) The Somatic Gonad

Male somatic gonad development -The Distal tip cells -The Seminal Vesicle -The Vas Deferens -Cell list -Back to Contents

Male somatic gonad development

1. Gonad cell generation

The male somatic gonad refers to the non-germline component of the gonad. It consist of 3 tissues: the distal tip cells (DTCs), the seminal vesicle (SV) and the vas deferens (Vas) (ReprodMaleFig2, Kimble and Hirsh, 1979). All cells of the somatic gonad develop from Z1 and Z4 of L1 gonad primordium. By L2, Z1/Z4 have generated 10 descendents: 2 DTCs, terminal somatic cells that will regulate germline patterning, 3 vas deferens and 4 seminal vesicle blast cell precursors and 1 linker cell (LC), a transient cell that functions in gonad elongation (Kimble and Hirsh, 1979). These cells organize to form the Somatic gonadal Primordium of the male (SPm) which establishes asymmetry of the future male gonad (ReprodMaleFig2). Between early-L3 and mid-L4 the SV and Vas precursors divide to generate a total of 53 progeny (23 SV cells and 30 Vas terminal cells). During this period, the primordium expands due to cell proliferation of the somatic and germ line lineages.

 

2. Gonad elongation

The gonad also elongates during this period guided by the male-specific linker cell (LC). The trajectory of its migration establishes the characteristic J-shape of the male gonad (MaleReprodFig3A; Hedgecock et al., 1987; Antebi et al, 1997). In hermaphrodites, this leader cell function is performed by the DTCs, which also regulate germ cell patterning. The male linker cell, however does not share this second function with the hermaphrodite DTCs and is required only for gonad migration. Possibly because of this functional difference, the male linker cell and DTCs have different morphologies. The linker cell is rhomboid shaped and does not extend cytonemes (tendril-like processes) proximally over the germ line (MaleReprodFig3B).

Progression of gonad elongation can be used to roughly estimate male larval stage. Towards the end of its migration the linker cell passes between U lineage cells of the developing cloaca (MaleReprodFig4). One of these cells (U.lp or U.rp, or their fusion products) engulfs the linker cell. This opens the passageway between the lumens of the vas deferens and the cloaca (for more about U cells see THE PROCTODEUM - EPITHELIAL SYSTEM OF THE MALE, PARTIII).

The Distal Tip Cells

In males, the two distal tip cells (DTC's) are found together at the distal end of the gonad (MaleReprodFig5A). This arrangement is established early in gonad development during formation of the SPm (MaleReprodFig2; Kimble and Hirsh, 1979). As in the hermaphrodite the DTCs in the male are required to maintain the nearby germ cells in a mitotic state or to prevent them from entering meiosis (for more on meiotic regulation by the DTC see HERMAPHRODITE HANDBOOK-Reproductive System Part II; Kimble and White, 1981). In contrast to the hermaphrodite, however, the male DTCs do not migrate during gonad development; this function is fullfilled by the male linker cell (see above MaleReprodFig3-4).

Superfically male DTCs appear to have a similar morphology to those of the hermaphrodite (for a detailed description of hermaphrodite DTCs see HERMAPHRODITE HANDBOOK-Reproductive System Part II). The DTC is a single, large somatic cell that has a large nucleus located at its distal edge. The cell forms a close-fitting cap over the most distal 6-10 germ cells (MaleReprodFig5B). No intervening basal lamina or specialized intercellular junctions are found between the DTC and germ cells.

The Seminal Vesicle

The seminal vesicle (SV) consists of an inner tube of 20 apparent secretory cells that are surrounded by a thin sheet of the cytoplasmic processes formed by 3 large cells. Shortly after their differentiation the small cells have a granular appearance when viewed with DIC optics and small blebs are apparent on their lumenal surfaces (MaleReprodFig6A; Kimble and Hirsh, 1979). The seminal vesicle has a larger outer diameter than either the testis or vas deferens. The small cells derived from the four seminal vesicle precursors of the SPm while the large, thin cells are the most distal daughters of the vas deferens precursors (MaleReprodFig2). These large cells are born proximal to the seminal vesicle and then enlarge, flatten and spread over smaller cells encapsulating them and germline cells. Their thin, sheet-like morphology and extension over the germ line is reminiscent of the distal sheath cells of the hermaphrodite gonad. (MaleReprodFig6B, 7A, 7B).

The seminal vesicle stores spermatids until ejaculation. Spermatids are the sessile precursors to ameboid mature spermatozoa. Spermatids are formed by the process of spermatogenesis where spermatocytes detach from the rachis of the proximal germ line and undergo 2 meiotic divisions (MaleReprodFig6A, 7A, 7B; Wolf et al., 1978; see THE GERM LINE - REPRODUCTIVE SYSTEM OF THE MALE, Part III).

 

The Vas Deferens

The vas deferens (vas) is a complex secretory tube that extends from the seminal vesicle to the cloaca. It is composed of 30 cells that can be divided into at least 3 cell types based on morphological differences in secretory granules alone (N. Wolf, J. Kimble and D. Hirsh unpublished observations). The anterior end of the vas appears to act as a valve that may regulate sperm release from the seminal vesicle during ejaculation (MaleReprodFig9A, 9B). The anatomical characteristics of valve cells are not well defined. However, Gower et al., 2005 have identified a potential molecular marker, the itr-1 reporter gene, which exhibits elevated expression in cells of this region (MaleReprodFig9A). Differences in cell shape are also apparent along the length of the vas. Posterior to the valve, some cells are cuboid in shape while closer to the cloaca, vas cells are elongated (MaleReprodFig9A).

Consistent with a secretory function, cells of the vas deferens are rich in ribosomes, ER and secretory granules and potential storage vacuoles (MaleReprodFig10A-D).

The most proximal, ventral cell of the vas and its neighbor, cloacal cell Bε(l/r).aav, are innervated by neurons of post-cloacal sensilla (PCBL/R and PCCL/R) and spicule neurons (SPCL/R) (MaleReprodFig11A, 11B; Male Wiring Project). Synapses occur where axons run in the dorsal PAG (pre-anal ganglion), directly beneath these gonadal cells. The post-cloacal sensilla and spicule function in the steps that preceed ejaculation, namely vulval location and spicule insertion (see Mating Behavior Movie in MALE ANATOMY INTRODUCTION, Part I - MALE HAND BOOK). Direct gonadal innervation by SPCL/R, PCBL/R and PCCL/R suggests that vulval contact and fully extended spicules may trigger this step (reviewed in WormBook: Male Mating Behavior - Barr and Garcia).

The innervated posterior vas cell and Bε(l/r).aav contain numerous electron-dense vesicles that may be secretory (MaleReprodFig11B). These cells line the vas and cloacal lumens, respectively. Their lumenal (apical) surfaces are extensively folded and bear septate junctions (MaleReprodFig11B, 11CD). This is reminiscent of the hermaphrodite spermatheca in hermaphrodites where the lumen may close and open by unzipping of septate junctions. Possibly opening and closing of the vas/cloacal lumen may be regulated in response to inputs from the dorsal PAG.

 

Cell list

(under construction)

DTC (gon male dtc) :
Z1.a
Z4.p


Seminal vesicles (gon male sves)
Z1.pap
Z1.pp (x10)
Z4.paa or Z4.aaa
Z4.aap
Z4.ap (x10)

Vas deferens (gon male vdef)
Z1.pap(x10)
Z4.aa (x20)
or
Z4.pa (x20)
Z4.aap (x10)

Linker cell
Z1.paa or Z4.aaa

Acknowledgements

We would like to thank Steven L'Hernault (Emory U., GA) for critically reviewing this chapter for us.


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