REPRODUCTIVE SYSTEM - (Part II) The somatic gonad

General description - The distal tip cell - Gonadal sheath cells -The spermatheca - The spermatheca-uterine valve-List of somatic gonad cells-Back to Contents

General description

The somatic gonad refers to the non-germ-line component of each arm (GonFIG1 - ADULT). It consists of five tissues, each with specific functions and distinct anatomical features: the distal tip cells (DTCs), the gonadal sheath, the spermatheca (sp), the spermatheca-uterine (sp-ut) valve and the uterus (covered in Part IV - the egg-laying apparatus). These tissues, in particular the sheath and DTC, are intimately associated with the germ line and play a critical role in its development and its organization and function in the adult (Kimble and White, 1981; McCarter et al., 1997; Hall et al., 1999). All cells of the somatic gonad derive from two founder cells, Z1 and Z4, present in the L1 gonad primordium (GonFIG1- L1). By L2, Z1/Z4 have generated 12 descendents: 2 DTCs, required for gonad elongation and germline patterning; 9 blast cells that will, collectively, generate all other adult somatic gonad cells; 1 anchor cell (AC), a transient cell that functions to pattern the cells of the vulva. Somatic and germ cells are intermingled until the L2/L3 molt where upon they rearrange establishing the general organization of the future gonad (GonFIG1- L2/L3 MOLT). The DTCs are positioned at the anterior and posterior of the developing gonad. The 10 remaining cells gather at the center to form the SPh (Somatic gonadal Primordium of the hermaphrodite), thus dividing the germ line into anterior and posterior populations of cells or arms (Kimble and Hirsh, 1979).

The distal tip cell

The DTC is a single, large somatic cell located at the tip of each gonad arm. It forms a close-fitting cap over the most distal 6-10 germ cells. No intervening basement membrane or specialized intercellular junctions are found between the DTC and germ cells. Several thin cytoplasmic arms (tentacle-like cytonemes), less tightly associated with germ cells, extend from the cap for an average of 8±4 cell diameters (but can extend as far as 20 cell diameters; GonFIG2A, 2B). The gbl (gonadal basal lamina), which covers the entire outer surface of the gonad, is thickened in the region of the DTC (GonFIG2C). Fragments of the gbl appear to be shed from the trailing arms of the DTC into the pseudocoelom (GonFIG2D). The DTC has a large nucleus located at its leading (distal) edge and its cytoplasm is filled with distinctive membrane-bounded vacuoles, some RER (rough ER) and mitochondria. The plasma membrane sometimes displays “omega” figures where it faces the gbl, indicative of active endocytosis or exocytosis (GonFIG2E). The gross anatomy of DTC cell, born in the L1, does not alter significantly during the course of development, although the adult cell appears to have more and longer cytonemes extending over the germ line (Hall and Hedgecock, unpublished).

The hermaphrodite DTC has two major functions:
1. Gonadal arm elongation during development.
2. Promoting mitosis and/or inhibiting meiosis of the germ cells, both during development and in the adult.
1. Gonad arm elongation. The gonad arms acquire their U-shape by the directed migration of the DTC, which acts as a leader cell (GonFIG3A, 3B). Arm elongation begins in L2 (21hrs, 20°C) and continues, proceeding at variable rates, until the L4 molt (45 hrs) (Hall and Hedgecock, unpublished; Antebi et al., 1997). During migration, the DTC glides along basal laminae of body wall muscles and hypodermis. The DTC produces a metalloprotease (GON-1) that is hypothesized to facilitate migration by remodeling the basal laminae during gonad extension (Blelloch and Kimble 1999; Blelloch et al., 1999). In contrast to neuronal growth cones, the leading edge of the DTC is broad and blunt during migration and bears no fingers or lamellipodia (GonFIG2B). Genetic functions that control elongation include global guidance molecules (such as unc-6, unc-5 and unc-40) (Hedgecock et al., 1987, 1990) and cell recognition functions such as those defined by the programmed cell death corpse engulfment genes (e.g., ced-2, -5 and -10) (Conradt, 2001). DTC migration also appears to be sensitive to global signals of developmental stage since migration halts during dauer arrest and is advanced or delayed in heterochronic mutant backgrounds (Ambros,1997; Antebi et al., 1997).

2. Regulation of germ line mitosis versus meiosis entry. The adult germ line (see Part III - germ line) exhibits distal-proximal polarity with mitotic cells at the distal most end and meiotic cells filling the remainder of the gonad. Among the meiotic cells there is also a gradient of maturation with the most mature stages of meiosis and gametogenesis at the proximal end of the gonad. Ablation of the DTC causes all mitotic nuclei to become meiotic and all meiotic nuclei to mature into gametes, the fate of the most proximal meiotic germ cells (see Part III - germ line; Kimble and White, 1981, Austin and Kimble, 1987; Lambie and Kimble, 1991). DTC regulation of mitosis versus meiosis entry is mediated by a Notch/LIN-12 signal transduction pathway (Lambie and Kimble, 1991; Crittenden et al., 1994; Henderson et al., 1994, Tax et al., 1994) . The DTC expresses the pathway ligand LAG-2 (Lin And Glp), while the germ line expresses the pathway receptor (GonFIG3B, 4), GLP-1 (germline proliferation abnormal mutant phenotype) and downstream effectors. Pathway activation blocks entry into meiosis (or promotes mitosis), maintaining germ cells near the DTC in a mitotic state (Hansen et al., 2004). It is hypothesized that germ cells enter meiosis by default due to their increased distance from the DTC. The exact position and timing of the initial onset of meiosis in the L3 stage (Part III - germ line, GerFIG6) is influenced by both the DTC and non-DTC somatic cells (Pepper et al, 2003;Killian and Hubbard, 2004). Much of the DTC-germ line contact region falls short of the mitotic zone proximal boundary (GonFIG4), suggesting that pathway activation must be propagated in some way to explain how cells not in direct contact with DTC stay in a mitotic state (for models see Crittenden et al., 1994; Crittenden et al., 2003).

Gonadal sheath cells

Five pairs of thin gonadal sheath cells (0.4mM thick) form a single layer covering the germ line component of each arm, each pair occupying a stereotyped position along the gonad proximal-distal axis. Neighboring sheath cell borders partially overlap and occasional gap junctions and macular adherens junctions are observed between cells in these regions (see below). Sheath cells are intimately associated with the germ line and are necessary for several aspects of germline development. Sheath cells or their precursors promote germline proliferation, exit from pachytene, gametogenesis and male gamete fate during germline sex determination (Seydoux et al., 1990; McCarter et al., 1997; Rose et al., 1997;Killian and Hubbard, 2004). In the adult, distal sheath cells engulf germ cells eliminated by programmed cell death (see Part III - germ line). Proximal sheath cells are necessary for oocyte maturation, ovulation and function permissively in the process of yolk protein uptake by oocytes (McCarter et al.,1999; Hall et al., 1999; Grant and Hirsh, 1999).

Sheath cells arise from the SS blast cells present in the L2/L3 SPh (GonFIG1, 5B). During gonadogenesis, sheath cells reach their final distal-proximal location either by being pulled along with or crawling along the growing germ line (Kimble and Hirsh, 1979; McCarter et al., 1997). Distal and proximal sheath cells of the adult express quite different characteristics. Sheath cell pair 1, which overlies the distal germ line, in particular, is strikingly different from the more proximal pairs 3 to 5, which overlie developing oocytes. Pair 2, located over the loop, appears to express properties intermediate to the distal and proximal pairs.

Distal sheath cell pair 1. The cytoplasm of these cells forms concentrated wedges between germ cells and a thin layer over them, giving pair 1 a net-like appearance (GonFIG6A, 6B). Distally, cells extend filopodia that form an irregular meshwork running between germ cells (GonFIG6C; Hall et al., 1999). Beneath this distal sheath pair, germ cells are gradually flowing proximally towards the loop, propelled both by the generation of new germ cells distally and by loss of some germ cells to apoptosis and/or ovulation more proximally. Thus the distal sheath cells may be in a perpetually crawling phase, just to keep their place over the moving germ line.

Proximal gonad sheath cell pairs. Pairs 3 to 5 differ from sheath cell pair 1 dramatically in their morphology and ultrastructural characteristics (GonFIG7A). Pairs 3 to 5 express muscle components such as the filament proteins actin (visualized with rhodamine phalloidin) and myosin, and the thin filament associated muscle protein UNC-87 (Hirsh et al., 1976; Strome, 1986; McCarter et al., 1997; Goetinck and Waterston, 1994). These filaments are organized into dense networks. In pairs 3 and 4, filaments are predominantly longitudinally-oriented, whereas in pair 5, filaments are both longitudinally and circumferentially-oriented (GonFIG7B,7C). Filaments are also present in the distal sheath cells but are much less abundant. The presence of dense networks in proximal cells is consistent with their contractile properties. Proximal sheath contraction is required for ovulation, transfer of the oocyte into the spermatheca for fertilization. During ovulation proximal sheath cell contraction pulls the dilated spermatheca over the most proximal ooctye. Neither the sheath nor spermatheca (see below) appears to be innervated. These tissues may therefore be similar to arterial smooth muscle and potentially regulate contraction and relaxation through calcium sparks (see Bui and Sternberg, 2002 and references therein).

Gap junctions are seen occasionally between sheath cells and between the apical sheath cell surface and oocytes (GonFIG7D-F; Hall et al., 1999). Contraction of sheath cells is coupled to oocyte maturation and the presence of sperm (see Part III - germ line; McCarter et al.,1999; Miller et al., 2001; 2003). Sheath:sheath and sheath:oocyte gap junctions may therefore facilitate the coordination of oocyte stage and sheath contraction rate with the presence of sperm. Sheath cell pairs 4 and 5 also contain numerous pores. Yolk particles produced in the intestine pass through the gbl and the sheath pores, gaining entry to the oocytes by a process of endocytosis (Grant and Hirsh, 1999; Hall et al., 1999).

The spermatheca

The spermatheca (sp) is an accordion-like tube that contains sperm and is the site of oocyte fertilization. It is composed of 24 cells organized into two regional groups: distally, 8 cells aligned in 2 rows that form a narrow corridor (or neck) and proximally, 16 cells which form a wider bag-like chamber that contains sperm and is the site of fertilization (Kimble and Hirsh, 1979; McCarter et al., 1997).

In the absence of an oocyte, the adult spermatheca lumen is narrow and the apical surfaces of cells lining it are highly convoluted, providing the potential for expansion and an adherent surface for sperm (GonFIG8C; compare GonFIG9A cf. 9B). The outer (basal) surface displays numerous longitudinal folds of collapsed membranes (GonFIG8A) that may also allow for the radial expansion of the spermatheca during oocyte passage.

Spermathecal cells are rich in actin microfilaments that are organized into circumferentially-oriented networks (GonFIG9C) . Myosin, however, has not been detected (Strome, 1986; McCarter et al., 1997). Circumferential dilation of the distal spermatheca during ovulation is triggered in response to activation of the lin-3/let-23 Receptor Tyrosine Kinase (RTK) pathway by the maturing 1° oocyte (see Part III - germ line). Pathway activation causes an increase in IP3 levels, which leads to dilation, possibly by a mechanism involving calcium release (Clandinin et al., 1998; McCarter et al.,1999; Bui and Sternberg, 2002). Tight regulation of IP3 levels appears to be necessary to ensure that dilation is strictly controlled so that only one oocyte at a time is enveloped by the spermatheca (Bui and Sternberg, 2002). Gap junctions are located on the lateral borders between sheath and spemathecal cells. These could serve to synchronize spermatheca dilation and relaxation with contraction of the sheath.

Spermathecal cells arise from SS and DU blast cells of the HSP; 18 cells are the products of the SS cells and 6 derive from the DUs (GonFIG1, 5B; Kimble and Hirsh, 1979; McCarter et al., 1997; Newman et al., 1996). Terminal cells form a spermatheca with a lumen by late L4, however, this organ does not achieve its adult form until the first oocyte has passes through it (J. White, unpublished). Prior to the first ovulation the newly formed spermatheca is devoid of sperm (GonFIG10C). Male gametes are generated in the oviduct and remain there until passage of the first mature oocyte pushes them into the spermatheca (see Part III - germ line). This first ovulation event also results in loss or reduction of numerous filapodia that extend from apical membranes into the spermathecal lumen (GonFIG10B, 10C; J. White unpublished; Hall, unpublished).

Maturing spermathecal cells (GonFIG10B, 10C) have a dark cytoplasm (granular by DIC) and are covered by a thick basal lamina on their basal surface. Cells are organized into a spiral structure with a single left-handed twist along the organ's A-P axis. This arrangement likely contributes to the complex twisting of cell borders, which are hard to resolve, even at high magnification. Each cell contributes a portion of its apical side to the lumenal surface and its basal side to the outer surface of the tissue. Cell surfaces bear a variety of junction types that are recognized by MH27 (see GonFIG9A; GonFIG10C-F; Hall, unpublished). Apical surfaces bear adherens junctions (aj) and pleated septate junction (psj) while lateral surfces bear smooth/continous septate (s/csj) and gap junctions (gj). The pleated septate and continuous junctions, on either side of the adherens junctions, may zip and unzip as oocytes pass through the organ (GonFIG10G; J. White, personal communication).

 

The spermatheca-uterine valve

After oocyte fertilization the newly formed embryo passes from the spermatheca to the uterus via a connecting valve called the spermatheca-uterus (sp-ut) valve. The adult valve consists of a toroidal syncytium generated by the fusion of four cells (sujns) (GonFIG11A-C; Kimble and Hirsh, 1979).

Like the spermatheca, the morphology of the sp-ut valve is altered by passage of the first fertilized oocyte. Prior to the first ovulation, the center of the toroid is occupied by two junctional core cells, also syncytial (sujcs) (Kimble and Hirsh, 1979). The core cells extend pseudopodia into the apical folds of the sujn cells and core cell nuclei protrude into the uterus lumen (GonFIG12A,12B; Kimble and Hirsh, 1979). Passage of the first fertilized oocyte apparently pushes the core cell bodies away to open the passage. The fate of the displaced core cells is not known.

The anatomy of the sujn valve cells has been followed in serial sections by Eileen Southgate and John White (pers. comm.). The outward (basal surface) of the valve is encircled by a thick basal lamina. At its apical (luminal) face, valve membranes appear to zip together by pleated septate junctions, in the same manner as the spermatheca, thus sealing the lumen when empty (GonFIG11B). Valve cells extend many interlocking fingers into the valve-spermatheca interface. These, together with possible adherens and septate junctions, may serve to hold the adjacent tissues together. On the opposite side, where the valve faces the nearest uterine epithelial cells (ut4), the lateral cell borders contain extensive septate junctions and possibly some adherens junctions.

List of somatic gonad cells

1. Late L2/early L3 stage SPh

Z1.aa (Distal Tip Cell, anterior)

 

SS, Z1.ap (Somatic sheath and Spermatheca precursor)

SS, Z1.paa (Somatic sheath and Spermatheca precursor)

DU Z1.pap (Dorsal Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

 

VUs and AC are of either the 5R or 5L configuration:

5R configuration

VU, Z1.ppa (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

AC, Z1.ppp (Anchor Cell)

VU, Z4.aaa (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

VU, Z4.aap (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

5L configuration

VU, Z1.ppa (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

VU, Z1.ppp (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

AC, Z4.aaa (Anchor Cell)

VU, Z4.aap (Ventral Uterine precursor; generates uterus, spermatheca and spermatheca-uterine valve cells)

 

DU, Z4.apa (Dorsal Uterine precursor, generates uterus, spermatheca and spermatheca-uterine valve cells)

SS, Z4.app (Somatic sheath and Spermatheca precursor)

SS, Z4.apa (Somatic sheath and Spermatheca precursor)

 

Z4.pp (Distal Tip Cell, posterior)

 

2. Adult anterior gonad arm

Distal Tip Cell of anterior gonad arm:

Z1.aa

 

Somatic Sheath (10 cells/5 pairs) of anterior gonad arm:

Z1.apa (sheath cell 1)

Z1.appaaa (sheath cell 2)

Z1.appaap (sheath cell 3)

Z1.appapa (sheath cell 4)

Z1.appapp (sheath cell 5)

Z1.paaa (sheath cell 1)

Z1.paapaaa (sheath cell 2)

Z1.paapaap (sheath cell 3)

Z1.paapapa (sheath cell 4)

Z1.paapapp (sheath cell 5)

 

Spermatheca (24 cells) of anterior gonad arm:

Z1.apppaaaa

Z1.apppaaap

Z1.apppaapa

Z1.apppaapp

Z1.apppapaa

Z1.apppapap

Z1.apppapp

Z1.appppa

Z1.appppp

Z1.paappaaaa

Z1.paappaaap

Z1.paappaapa

Z1.paappaapp

Z1.paappapaa

Z1.paappapap

Z1.paappapp

Z1.paapppa

Z1.paapppp

Z1.papaaad

Z1.papaaav

Z1.papaapv

Z4.apaaaad

Z4.apaaaav

Z4.apaaapv

 

Spermatheca-uterine valve and core (both syncytial) of anterior gonad arm:

Z1.papaapd; Z4.apaaapd (2 sujc cells that fuse to make the "core" syncytium, lost after the 1st ovulation)

Z1.papapaaa; Z1.ppaaaaa; Z1.ppaaapa; Z4.apaapaaa (4 sujn cells that fuse to make the valve syncytium)

 

3. Adult posterior gonad arm

Distal Tip Cell of posterior gonad arm:
Z4.pp

 

Somatic Sheath (10 cells/5 pairs) of posterior gonad arm:

Z4.pap (sheath cell 1)

Z4.paappp (sheath cell 2)

Z4.paappa (sheath cell 3)

Z4.paapap (sheath cell 4)

Z4.paapaa (sheath cell 5)

Z4.appp (sheath cell 1)

Z4.appappp (sheath cell 2)

Z4.appappa (sheath cell 3)

Z4.appapap (sheath cell 4)

Z4.appapaa (sheath cell 5)

 

Spermatheca (24 cells) of posterior gonad arm:

Z1.papppav

Z1.pappppd

Z1.pappppv

Z4.apappav

Z4.apapppd

Z4.apapppv

Z4.appaaaa

Z4.appaaap

Z4.appaapaa

Z4.appaapapa

Z4.appaapapp

Z4.appaappaa

Z4.appaappap

Z4.appaapppa

Z4.appaapppp

Z4.paaaaa

Z4.paaaap

Z4.paaapaa

Z4.paaapapa

Z4.paaapapp

Z4.paaappaa

Z4.paaappap

Z4.paaapppa

Z4.paaapppp

 

Spermatheca-uterine valve and core (both syncytial) of posterior gonad arm:

Z1.papppad; Z4.apappad (2 sujc cells that fuse to make the "core" syncytium, lost after the 1st ovulation)

Z1.pappappp; Z4.apapappp;Z4.aapppap; Z4.aappppp (4 sujn cells that fuse to make the valve syncytium)

 


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