Uterine and vulval muscles - Egg-laying neurons - List of cells - Back to Contents
The uterine (um1, um2) and vulval (vm1, vm2) muscles (EggFIG12), collectively referred to as the sex muscles, are required for moving eggs through the uterus and vulva (described in PartIVa). Only 4 the 16 sex-muscles receive direct inputs from the egg-laying neurons. The remaining sex muscles are electrically coupled, either directly or indirectly, to these innervated muscles. This configuration may serve to coordinate uterine and vulval contraction. The sex muscles are classified as "non-striated" muscles because they do not have a "striated" appearance (e.g. as in body muscle), normally attributed to the presence of an ordered array of multiple sarcomeres (muscle contractile units; see Muscle System). Vulval muscles have a single sarcomere that extends along the entire muscle length and attaches to a discrete zone in the body wall, at one end, and in the vulva, at the other (White, 1988). The attachment sites for the uterine muscles have not been extensively characterized so it is not clear whether they are also single sarcomere cells or are more like smooth muscle with randomly arrayed sites of attachment.
There are 8 uterine muscles arranged in 4 bands around the uterus lobes; 2 bands per lobe; 2 muscle cells per band (EggFIG12; 13A). A left/right pair of um2 type muscles (um2L/R) encircles the more distal ut toroids of each lobe. A left /right pair of um1 type muscles (um1L/R) cup the ventral half of the uterus over the more proximal ut toroids and at their dorsal edges attach to the lateral seam. The ventral-proximal edges of the um2 muscles overlap with the um1 muscles (EggFIG12A). Uterine muscles are covered in a thin basal lamina (bl; EggFIG13B). Dense bodies anchor the muscle myofilament network to the basal lamina on the surface facing the uterus. The muscle filaments are circumferentially oriented so their contraction may move eggs by squeezing on the uterus (EggFIG13B; Sulston and Horvitz, 1977). The uterine muscles are not directly innervated and instead are coupled via gap junctions, either directly or indirectly, to vulval muscles that are innervated by the egg-laying neurons (see EggFIG15 below; MOW - White et al., 1986).
There are 8 vulval muscles organized into 2 layers of 4 cells around the vulva: one layer contains 4 vm1 type muscles and the other 4 vm2 type muscles. The muscles run at diagonal angles from the vulval lips to the subventral body wall (EggFIG14). Their proximal ends are linked to the vulval cuticle by specialized attachments called FOs (Fibrous Organelles, a.k.a. hemidesmosomes, described in THE CUTICLE; EggFIG14C).
The vm2 muscles attach between the uterus and vulF (EggFIG14D,14F). Their distal ends insinuate between adjacent members of the ventral body wall muscle (BWM) quadrant (EggFIG14E). vm1 muscles attach to the vulva more ventrally than the vm2s, between vulC and vulD toroids (EggFIG14D, 14F), but join the body wall more dorsally, attaching near the dorsal edge of the ventral body wall muscle quadrant (EggFIG14E).
The vm2s are the only sex muscles that are directly innervated (EggFIG15; MOW - White et al., 1986). The muscles extend arms into the regions of neuropil formed at the vulva where they receive synaptic inputs (EggFIG17B, 17C below). vm1 connects to vm2 by gap junctions. Coordinated foreshortening of the vulval muscles pulls the lips apart allowing eggs to pass through the lumen and out into the environment.
Uterine and vulval muscles derive from a common precursor, the SM (Sex Myoblast). During L1 the M (Mesoderm blast cell; EggFIG16A, 16B) lineage produces a left and a right SM (SML/R; see M Lineage Tree; Sulston and Horvitz, 1977). In L2 the SMs migrate anteriorly along ventral muscle quadrants to the precise center of the developing gonad and future vulva (EggFIG16C). There, the SMs undergo 3 rounds of division to produce the vulval and uterine muscle cells (EggFIG16D,16E; for studies of muscle specification see Harfe et al. 1998; Liu and Fire, 2000; Corsi et al., 2000; Kosta and Fire 2001; Eimer et al., 2002).
SM migration and positioning at the gonad center is guided by the balance of several forces: a gonad-dependent attractive (GDA) mechanism (Thomas et al., 1990), a gonad-dependent repulsive (GDR) mechanism (Stern and Horvitz, 1991) and a gonad-independent attractive mechanism (GIM) (Chen et al., 1997; Huang et al., 2003). The DUs, VUs and AC of the SPh (PartI) and 1°-fated P6.p vulval cells (PartIVa) express FGF-related ligand EGL-17 which is likely to be the gonad-dependent attractant for SM migration. Interestingly, these same cells also appear to be the source of the GDR mechanism (Burdine et al., 1997; 1998; Branda and Stern, 2000).
The vm2 muscles receive major inputs from two groups of motor neurons, the VCn neurons (VC1 to 6) and the HSNs (HSNL/R) (EggFIG17A; MOW - White et al., 1986). The precise role of each neuron and the neurotransmitters they release in egg laying appears to be complex and several models have been proposed (see Weinshenker et al. 1995; Waggoner et al., 1998; 2000; Shyn et al, 2003; Bany et al., 2003).
VC4 and VC5 cell bodies flank the vulval epithelial tube and have short processes in the VNC. VC1, VC2, VC3 and VC6 neuron cell bodies are spaced along the length of the VNC. They each send out a single main axon that runs in the dorsal "neighborhood" of the cord and make similar synaptic contacts to one another. When VCn neuron axons reach the vicinity of the vulva they send processes dorsally along the ventral basal (outer) surface of vulE (EggFIG18). The neurons branch, synapse with each other, with the HSNs and vm2 muscle arms forming a local neuropil (EggFIG17B, 17C). VC4 and VC5 branch more extensively than other VC neurons in this region.
The VCn neurons are derived from the anterior daughters of ventral hypodermal blast cells P3 to P8, the same cells that produce VPCs (PartIVa) from their posterior branches (Sulston and Horvitz, 1977). The neurons are born in L1, begin to send out processes in late L3 and branch in the region of the vulva during L4 (EggFIG16A). Surprisingly, VCn branching depends on cells of the vulva and not on the presence of their targets (Li and Chalfie, 1990; Colavita and Tessier-Lavigne, 2003).
HSNL/R cell bodies are situated subventrally, just posterior to the vulva (EggFIG17B). Each HSN axon projects ventrally to the mid-line to join the ipsilateral VNC (VNCR or VCNL) and from there extends into the nerve ring. As they pass the vulva the HSNs defasciculate dorsally, branch and form synapses with VCn neurons and vm2 muscle arms thereby contributing to the neuropil.
HSNL/R are born in the tail of the embryo and migrate prior to hatching to the mid-body, near to the gonadal primordium (EggFIG16A; Sulston and Horvitz, 1977; Sulston et al., 1983; Desai et al., 1988). Axon outgrowth begins during L2 and L3 and is guided by 1°-fated P6.p lineage cells (see PartIVa) and VNC neurons (Thomas et al., 1990; Garriga et al., 1993). Synapse formation takes place in the L3 and L4 stages. Primary vulval cells (from which vulE is formed) produce a signal that acts as a synaptic guidepost, promoting the correct pattern of presynaptic vesicle clustering in the HSNs (Shen and Bargmann, 2003; Shen et al., 2004)
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