GLR cells - Function of GLR - Cell list - Back to Contents
Three pairs of GLR cells ( GLRDL/R, GLRL/R, GLRVL/R) lie within the pseudocoelom slightly posterior to the nerve ring at the level of the pharyngeal isthmus. They are arranged in a six-fold symmetrical manner (glrFIG1 and 2, glrFIG11, glrFIG13). The cell bodies of the GLR's lie near the dorsal and ventral insertions of the muscle arms of the head where the muscle arms are enlarged to dive from the muscle belly towards the isthmus of the pharynx (glrFIG11 and 13). This physical closeness may reflect the common lineage of the GLR's and the bodywall muscles since both derive from the MS lineage. Their sister cells also include the head mesodermal cell, the pharyngeal musculature and the coelomocytes.
Anteriorly, each cell extends a thin, sheet-like process that lies inside the nerve ring and the muscle arm layer and is closely apposed to the basement membrane of the pharynx (glrFIGs 2-4 and 9-12). Each of these processes wraps around roughly one third of the circumference of the isthmus, touching its neighbor on each side (glrFIG5). The nerve ring is tilted at an angle with respect to the body axis, in a pattern that reflects the position of the dorsal and ventral GLR cells (glrFIG1). On the outside, the GLR cells are surrounded by the muscle arms that lie interior to the motor end plate layer of the nerve ring where muscles receive synaptic input from the nerve ring motor neurons (glrFIG5). The neurites of the nerve ring spread tightly over the basement membrane of the muscle arms, and completely encircle the isthmus of the pharynx. There is no direct connection of the GLR's to the pharyngeal basement membrane which lies between the GLR cells and the pharynx and there is no basement membrane separating the muscle arms from the GLR processes (White J. G. et al, 1986). Arms from each of the 8 longitudinal rows of the head muscles run along specific GLR cells such that each GLRDL/R and GLRVL/R is associated with muscle arms from a single row and each GLRL and GLRR is associated with muscle arms from two rows (glrFIG6). There are gj's between GLR cells and muscle arms and GLR's and RME motor neurons (glrFIG6). However, GLR cells do not make gj's to each other and do not make or receive any chemical synapses to or from any cells.
Anterior to the nerve ring, GLR processes narrow down into thin cylinders and continue to peter out at the level of the junction of the pharynx and the buccal cavity without any terminal specializations (glrFIG2 and 8). Throughout their length they run in the inner labial bundles, closely apposed to the IL1 dendrites (glrFIG8, note that in this cross section only GLRVL/R processes have joined the labial bundles).
Below are 6 transverse sections shown from anterior to posterior.
Click on the images to see full layout.
The cytoplasm of the GLR's is electron dense and contains a distinctive collection of membrane-bound vacuoles. These vacuoles formally look very similar to inclusions in the distal tip cells and the coelomocytes (glrFIG14). This suggests an active endocytic or secretory function.
Because of their location, connectivity pattern, as well as lineage, GLR cells are suggested to be mesodermal scaffolding cells that guide muscle arms to appropriate territories during development (White J. G. et al., 1986). The C. elegans myoD homolog, hlh-1, which is expressed by lineal precursors of body wall muscle and is required for normal muscle function is also expressed in GLR cells in late embryogenesis and larval stages. It has been suggested that hlh-1 might drive expression of cell surface proteins that mediate recognition and contact between GLR's and head muscles (Krause M., et al., 1994). Similar to body wall muscle cells, GLR cells secrete type IV collagen which is integrated into the basement membrane underlying the muscle (Norman K. R. and Moerman D. G,. 2000, and Graham P. L. et al., 1997).
At the anterior end of the nerve ring, the sheet-like anterior processes of GLR's seal the space between the end of the somatic basement membrane of the muscle and the basement membrane of the pharynx and the pseudocoelom. When any one of the parental cells of the GLR's (MSaaaaaa for GLRDL/R; MSapaaaa for GRLL/VL; MSppaaaa for GLRR/VR) is killed in the embryo, the worms hatch late and arrest as starved L1s. In these animals, nerve rings are displaced anteriorly and there is widespread degeneration and vacuolation in neurons and hypodermis which may result from disruption of this seal (Chisholm A. and Horvitz B., 1991, also Chisholm A., pers. comm).
GLRL GLRR GLRVL GLRVR GLRDL GLRDR
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