MESODERMAL ORGANS- Head Mesodermal Cell

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Head Mesodermal Cell

This is a single cell in the head that lies in the pseudocoelom with the body muscles. The head mesodermal cell (hmc) and its homolog originate from the embryonic MS lineage and are sisters of the gonad cells Z4 and Z1, respectively (Sulston et al., 1983). In the embryo, hmc and hmc homolog migrate circumferentially to the dorsal midline where they meet and align anterior-posteriorly (hmcFIGs 1and 2). The anteriorly located cell (hmc homolog) dies late in embryogenesis.

Postembryonically, hmc cell body lies dorso-medial to the terminal bulb of the pharynx in the head (hmcFIGs 3-5, hmcFIG8). It has posteriorly and ventrally extended processes. The ventral process splits at the pharynx and the two branches grow along the sides of the terminal bulb of the pharynx making a loop around the bulb. Ventral to the bulb, these processes first merge and then split to form a ventral anterior and a posterior arm. The ventral anterior process runs inside the anterior loop of the right excretory gland process and adjacent to the ventral hypodermal ridge. The ventral posterior arm runs in conjunction with ventral body wall muscle arms and the hypodermal ridge and makes gap junctions with ventral body wall muscle arms (hmcFIGs 6-9. Sections are from anterior to posterior). The posterior process runs some distance adjacent to the dorsal hypodermal ridge and makes gap junctions with arms from dorsal muscles. Hmc cell body is flattened and contains a nucleus much like that of body wall muscles and a smaller nucleolus. In the adult, this cell has very few actinomyosin fibrils, all of which seem to lie within the circular loop of the two ventral processes which wrap around the bulb (hmcFIG 3B). These fibrils appear to be too few to perform any significant motor function. Unlike body wall muscles in the nematode, there are no places where the cell maintains any obvious anchorage to the cuticle. The extensive gap junctions that the head mesodermal cell forms with adjacent bodywall muscle arms on both the dorsal and ventral sides (White J. G. et al., 1976; Sulston J. E., unpublished notes), suggest that it might be useful in synchronizing simultaneous contractions of the dorsal and ventral head/neck muscles (hmcFIGs 10-12). Such a function has been postulated to be important in initiating and coordinating the “flipping” motions of the late embryo, prior to the onset of the larval motor pattern (Hall D. H. and Hedgecock E., 1991; Hedgecock E. and Hall D. H. WBG 11(5):96).

WA editors' Note: the position of hmc soma in N2U animal is seen to be slightly more anterior than the position of hmc cell body seen in live animals. In N2U, hmc soma is seen at the level of the pharyngeal grinder (hmcFIG8) whereas in hmcFIGs 4 and 5 hmc somata are located at the level of pharyngeal-intestinal valve which is slightly posterior to the pharynx.

Similar “stomatal” muscles in other nematodes have been postulated to help the digestive function of the grinder, since the mesodermal cell surrounds the pharynx where the "teeth" are located (Chitwood and Chitwood, 1950). This function seems unlikely in C. elegans due to the inconsequential nature of its motor elements.

It is also conceivable that this cell could facilitate the function of the excretory system. The right excretory gland process remains in intimate contact with the head mesodermal cell over 5-10 microns and may be a site for electrical coupling, although evidence for gj's between the two cells remain ambiguous to date (See hmcFIG11). Since the muscle elements of hmc are too wispy and the hmc process lies inside of the gland process loop, it can not be accomplishing any squeezing activity directly on the gland. Nonetheless, perhaps by synchronizing local contraction of all head muscles, this cell could facilitate excretion of granules from the excretory gland, or excretion of the liquid contents of excretory canal sinus.

Cell List

  • hmc
  • hmc homolog

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