SEAM CELL MATURATION IMAGES

May 2012, DHH interpretation

This set of EM prints were produced by John Sulston, R.N. Singh and Nichol Thomson at the MRC to study late stages of seam cell maturation between late L3 stage and the 4^th larval molt (Singh and Sulston, 1978). Several micrographs in that publication derive from these animals, and it is worth reading this paper for its description of the cytoplasmic changes. During L3 and L4 stages, seam cells (H1, H2, V1 – V6 and T) are undergoing their terminal divisions to produce a chain of 16 seam cells along the length of the body (see further details at http://www.wormatlas.org/hermaphrodite/seam%20cells/Seamframeset.html and http://www.wormatlas.org/hermaphrodite/hypodermis/Images/hypfig1leg.htm).

By preparing a set of animals with closely spaced times of development, Singh and Sulston have tried to compare the cytoplasmic contents within the hypodermis and seam cells at varying times relative to the molts. In particular, they observed a rise in the number and size of Golgi bodies within the seam cells just prior to molts, as if Golgi might be important for new cuticle (and alae) deposition. As the seam cells finally mature at the time of the 4^th larval molt, they become a chain of syncytial cells just beneath the alae of the adult cuticle, although in these prints it is difficult to see the cell borders well enough to score the presence or absence of syncytia. Note that in some animals, marked “41 hrs” and “44 hrs”, one can still see the L4 cuticle in place overlying the new adult cuticle with alae – demonstrating that both represent late L4 stage animals, near the time of syncytial formation.

The prints are often annotated with blue tracings of the seam cell borders – in prints without those annotation, look for the two small adherens junctions that mark where the seam cell borders upon the hypodermis near the cuticle layer. In both animals, the seam seems well aligned with the alae, and some sequential prints alternately display either the left seam cell or the right seam cell within the same thin section. Based upon seeing the pharynx beneath the bodywall in “44 hr” animal, we judge that the cells shown here are the daughters of the original H1 seam cell. By the same reasoning, the seam cells shown in the “41 hr” animal are likely to represent daughters of the original H2 or V1 seam cells.

In some prints internal organelles are circled in blue inside the seam cell and/or the neighboring hypodermis. These features mostly represent Golgi bodies, although there may also be some autophagosomes or late stage endosomes present. These organelles become common in some steps in seam cell maturation, as well as in hypodermis (see Singh and Sulston, 1978; Melendez et al., 2003) to permit production of the alae and reorganization of the size, shape and contents of the seam cells during maturation. In a few prints the border of the nucleus of the seam cell is marked in red (prints 8-12 of “41 hr” animal). Many more nuclei can be seen in the syncytial hypodermis in these prints.