The histochemistry and ultrastructure of the clitellum of the enchytraeid Lumbricittus rivalis (Oligochaeta: Annelida)

The monostratified clitellar epidermis of Lumbricillus rivalis consists of supporting cells, granular secretory cells, and globular secretory cells, together with the acid mucous cells typical of normal skin in the anterior and posterior transition zones. The secretion of the granular cells is a neutral glycoprotein with low levels of bound lipid, and that of the globular cells is a sulphated acid mucopolysaccharide lacking detectable protein or lipid. Ultrastructurally the granular cells possess 1 μm, membrane-bound granules of variable electron density suggestive of maturation changes. The formative granules arise from Golgi vesicles and are moderately electron translucent. They contain parallel-aligned, tubule-like inclusions of 14 nm diameter, observed also in the mature regions of the Golgi. Granules in the mid or apical part of the cell show increased electron density, the formative pattern being wholly or partially obscured and the subunit alignment, when discernable, no longer uniform. Moire fringe patterns are evident in some granules. The patterns and electron density are lost after pronase digestion. The globular cells contain electron translucent, membrane-bound globules of 2–3 μm diameter with finely fibrous contents. They arise from mature Golgi vesicles and are unaffected by pronase treatment. The results suggest that the granular cells secrete the cocoon wall and the globular cells the material that surrounds the developing embryos, and are compared with published accounts of other microdrile clitella and with those of the clitellum of lumbricid earthworms.