Ultrastructural and cytochemical study of the kidney and nephridial gland cells of the marine prosobranch mollusc Nucella lapillus (L.) in relation to function

There are three epithelial cell types over the dorsal wall folds of kidney of Nucella lapillus (L.). The most numerous, excretory cells, are unciliate d columnar cells, characterised by the presence of a variety of large membr ane-bound cytoplasmic vacuoles, some containing granular material that give s a positive reaction for periodate-reactive carbohydrates.'The second type , resorptive cells, are usually cone-shaped ciliated cells, with a well-dev eloped endocytotic canal system and numerous vesicles in their apical cytop lasm. The apical cytoplasm contains large clusters of periodate-reactive gr anules, possibly glycogen and occasional large vacuoles containing some gra nular material. The third type, "small mucous cells," are carbohydrate-cont aining cells with many small mucous granules, which react positively for su lphated and carboxylated carbohydrates showing a reticulate positive reacti on. In the "secondary" folds, which are interspersed with the primary ones, the epithelium is cuboidal and in some places almost squamous, and most of the cells display some cilia, well-recognizable microvilli, many pinocytotic v esicles, phagosomes and elements of the canal system on their apices. Many mitochondria and deposits of glycogen are also observed. The nephridial gland epithelium is composed of ciliated resorptive cells an d "small mucous cells," very similar to those in the dorsal wall folds. The significance of the intracellular presence of carbohydrates in relation to the ability of the tissues to absorb and store sugars is discussed.