Structure and possible functions of the calcospherite-rich cells (R* cells) in the digestive gland of the shore crab Carcinus maenas.

R*-cells of the digestive gland of Carcinus maenas have been investigated functionally and morphologically. A comparison of the capacity of separated cell suspensions to synthesize glycogen gave support to the hypothesis that R and R* cells belong to the same cell line. The unexpected observation of R* cells in gastric juice suggests that their release could represent a mode of redistribution of carbohydrate stores when the feeding activity of the crab is lower. Under electron microscopy, the calcospherites of R* cells appeared to be surrounded by multiple membranous layers, and displayed tubular and vesicular structures in their core. High glucose-6-phosphatase (G6Pase) activity in the subcellular fraction that is enriched in calcospherites suggests that these membranes are derived from the endoplasmic reticulum, via a process in which the enzyme plays a key role. We propose that this is the way by which the R cell differentiates into R* cell.