Glycogen deposits in the pyloric gland of the ascidian Styela clava (Urochordata).

The pyloric gland of Styela clava contains large glycogen deposits that are digested by treatment with alpha amylase and depleted by 15 days starvation. The deposits are surrounded by cytoplasmic regions containing smooth endoplasmic reticulum and mitochondria. The cells also have rough endoplasmic reticulum, Golgi cisterns, lysosomes, microvilli, cilia, and lateral infoldings of the plasma membrane. The fine structure of the pyloric cells and the position of tubules between the absorptive epithelium and general circulation suggest that the gland functions as the vertebrate liver in carbohydrate metabolism. The pyloric cells of Styela do not appear to be excretory in a "renal" sense, since there is no infolding of the basal plasmalemma and mitochondria are usually associated only with the glycogen deposits. However, a hepatic-like excretory role is consistent with current findings. In light of the phylogenic affinities of vertebrates and ascidians, it is possible that the pyloric gland is homologous to the liver.