Engagement of the periesophageal ring during Holothuria polii response to erythrocyte injection.

In Holothuria polii, the periesophageal ring is an important organ supplying spherule cells after stimulation with foreign material. In animals injected with formalinized sheep erythrocytes, in fact, a depletion of spherule cells is observed in the periesophageal ring, whereas in the connective tissue, in the external epithelium and around the antigen- injected site, small, transparent cells can be visualized. It is supposed that the latter are stem cells of spherule cells.