Studies on colony specificity in the compound ascidian, Botryllus primigenus Oka. I. Initiation of "nonfusion" reaction with special reference to blood cells infiltration.

When two incompatible colonies of the compound ascidian, Botryllus primigenus Oka come into contact, a specific rejection, called "nonfusion" reaction (NFR), always occurs in the contact area between them. At the beginning of the NFR, concentrations of cells were always observed near the ampullae tips. In the present study, it was revealed that these cells appear to be blood cells infiltrating through the ampullae and were identified as morula cells (vanadocytes). When India ink was injected into vessels prior to the occurrence of NFR, it leaked from the tips of the ampullae into the test matrix, suggesting an increase in the permeability of the ampullar epidermis. Experimental studies to elucidate the mechanism of NFR were carried out. Contact between a colony piece and a zooid-free ampullae fragment or overlapping of one colony to another, respectively, resulted in NFR only at one side of a pair. Furthermore, in stage 3 of NFR, when an opaque area was pushed with a pair of forceps and the blood in this area was transferred to healthy part of vessels, the stoppage of blood flow and subsequent contraction of vessels always took place in the latter. These observations suggest that ampullae penetration into the opposite colony is essential to NFR and that NFR is directly initiated in the ampullar lumen.