DEVELOPMENT OF THE CHICK THYMUS MICROENVIRONMENT - A STUDY BY LECTIN HISTOCHEMISTRY

The microenvironment of the chick thymus has been examined during deve lopment using lectin histochemistry. We have assayed WGA, Con A, RCA-I and TPA on thymic sections from 13, 15, 17 and 19 d chick embryos and 0, 5, 10 and 15 d chicks. All lectins were immunoperoxidase and collo idal gold-conjugated for transmission electron microscope observations . WGA labelled both the cortical and medullary thymic stroma at all th e stages analysed. An intense reaction to WGA was observed in the subc ortical region from stage 18 embryos to 5 d chicks. On the other hand, WGA did not stain medullary areas of the chick thymus. Con A lectin d etected several cell clusters of stromal cells and thymocytes in corti cal regions. These clusters could represent a lymphostromal complex wi th which Con A receptors are associated, probably in relation to cell adhesion. The residues detected by RCA were distributed both in stroma l cells and thymocytes of the developing chick thymus. There was an in crease of the reaction to RCA between the 19 d embryos and the 5 d chi cks. This increase might be interpreted in terms of the secretion of t hymic humoral factors at these stages. The thymic stromal cells staine d with immunoperoxidase conjugated-TPA showed a reticular pattern in t he medulla. There is a possibility that the fucosyl residues may be ex pressed in the Ia antigen as has previously been suggested in other sp ecies.