The masking effect of sialic acid on Con A, PNA and SBA ectoderm binding sites during neurulation in the bantam chick embryo.

The masking effect of sialic acid on cell surface carbohydrates localized on the ectoderm in stage 6-11 bantam embryos was examined using fluorescein isothiocyanate-labeled Con A, PNA, SBA, LFA, and LPA before and after neuraminidase treatment. The results showed selective lectin binding on both the neuroectoderm and the surface ectoderm. In general, these lectin-binding sites increased or were at least expressed on neuroectoderm during neurulation. On the apical surfaces of the developing neuroectoderm, masked Con A-binding sites were evident from the earliest stage and rapidly increased. These sites coexisted with unmasked binding sites which gradually increased. Masked PNA sites were rarely observed but became abundant in later stages, even though coexistent unmasked sites also rapidly increased. Masked SBA sites were poorly observable in the early stage and gradually increased thereafter, whereas unmasked sites were expressed at later stages. On the basal surfaces masked Con A sites were evident in the early stages but gradually decreased in later stages, whereas unmasked sites were relatively abundant and increased thereafter. Masked PNA sites were evident and increased very rapidly, whereas unmasked sites became observable up to the latest stage. Masked SBA sites were minimal in all three stages, and unmasked sites expressed themselves slightly at later stages. The change in composition of carbohydrates on the developing neuroectoderm was obviously different from that on the developing surface ectoderm. On the contact surface of the neural ridge, the number of masked sites of penultimate sugars was large at Con A sites, slight at PNA and SBA sites, which coexisted with unmasked sugar chain terminals in the areas where Con A sites were moderate and where PNA and SBA sites were poor. Finally, the role of masking on binding sites for Con A, PNA and SBA during neural tube closure is discussed, and the observation that the apparent masking effect on three lectin binding sites did not correspond to the content of sialic acid detected by LFA and LPA is a subject for further study.