| Abstrakt: |
Superficial cells from early amphibian embryos display regional specializations of their cell surfaces. That portion of the plasma membrane facing the perivitelline space (apical surface) is nonadhesive, whereas, in the same cell, the lateral and basal portions of the plasma membrane will adhere to other cells. These adhesive differences are maintained on single cells that have been dissociated from the intact embryo. Extracts of cleavage-stage Rana pipiens embryos are capable of agglutinating formalinized sheep erythrocytes. The hemagglutination activity can be blocked by a yeast mannan and a family of glycoproteins containing high levels of mannose, indicating the presence of a lectin with oligomannosyl specificity. The cell surface location of this carbohydrate-binding component can be demonstrated by the ability of the formalinized sheep erythrocytes to form rosettes with living dissociated embryonic superficial cells. Rosette formation is blocked by the same inhibitors that are effective in blocking the activity of the crude extracts. The formalinized sheep erythrocytes form rosettes only to those cell surface regions of the superficial cells that are capable of adhering to other amphibian embryo cells. Receptors for concanavalin A, a lectin that binds D-mannose and D-glucose residues, have also been shown to be present exclusively over the adhesive regions of the superficial cells. The involvement of a carbohydrate-binding component with oligomannosyl specificity in the adhesive mechanisms of these cells is suggested by this restriction of both the embryonic amphibian lectin and its possible receptors (concanavalin A receptors) to adhesive regions of the cell surface. |
