Immunolocalization of basement membrane components and beta 1 integrin in the chick wing bud identifies specialized properties of the apical ectodermal ridge

To examine whether the extracellular matrix (ECM) plays a role in mediating interactions between the apical ectodermal ridge (AER) and the subjacent mesoderm in the chick wing bud, we used immunohistochemistry to locate the following tissue components during wing morphogenesis: types I and IV collagens, fibronectin, the basal lamina form of heparan sulphate proteoglycan (HSPG), laminin, and the beta 1 integrin subunit. The notch region at the base of the AER exhibits particularly strong labelling for type IV collagen, fibronectin, laminin, and beta 1 integrin. This suggests that the ridge cells are firmly anchored to their underlying basement membrane. In nonridge ectoderm, the beta 1 integrin subunit is present only at the basal cell surface, whereas in the AER it has a pericellular distribution. The localization of beta 1 integrin receptors at the lateral ridge cell surfaces, in the apparent absence of fibronectin, collagens I and IV, and laminin, suggests that they may function in cell-cell adhesion in the AER. The normal AER-mesenchyme interface was compared to an experimental situation in which the AER flattens. This was induced in the anterior region of the wing bud by inserting an impermeable barrier at intersomite level 17/18, at stage 21. At 12 hr (stage 23) and 24 hr (stage 25) after the operation, each of the ECM components listed above is uniformly distributed along the experimental epithelial-mesenchymal interface. By 24 hr postoperation, the beta 1 integrin subunit is restricted to the basal surface of the flattened apical ectoderm. Similar changes occur in the AER as it flattens during later stages of normal development. These results point to a possible role for the ECM and integrin receptors in maintaining the thickened structure of the AER.