Objectives: At a late stage of amelogenesis a basement membrane-like structure appears between ameloblasts at the maturation stage and the surface of enamel. Although it was known to contain certain basement membrane components, its detailed nature as well as its role had not been well defined. Methods: This structure was examined by high-resolution electron microscopy combined with immunohistochemical staining. Mandibular incisors of male Sprague Dawley rats (75-100 g) were processed for the preparation of conventional Epon thin sections for ultrastructural observations, and frozen thick sections were used for immunoperoxidase and immunofluorescence staining for laminin, heparan sulfate proteoglycan (HSPG) and type IV collagen. Results: This basal lamina-like structure was characterized by the presence of unusually abundant 4.5-5 nm wide ribbon-like “double tracks”, the form known to be taken by HSPG in basal laminae. In addition, the double tracks also appeared to extend from the basal lamina into the enamel. The main ultrastructural component of basal laminae, known as “cords” was replaced by a network of fine filaments; these have been previously identified as cord components and consist of type IV collagen. Immunohistochemical staining of the membrane showed an intense reaction for HSPG, a moderate stain for type IV collagen, and negligible staining for laminin. Conclusion: These observations indicate that this structure is an atypical basement membrane in which the cord network is replaced by a network of type IV collagen filaments, a change attributed to a deficiency of laminin. However, the membrane was found to be unusually rich in HSPG. A possible role for this basement membrane is to mediate firm attachment of maturation ameloblasts to the surface of enamel.

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