This paper discusses a new hypermedia-based model known as IUHM. IUHM emerged as a result of the development of the OPALES system, a collaborative environment for exploring and indexing video archives in a digital library. A basic design requirement of OPALES is that it must permit and support the integration of new services throughout its life cycle. Thus, IUHM depends heavily upon the notions of extensibility and openness. Support for openness, extensibility and late binding of services is provided in the IUHM model by a single reflexive mechanism. This uniform mechanism is used for describing all relationships between arbitrary system entities, including services, data and metadata. The mechanism in question consists of a generic, computable hypertext structure with typed links, known as the Information Unit, and is the minimal structural scheme to which all encapsulated entities comply. We describe and justify the design of the Information Unit, as well as the semantics of its four link types, namely role, type, owner, relative. We further describe the minimal kernel of the runtime layer responsible for the dynamic behaviour specified by the IUHM compliant hypertext network. We discuss the mechanisms involved in the dynamic binding of services and service composition. We illustrate these notions by real-world examples of the integration of metadata services within the OPALES system. Categories and Subject Descriptors H.3.7 [Information Storage And Retrieval]: Digital libraries - systems issues, user issues. I.7.2 [Computing Methodologies]: Document preparation - hypertext/hypermedia, languages and systems. General Terms Management, Design, Reliability, Human Factors, Languages