Invited Review: Structure and Composition of Basement Membranes and Other Basal Matrix Systems in Selected Invertebrates

Abstract

Abstract Problems concerning terminology and relationships of basement membranes to extracellular matrices (ECMs) and connective tissue systems in invertebrates are discussed, and recommendations regarding nomenclature are presented. It is suggested to use ‘basal matrix’ as a comprehensive term for various basal ECM structures, and apply the term ‘basement membrane’ (BM) for a unique ECM structure that is fully resolved only by electron microscopy and as a general and descriptive word in both light microscopy, biochemistry and electron microscopy. When possible and relevant it is recommended to apply the subdivisions: ‘lamina lucida’, ‘lamina densa’ (= ‘basal lamina’) and ‘lamina fibroreticularis’ (sometimes missing). A brief and non‐technical account of the recent concepts of BM macromolecules and their assembly is given, based on mammalian models. Basement membranes probably first appeared in cnidarians, and from then on became obligatory ECM structures. It is emphasized that this event was crucial for evolution of the multicellular state, and for construction and morphogenesis of tissues and organs. Lower worms, such as turbellarians and nemertines, display the most varied and complex basal matrices. First, in annelids, complete correlation between BM structure and composition has been demonstrated, with isolation and consecutive localization of basement‐specific molecules such as, for example, collagen IV and laminin. Both in arthropods, especially insects, and echinoderms, the introduction of modern techniques corresponding to those used in mammalian BM research has now given equally valid results. An interesting and fundamental outcome of the reviewed work on invertebrate BMs is the realization of the early evolution of BM components. Macromolecules such as laminin, collagen IV and fibronectin are highly conserved, but with fine modulations, and in lower invertebrates they may exist without being assembled into true BM structures. Laminin and fibronectin are expressed with certainty in cnidarians, and fibronectin possibly already in sponges.