Epithelium--The Primary Building Block for Metazoan Complexity

Abstract

In simplest terms, the complexity of the metazoan body arises through various combinations of but two tissue types: epithelium and mesenchyme. Through mutual inductions and interactions, these tissues produce all of the organs of the body. Of the two, epithelium must be considered the default type in the Eumetazoa because it arises first in embryonic development and because mesenchyme arises from it by a switching off of the mechanisms that underly differentiation and maintenance of epithelial cells. In the few model metazoans whose epithelia have been studied by molecular techniques (largely Drosophila, Caenorhabditis, mouse), the molecular mechanisms underlying differentiation of epithelia show remarkable similarity. Extrapolating from these studies and from comparisons of the morphology of epithelia in lower metazoans, I propose how epithelia arose in the stem metazoan. Steps in epithelial differentiation include 1) establishment of cell polarity by molecular markers confined to either apical or basolateral domains in the plasma membrane; 2) aggregation of cells into sheets by localization of cell-adhesion molecules like cadherin to the lateral membrane; 3) formation of a zonula adherens junction from the cadherins by their localization to a discrete belt; 4) cell-to-cell linking of certain transmembrane proteins (primitively in the septate junction) to produce gates that physiologically isolate compartments delimited by the cells; and 5) synthesis of a basal lamina and adaptation of receptors (integrins) to its components. Despite morphological differences in the variety of cell junctions evident in various epithelia, the underlying molecular markers of these junctions are probably universally present in all eumetazoan epithelia.

In vivo imaging of basement membrane movement: ECM patterning shapesHydrapolyps

“…Here, we have investigated the behaviour of the basement membrane system in a simple invertebrate animal, the cnidarian Hydra. The epithelium of Hydra is one of the earliest examples of true epithelial organisation in metazoan evolution (Tyler, 2003). The surprisingly dynamic nature of Hydra ECM observed in our experiments -together with similar observations in the distantly related vertebrate embryo -suggests that ECM in the tissues of all multicellular animals has a more dynamic nature than so far assumed.…”

“…The first ECM is laid down early in development and gives rise to basic epithelial organization (Tyler, 2003). This usually happens before tissue folding and morphogenesis (Ingber, 2006).…”

The contractile sponge epitheliumsensu lato– body contraction of the demospongeTethya wilhelmais mediated by the pinacoderm

“…Ultrastructural research allowed for the definition to be altered in order to meet homology assumptions. Following Rieger (Rieger, 1986), Ax and colleagues (Ax et al, 1989;Ax, 1995) and Tyler (Tyler, 2003), such an extended short definition sensu stricto would be: a polar tissue underlined by basal membrane, with close lateral contacts between cells mediated by belt desmosomes. This fairly recent re-definition resulted in the occupation of the established terminus by new attributes and makes sense only in the context of phylogenetic systematics.…”

“…Semantically, however, such re-definitions of existing terms generally create difficulties, such as in the functional usage of the term epithelium regarding the Porifera. To date, most members of the Porifera lack a basal membrane and the presence of belt-like junctions is controversial (Tyler, 2003;Leys et al, 2009;Ereskovsky et al, 2010); therefore, their characters were addressed as non-epithelial, epitheloid or non-tissue grade of organization, which is semantically imprecise. This specific semantic problem is part of the 'linguistic problem of morphology', which is currently being widely discussed (i.e.…”

¿Es la endosimbiosis seriada una alternativa a la teoría sintética de la evolución? implicaciones epistemológicas y didácticas

“…Las bacterias fueron las únicas pobladoras del planeta durante los siguientes 2000 millones de años, es decir un poco más de la mitad de la historia de la vida en la Tierra (Sampedro, 2007). Luego, hace unos 1800 millones de años, aparecieron los primeros eucariotas, según la descripción de la microflora fósil de Gunflit (Canadá) (Tyler, 2003). Teniendo en cuenta estos hechos, uno de los acontecimientos más relevantes en la evolución biológica sería la aparición de los eucariotas, cuyo origen, se pensaba hasta la década de 1960 había sucedido por un proceso gradualista y neodarwiniano, es decir por la acumulación de pequeñas variaciones en las células procariotas.…”