Fine Structural Observations on the Extracellular Matrix (ECM) of Xenoturbella bocki Westblad, 1949

Abstract

Fine structural observations on the extracellular matrix (ECM) and connective tissue system of the enigmatic vermiform animal Xenoturbella bocki have demonstrated a complex and interesting organization of the ECM. Most conspicuous is the subepidermal membrane complex (SMC), and the major part of the ECM is present in this structure, which consists of a limiting basal lamina on each side of a central thick filamentous layer, probably of a collagenous nature. Distinct anchoring filaments are found as part of the SMC. A basal lamina is also observed in connection with the gastrodermal cells. The mesoderm is represented by a loose, ill‐defined parenchyma, without filaments. The SMC is discussed in relation to subepidermal basal matrices occurring in turbellarians and enteropneusts, the two groups to which a relationship to Xenoturbella previously has been suggested. Comparisons between the ECM in Xenoturbella and in turbellarians do not support the notion of a relationship between these groups. Conversely, the present study strengthens previous indications of distinct similarities between certain characteristics of the epidermis and the SMC present in both enteropneusts and in Xenoturbella.

Xenacoelomorpha: a case of independent nervous system centralization?

“…Neither brain or brain-like structures as nerve rings are present; nor are neurite bundles or similar arrangements [18,31,34]. The presence of nerve processes is completely restricted to the epidermis and the subepidermal membrane complex (SMC) [18], with a lack of nerve structures in the subepidermal muscle layer and in the parenchyma [18,34]. The absence of neural structures below this layer and the lack of obvious direct musculature innervation led some authors to suggest that neuronal substances were transmitting individually from the basiepidermal nerve net to the muscle fibres, or that these animals relied on the use of muscular pacemakers [34].…”

“…The statocyst consists of a vesicle formed by an outer cellular capsule and an internal layer of parietal cells. It is located in the anterior part of the body and inside the SMC [18,35] fenced by some neural projections that reach its periphery [31,35] (H. Nakano 2015, personal communication). Due to the composition and the structure of this organ, it has been speculated that it cannot work as a 'true' georeceptor; hence its function still remains unclear [35].…”

Two Types of Endosymbiotic Bacteria in the Enigmatic Marine Worm Xenoturbella bocki

“…In Xenoturbella, which is hermaphroditic, eggs and spermatids develop within the parenchymal cell layer (Fig. 1A) and are emptied into the gut via the gastrodermis (27). Therefore, spermatid clusters likely contain gastrodermal cells that harbor endosymbionts.…”

Marine ecological genomics: when genomics meets marine ecology

“…Owing to a simple body plan, their phylogenetic position has long remained puzzling. Based on morphology, they have been suggested to be a primitive flatworm (Westblad 1949), unique representatives of a plesiomorphic metazoan group (Jagersten 1959), an enteropneust, holothurian, or unique representatives of a deuterostome group (Reisinger 1960), a hemichordate (Pedersen & Pedersen 1986), an acoel flatworm (Franzen & Afzelius 1987, Lundin 1998, 2001, a primitive metazoan (Ehlers & Sopott-Ehlers 1997, Raikova et al 2000, a bivalve (Israelsson 1997(Israelsson , 1999, or a bryozoan (Zrzavy 1998). This example shows that, even when using rather powerful techniques such as scanning or transmission electron microscopy, morphology alone cannot resolve the phylogenetic position of an animal.…”