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

Abstract

AbstractFine 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.