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Ultrastructural study of the proboscis endothelium of Riseriellus occultus (Nemertea, Heteronemertea)

Abstract:

We have examined with transmission electron microscopy the epithelial layer exposed to the rhynchocoel fluid of the proboscis in the heteronemertine Riseriellus occultus. This epithelium is organized as a monociliated, pseudostratified myoepithelium consisting of two cell types: apically situated monociliated supportive cells and subapical myocytes lacking cilia. The low supportive cells form a continuous adluminal sheet and reach with numerous cytoplasmic processes into the extracellular matrix; these cells are characterized by numerous, irregularly shaped, apical folds projecting into the rhynchocoel fluid, delimiting broad extracellular spaces. The authors suppose that both apical and basal folds could accommodate stretching of the endothelium when the proboscis is everted. The apical folds of the supportive cells increase the interface of these with the rhynchocoel fluid; this feature, together with the presence of pinocytotic vesicles in such cells, suggest that they could be involved in the exchange of substances between the rhynchocoel fluid and the proboscis. The myocytes are scattered singly within the monociliated pseudostratified myoepithelium. They are situated between the supportive cells and the subjacent extracellular matrix. Basement membrane separating both cells types is lacking. Myofibrillar parts protrude basally from the myocyte somata. The myofibrillar parts lie in direct apposition to the extracellular matrix, and are oriented circular to the longitudinal axis of the proboscis. We consider the myocytes to be intra-epithelial, myoepithelial cells.