Sensory Cells Associated with the Tentacular Tunic of the Ascidian Polyandrocarpa misakiensis (Tunicata: Ascidiacea)

Abstract

New probable mechanosensory cell groups were found in Polyandrocarpa misakiensis. In this species, the tunic with epithelium penetrates into the oral and atrial tentacles (oral and atrial tentacular tunic), which is continuous with a tunic layer intervening between the descending and ascending epithelium of the siphons. In the oral tentacles, the tunic only extends into the basal part, but in the atrial siphon the tunic extends the full length of the tentacle. Intraepithelial sensory cells were found in the basal part of the oral and atrial tentacular tunic. These sensory cells are usually solitary or paired in the atrial tentacles, and a few cells are grouped together in the oral tentacles. In the oral tentacles, the apicolateral parts of the sensory cells are joined together by two types of junctions, i.e., adherent junctions and modified tight junctions. The supporting cells and sensory cells are connected by adherent junctions. Certain sensory cells are coupled to what seem to be neurosecretory cells. The sensory cells have one apical cilium surrounded by microvilli and a basal axonal process. The apical cytoplasm of the sensory cells has a few organelles. The basal cytoplasm of the sensory cells is dense with rough endoplasmic reticulum, mitochondria, lipid droplets, and dense bodies. The morphology of these sensory cells is comparable with that of the ciliated intraepithelial sensory neurons found in many tunicates, such as those of cupular organ and capusular organ.