Ultrastructure of lineus ruber (rhyncocoela) epidermis

Recent evidence indicates that nemertean epidermis is capable of absorbing certain organic solutes from sea water via mediated transport mechanisms, as well as secreting mucoid substances. Morphological studies suggest that these functions may be restricted to distinct epidermal cell populations.Mucous secretion at the free surface of the epidermis is the result of synthesis and release activities of cells in both the epidermis and dermis (cutis). Secretion of dermal origin passes through the epidermis to the worm's exterior in slender cytoplasmic processes (canaux d'evacuation) in the form of membrane bound vesicles. A single gland cell type, located entirely within the epidermis, releases externally a granular product histochemically identified as largely protein plus some amount of carbohydrate with low periodic acid-Schiff's reactivity. The close juxtaposition of granular endoplasmic reticulum and Golgi apparati to the secretory material is consistent with the composition of this secretory product.Interstitial cells possess microvilli projecting from their apical surface, in addition to cilia. The outer surface of the plasmalemma covering these ciliary projections is unadorned, but microvilli possess a fuzzy coat. At the peripheral ends of the microvilli, the coat is filamentous, while at their base the coat consists of foliate structures. Cationic colloidal iron binding suggest that the filamentous portion of the fuzzy coat contains the greatest proportion of the acidic surface charge. The presence of periodic acid-Schiff's positive material in this region suggests that the fuzzy coat also contains carbohydrate. Lateral boundaries of the interstitial cell lacks obvious junctional specializations; however, the apical 150 nm intracellular space narrows to 40 nm and continues in a tortuous interdigitating path to the base of the adjacent interstitial cells. Where the apex of these cells is broad, the interdigitations are shallow, but the basal half of the interstitial cells have deep complex infoldings. Cytoplasmic organelles other than the nucleus, mitochondria and some granular endoplasmic reticulum, are restricted to the apical half of the cytoplasm. The presence of closely apposed Golgi complexes and smooth endoplasmic reticulum, multivesicular bodies, lysosome-like dense vesicles and coated vesicles suggests that these cells may play a role in intracellular digestion of phagocytized paniculate matter from the external environment. The amplification of the interstitial cell's free surface suggests that these cells are primarily responsible for mediated solute transport across the epidermis.