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Ultrastructural study of the bacillary, granular and mucoid proboscidial gland cells of Riseriellus occultus (Nemertini, Heteronemertini)

Abstract:

The ultrastructure of six types (G<inf>5</inf>-G<inf>10</inf>) of proboscidial gland cells whose cell necks emerge independently on the epithelium surface is analysed and compared with data from other nemerteans. These types differ in cytological features, as well as in the morphology of their respective secretory granules. Secretory granules of the types G<inf>5</inf> and G<inf>6</inf> have a bacillary shape, and differ from each other based on their contents and dimensions. Secretory granules of the types G<inf>7</inf> and G<inf>8</inf> are spherical to ovoid; type G<inf>8</inf> gland cells are monociliated, and their secretory granules contain a paracrystalline material. Types G<inf>9</inf> and G<inf>10</inf> gland cells are typically goblet-shaped; secretory granules in the type G<inf>9</inf> have a spherical shape, contain a homogeneous electron dense material and maintain their individuality, whereas those of the G<inf>10</inf> type are elongate and have fibrillar contents, showing a tendency to fuse before they are extruded. The mucus sheet of the proboscis is responsible for lubrication of its epithelial surface. Secretion products of type G<inf>10</inf> gland cells form the background substance of this mucus, and those of the G<inf>5</inf> type confer stickiness to it. Type G<inf>9</inf> gland cells could provide the toxic component to the mucus, and type G<inf>7</inf> and G<inf>8</inf> gland cells could be concerned with the production of enzymatic secretions.