The spermatogenesis and spermatozoon of Dicrocoelium dendriticum were studied by transmission electron microscopy. Peripheric accessory cells project between germ cells. Each spermatogonium gives rise to 32 spermatozoa. The stages in spermiogenesis include development of the zone of differentiation, appearance of the intercentriolar body flanked by two centrioles from each of which a free axoneme and a striated rootlet grow, outgrowth of the differentiation zone to form the median cytoplasmic process and migration of the nucleus and mitochondria into it, and rotation of the flagella and subsequent proximodistal fusion of the three projections to form a monopartite spermatozoon. The spermatozoon possess two incorporated axonemes with the "9 + 1" pattern typical of those in trepaxonematid plathelminths. beta-Glycogen particles accumulate in the spermatozoa after they have separated from the cytophore as revealed by Thiery's method. This study confirms in a further family, Dicrocoeliidae, the constant pattern of spermiogenesis and spermatozoon structure in Digenea.
Light and transmission electron microscopy have been used to study the gastrodermal gland cells of the triclad Dugesia gonocephala s.l. The events involved in the ultrastructural transformation and the secretion process in these cells were followed at four different stages in both fasted and fed animals. During the feeding stage their secretory granules are directly discharged into the intestinal lumen by means of a secretion process of the holocrine type that is described in this paper. It is suggested that such secretions contribute to extracellular digestion and that disintegration of the gland cells is accompanied by a differentiation of neoblasts into new gland cells, reflecting a turnover of gland cells during the triclad digestive stages.
Junoy, J., Montalvo, S., Roldán, C. and García‐Corrales, P. 2000. Ultrastructural study of the bacillary, granular and mucoid proboscidial gland cells of Riseriellus occultus (Nemertini, Heteronemertini). — Acta Zoologica (Stockholm) 81: 235–242.
The ultrastructure of six types (G5‐G10) 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 G5 and G6 have a bacillary shape, and differ from each other based on their contents and dimensions. Secretory granules of the types G7 and G8 are spherical to ovoid; type G8 gland cells are monociliated, and their secretory granules contain a paracrystalline material. Types G9 and G10 gland cells are typically goblet‐shaped; secretory granules in the type G9 have a spherical shape, contain a homogeneous electron dense material and maintain their individuality, whereas those of the G10 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 G10 gland cells form the background substance of this mucus, and those of the G5 type confer stickiness to it. Type G9 gland cells could provide the toxic component to the mucus, and type G7 and G8 gland cells could be concerned with the production of enzymatic secretions.
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