The ultrastructure of the sperm funnel and non‐glandular vas deferens of Tubifex tubifex (Annelida: Oligochaeta)

Abstract

The sperm funnel, which consists of a ciliated epithelium bounded by a flattened peritoneum, is characterized by a “halo” of vertically orientated spermatozoa. The spermatozoa nuclei terminate within the ciliary zone of the funnel but junctional attachments between sperm and cilia have not been observed. The epithelial cells synthesize an electron dense secretion at centrally placed Golgi cisternae. The secretion tends to accumulate within the distal cytoplasm and appears to be released at the cell apex. It is hypothesized that the secretion, which is exclusive to this region of the male duct, may function as an attractant to newly differentiated spermatozoa. The active exchange of material between the funnel epithelium and the surrounding coelomic fluid is further indicated by the accumulation of mitochondria within the distal cytoplasm and the presence of microvilli and pinocytotic profiles, in addition to cilia, along the apical plasma membrane.The non‐glandular vas deferens, consisting of thin and thick walled regions, is highly coiled and is constructed of ring‐shaped epithelial cells invested in longitudinal muscle and peritoneum. The thin walled duct contains spirally placed cilia which possess multiple rootlets orientated at an acute angle to the lumen surface. Aggregates of 5 nm micro‐filaments encircle the cell and are inserted onto infoldings of the basal (outer) plasma membrane enclosing lateral extensions of the basal lamina. Microfilaments of comparable size and orientation also occur in the distal (luminal) epithelial cytoplasm of the thick walled duct and are inserted onto neighbouring regions of the lateral plasma membrane. The vas deferens microfilaments are interpreted as contractile units assisting in the passage of spermatozoa along the duct at the time of copulation.In the thick walled duct, cell apices protrude into the lumen resulting in a crenellated profile in longitudinal section. Lateral plasma membranes contain extensive radial folds or “pleats”. The lumen surface is unciliated but is characterized by laminate cytoplasmic projections which are loosely organized into groups displaying uniform orientation. The ultrastructural features of the thick walled vas deferens indicate a role in fluid transportation which is discussed with reference to sperm transmission.