Spermatogenesis and the fine structure of the mature spermatozoon in the free proglottis of Trilocularia acanthiaevulgaris (Cestoda, Tetraphyllidea).

Spermatogenesis and the structure of the mature spermatozoon of Trilocularia acanthiaevulgaris have been studied by light microscopic histochemistry and transmission and scanning electron microscopy. The primary spermatogonia occupy the periphery of the testis and undergo four mitotic divisions; incomplete cytokinesis results in two secondary, four tertiary and eight quaternary spermatogonia and sixteen primary spermatocytes. The quaternary spermatogonia and successive stages develop in a plasmodial rosette with their nuclei at the periphery and connected to the cytophore by cytoplasmic bridges. The primary spermatocytes undergo two meiotic divisions, resulting in 64 spermatids that develop into spermatozoa. Intranuclear synaptonemal complexes in primary spermatocytes confirm the first meiotic division. The onset of spermiogenesis is marked by an elongation of the spermatid nucleus and the appearance of "arching membrane-bound clefts" (after Rosario 1964) that delimit the differentiation zone. The latter contains a pair of axonemes with their associated basal bodies and rootlets and, as it extends, the nucleus migrates into it. At first, the axonemes grow at right angles to the extension, but during development they rotate through 90 degrees and come to fuse with the cytoplasmic extension. The mature spermatozoon has a broad head region, a middle piece and a narrow tail region. It consists of a highly coiled nucleus, paired axonemes of the 9 + 1 pattern typical of platyhelminths, one of which extends the full length of the tail and the other ends part-way along the tail, particles of beta-glycogen and peripheral microtubules. A crested body spirals around the outside of the sperm body in the tail region. T. acanthiaevulgaris is placed in context within the phylogenetic schemes for cestodes based on sperm ultrastructure.