Ultrastructural analysis of spermiogenesis in Admetus pomilio (Arachnida, amblypygi)

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Abstract
The mature spermatozoon of Admetus pomilio is a spherical cell containing nucleus and tightly coiled flagellum. In early spermatids the Golgi apparatus forms the acrosomal vesicle and at the opposite side the distal centriole gives rise to the axonemal complex of the sperm tail. As the nucleus elongates, chromatin forms twisted filaments and the spermatid nucleus takes on a helical form. Microtubules are juxtaposed with the nucleus envelope, which is separated from a central chromatin mass by an electron lucid region. A long perforatorium, located on the border of the chromatin mass, runs helically in the nucleus from the centriolar region to subacrosomal space. During tail elongation, the anterior part of the axoneme is surrounded by a long, spiral mitochondrial sheath. In the late spermatid, chromatin filaments appear twisted and become aggregated. The nucleus and flagellum undergo further contortions in which the nucleus coils and the flagellum winds up into the body of the cell and coils in a regular fashion. The mitochondrial sheath surrounds about 2/3 of the 9 + 3 axoneme. These features of spermatid ultrastructure resemble those in the primitive Liphistiomorpha.