Testicular spermatozoa and male germ cells evolution in "Dina lineata" (Hirudinea: Erpobdellidae) studied by scanning electron microscopy

Abstract

This paper studies the male germ cells evolution in the leech Dina lineata by scanning electron microscopy. A primitive spermatogonium is an oval cell measuring 1,6 per 6 µm. Seven not-finished consecutive anticlinal mitotic divisions lead to the formation of isogenic groups with 2, 4, 8, 16, 32, 64 spermatogonia and, finally, 128 primary spermatocytes. The cells are all clustered in only one group and they evolve gradually and synchronically, remaining connected by cytoplasmic bridges to a central cytoplasmic mass called cytophore. A last division, which is meiotic, gives rise to an isogenic group with 512 spermatids: the cellular body of each one is reduced to 1,3 µm in diameter.
By means of a complex spermiogenic process of cellular differentiation, each body becomes a filiform and helicoid spermatozoon measuring 48 µm in length and 0.3 µm in diameter. The morphogenetic and chronologic sequence of the various regions is the following: flagellum elongation; nucleus spiralization and elongation; mitochondrial region elongation; and finally, spiralization and elongation, first of the posterior acrosome and after of the anterior acrosome.
In the spermatozoon:
1) The posterior acrosome (7 µm) is a right-handed double helix integrated by two fibers: a narrow one and a wide one; each describes seven complete turns.
2) The anterior acrosome (6.5 µm) is a longitudinal axis which has a ribbonshaped expansion describing seven right-handed complete turns around the same axis.
3) The nuclear region (5.5 µm) is also a right-handed double helix integrated by two fibers. On their base they show the same trickness, but this is alternatively altered along it from the base to the other side.
4) The mitochondrial region (8.5 µm) and the flagellar one (20 µm) are externally smooth.