An ultrastructural investigation of the testes and spermiogenesis in Myobia murismusculi (Schrank) (Acari, Actinedida: Myobiidae)

Summary

The stages of spermiogenesis in Myobia murismusculi were investigated on the basis of ultrastructural analysis of both the testes and the female organs: receptaculum seminis and seminal duct. The walls of the testes consist of a thin epithelial layer. Germ and secretory cells lie free in the lumen of the testes. In the early stages of differentiation, both cell types represent clusters of sister cells joined by intercellular bridges. Each secretory cell contains prominent RER and Golgi complex, which produce single dense granule. Growing gradually the granule fills the whole volume of the cell's cytoplasm. Mature secretory cells disintegrate and the secretory product discharges into the testicular lumen. The germ cells are represented by the early, the intermediate and the late spermatids as well as the immature sperm (prospermia). Neither spermatogonia nor meiotic figures were observed in adult males. As spermiogenesis starts, numerous narrow invaginations of the outer membrane (peripheral channels) develop on the cell surface. They form a wide circumferential network connected to pinocytotic vesicles. Owing to the secretory activity of the Golgi complex, a large acrosomal granule is formed in the early spermatids. A long acrosomal filament runs along the intranuclear canal. Nuclear material condenses and forms two spherical bodies of different electron density. The lighter one can be observed until the stage of the late spermatids, when the nuclear envelope almost completely disappears. The electron-dense nuclear body transforms into a definite chromatin body, which is observed in the mature sperm as a cup-shaped structure. The late spermatids are characterized by the presence of a large electronlucent vacuole, which seems to be unique for the process of spermiogenesis in Actinedida. After the spermia enter the female genital tract, the peripheral channels disappear as well as the vacuole. The cells form long amoeboid arms with a special microtubular layer underneath the plasma membrane. The chromatin body is encircled by a large acrosomal granule of complex shape provided by long extensions running deep into the cytoplasm. The cytoplasm contains no organelles except for a group of unmodified mitochondria in the post-nuclear region. The main characteristics of the Myobia spermiogenesis are discussed with regard to other actinedid mites.

Key words:

• Testes
• spermiogenesis
• ultrastructure
• Myobiidae