Formation of the spermatozoon during and after spermiogenesis of Iguana iguana Linnaeus, 1758 (Reptilia, Iguanidae), with special attention to the middle piece and axonemic complex

Abstract

Spermiogenesis involves a series of cellular modifications, culminating in the production of a highly specialized cell, the spermatozoon. To understand the peculiarities of this process, specimens of the lizard Iguana iguana were collected in the Pantanal-Brazil; the testis and ducts processed according to routine methods for Light, Scanning and Transmission Electron Microscopy. After nuclear condensation and elongation, the cytoplasm and organelles flow towards the basal area, below the nucleus. An infolding occurs at the nuclear base, where the middle piece is implanted. This area contains a centriole pair, with the proximal one perpendicular and the distal one parallel to the nuclear axis. The distal centriole can be functionally compared to the basal corpuscle at the axoneme base. The mitochondria, that are round or elongated with transverse cristae, are initially organized around the centriole in continuous rings; later they alternate with dense bodies. Thus the dense bodies are apparently formed by mitochondrial modification with deposition of dense material. In I. iguana the formation of dense bodies is one of the last events of spermiogenesis. Marking the transition between middle piece and flagellum is the annulus. This structure blocks the displacement of mitochondria from the middle piece during flagellar movement. Halfway down the middle piece, a fibrous sheath is formed by the accumulation of an amorphous layer surrounding the axoneme, and this structure is now called the axonemic complex. In the spermatozoa of I. iguana, the flagellar end piece is reduced in diameter, since the fibrous sheath terminates and only the typical axonemal microtubule pattern (9+2) is present. A large amount of cytoplasm is observed around spermatozoa in the seminiferous tubule lumen, suggesting that these are "immature" spermatozoa. Structural modifications occur during their passage through the reproductive tract. These post-testis modifications have not been precisely located along the reproductive tract and are probably not essential for spermatozoon mobility. Some features of the male reproductive tract are shown.