Ultrastructure of the differentiating male germ cells in Haliotis asinina Linnaeus

Male germ cells in the testis of H. asinina can be divided into 14 stages b ased on the ultrastructure and patterns of chromatin condensation. The sper matogonium is a spherical or oval-shaped cell with diameter about 8 mum. It s nucleus contains mostly euchromatin with only a thin rim of heterochromat in along the inner facet of the nuclear envelope. Primary spermatocytes (Pr Sc) are divided into six stages, i.e., leptotene (LSc), zygotene (ZSc), pac hytene (PSc), diplotene (DSc), diakinesis (DiSc) and metaphase (MSc), The e arly cells are round and become increasingly larger, ranging in size from 1 2 to 14 mum from LSc to PSc; then their sizes gradually decrease from 10 to 7 mum from DSc to MSc, LSc contains small blocks of heterochromatin that a re scattered throughout the nucleus. These heterochromatin blocks are incre asingly thickened and lengthened in ZSc, and achieve their maximum sizes in PSc. DSc decreases in size, resulting in the close clumping of chromatin b locks; while in DiSc and MSc long and large blocks of chromosomes are forme d and then move to be aligned along the equatorial region. In the nuclei of ail stages of PrSc, heterochromatin blocks are formed by the tight aggrega tion of 30 nm chromatin fibers. The secondary spermatocyte (SSc) is a round cell about 8 mum in diameter. They are aligned in rows that separate sperm atids from primary spermatocytes. Their nuclei contain criss-crossing chrom atin cords in a reticulate pattern, whose individual 30 nm fibers are loose ly packed. All spermatids are freed from the epithelium, and can be divided into four stages: St(1) is a large round cell (about 5-6 mum), and its nuc leus contains evenly dispersed 30 nm chromatin fibers. In St(2) the nucleus decreases in size by a half and becomes oval; thus the chromatin fibers ar e packed closer together, particularly around the axis of condensation. In St(3) the nucleus is elongated with individual chromatin fibers enlarged to about 40 nm in cross section, and they are packed tightly together. In St( 4) (about 3x2 mum) the nucleus is increasingly elongated with the acrosome covering the anterior pole. Individual chromatin fibers are enlarged to 60 nm and appear in cross-section as closely aligned dense granules. The sperm atozoon has a cone-shaped head (about 3x1.5 mum) that contains completely c ondensed chromatin covered by the cup-shaped acrosome with the subacrosomal core. Each spermatozoon has globular mito-chondria surrounding a pair of c entrioles at the neck region and the tail consists of the axonemal microtub ules surrounded by the plasma membrane.