Ultrastructure of the differentiating male germ cells in Haliotis asinina Linnaeus

Abstract

Male germ cells in the testis of H. asinina can be divided into 14 stages based on the ultrastructure and patterns of chromatin condensation. The spermatogonium is a spherical or oval-shaped cell with diameter about 8 μm. Its nucleus contains mostly euchromatin with only a thin rim of heterochromatin along the inner facet of the nuclear envelope. Primary spermatocytes (PrSc) are divided into six stages, i.e., leptotene (LSc), zygotene (ZSc), pachytene (PSc), diplotene (DSc), diakinesis (DiSc) and metaphase (MSc). The early cells are round and become increasingly larger, ranging in size from 12 to 14 μm from LSc to PSc; then their sizes gradually decrease from 10 to 7 μm from DSc to MSc. LSc contains small blocks of heterochromatin that are scattered throughout the nucleus. These heterochromatin blocks are increasingly thickened and lengthened in ZSc, and achieve their maximum sizes in PSc. DSc decreases in size, resulting in the close clumping of chromatin blocks; while in DiSc and MSc long and large blocks of chromosomes are formed and then move to be aligned along the equatorial region. In the nuclei of all stages of PrSc, heterochromatin blocks are formed by the tight aggregation of 30 nm chromatin fibers. The secondary spermatocyte (SSc) is a round cell about 8 μm in diameter. They are aligned in rows that separate spermatids from primary spermatocytes. Their nuclei contain criss-crossing chromatin cords in a reticulate pattern, whose individual 30 nm fibers are loosely packed. All spermatids are freed from the epithelium, and can be divided into four stages: St1is a large round cell (about 5-6 μm), and its nucleus contains evenly dispersed 30 nm chromatin fibers. In St2the nucleus decreases in size by a half and becomes oval; thus the chromatin fibers are packed closer together, particularly around the axis of condensation. In St3the nucleus is elongated with individual chromatin fibers enlarged to about 40 nm in cross section, and they are packed tightly together. In St4(about 3×2 μm) 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 spermatozoon has a cone-shaped head (about 3×1.5 μm) that contains completely condensed chromatin covered by the cup-shaped acrosome with the subacrosomal core. Each spermatozoon has globular mitochondria surrounding a pair of centrioles at the neck region and the tail consists of the axonemal microtubules surrounded by the plasma membrane.