ScienceAsia 29 (2003): 241-254 |doi: 10.2306/scienceasia1513-1874.2003.29.241

Classification of Spermatogenic Cells in Rana tigerina Based on Ultrastructure

Sirikul Manochantr, Prapee Sretarugsa, Chaitip Wanichanon, Jittipan Chavadej and Prasert Sobhon^*

ABSTRACT: Germ cells in the spermatogenetic process of Rana tigerina can be classified into 14 steps based on the pattern and degree of chromatin condensation as studied by transmission electron microscopy. Primary spermatogonia contain a large spherical nuclei with mostly euchromatin that consists of 2 levels of fibers with 10 and 30 nm thicknesses. Secondary spermatogonia have small heterochromatin blocks consisting of tight aggregations of 30 nm fibers on the inner surface of the nuclear envelope, as well as in the central area of the nucleus. Primary spermatocytes are divided into 6 steps. A leptotene spermatocytes contain small loosely packed chromatin clumps that are resulted from the winding of 30 nm fibers around the condensation axes, each of which is a single electron dense line. Zygotene spermatocytes contain long and increasingly thickened heterochromatin blocks, which are joined together by synaptonemal complexes. Pachytene spermatocytes show long, thick, intertwined heterochromatin blocks cords, which are parts of the complete chromosomes. These chromatin blocks become distributed in a cartwheel pattern in diplotene spermatocytes. Long and large heterochromatin blocks are separated from each other during the diakinesis step and become aligned along the equatorial region in the metaphase spermatocytes. Throughout the transformation of primary spermatocytes, the 30 nm chromatin fibers become increasingly condensed into heterochromatin blocks of various sizes, while the 10 nm fibers are decreased in quantity until they are absent entirely in the metaphase spermatocytes. Secondary spermatocytes have nuclei that contain 4-6 large blocks of heterochromatin along the inner facet of the nuclear envelope, the 30 nm fibers are loosened up and the 10 nm fibers of which start to reappear. Spermatids can be divided into 4 steps. In spermatid I, the nucleus is round and contains more dispersed 30 nm chromatin fibers that are unraveled from the more tightly packed chromatin of the secondary spermatocytes. The 30 nm chromatin fibers are uniformly packed together and increasingly condensed in spermatid II, whose nucleus is decreased in size and becoming oval shaped. In spermatid III, the nucleus becomes elongated and it contains 30 nm chromatin fibers which are aggregated more tightly and evenly together, while 10 nm fibers disappear. In spermatid IV, the nucleus becomes highly elongated into a cylindrical shape, with a small acrosome covering the anterior pole. Its chromatin is highly condensed, but the outline of the 30 nm fibers could still be observed. In spermatozoa, the 30 nm chromatin fibers are packed so tightly together that chromatin becomes completely electron-opaque. The condensation of chromatin in Rana spermatozoa is, therefore, similar to the process of heterochromatization in fully differentiated somatic cells, where 30 nm fibers coalesce together without changing their initial size.

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Department of Anatomy, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400, Thailand.

* Corresponding author, Email: scpso@mahidol.ac.th

Received 27 Sep 2002, Accepted 18 Apr 2003