Untitled

Abstract

The process of chromatin condensation during the transformation from spermatids to spermatozoa in rat was observed by transmission electron microscopy. In Golgi and cap phase spermatids (stages 1-7), 2 sizes of chromatin fibers are evenly distributed in the nuclei, 10 nm and 30 nm thick (level 1 and 2). The latter are uniform fibers that appear as dense dots in cross section, while the former are thin zigzag fibers that link between 30 nm fibers. In earlier stages, level 1 fibers tend to predominate, while in later stages level 2 fibers do. In early acrosome phase spermatids (stages 8-9), the nuclei transform from round to pear shape with partially formed acrosomes covering the anterior ends, and the chromatin fibers, which are mostly at level 2, are packed closely and evenly together. There are increasing number of larger fibers about 40 nm in diameter (level 3) in the subacrosomal region of the nuclei. In mid acrosome phase spermatids (stages 10-12) the 40 nm fibers appear to grow in width to 50 nm (level 4) which transform into long straight fibers that are interlaced together in several directions, and become distributed evenly throughout the nuclei. In late acrosomal phase spermatids (stages 13-14) the chromatin appears as 60 and 70 nm thick knobs and branching cords (levels 5 & 6) in the anterior part of the nucleus, which becomes highly tapered, while the posterior part still contains mainly 50 nm straight fibers. The wave of transformation to thick chromatin cords continues from the anterior to posterior regions, until in maturation phase spermatids (stages 15-17) when chromatin in the anterior halves of some nuclei is completely condensed and the rest of chromatin appears as 90-100 nm thick (level 7) branching cords with narrow intervening light spaces. In immature spermatozoa (stages 18-19) the chromatin becomes completely condensed with only few pale spots scattered widely throughout each nucleus. The three main types (30, 50, 100 nm) of chromatin fibers observed during spermiogenesis correlate well with the sequence of the development of the spermatid to spermatozoa when histones, transition proteins and protamines are bound to DNA.

Putative molecular mechanism underlying sperm chromatin remodelling is regulated by reproductive hormones

“…Sperm nuclear chromatin is remodelled during mid-spermiogenesis in the rat testis [1,2]. Nuclear chromatin condensation is characterised by the displacement of somatic histones H4//H3/H2A/H2B in step9-12 elongating spermatids of stages IX-XII seminiferous tubules of rat [3].…”