Immunohistochemical distribution of hyperacetylated histone H4 in testis of paddlefish Polyodon spathula: ultrastructural correlation with chromatin condensation

The acetylation of core histones has been correlated with the deposition of free histones onto newly replicated DNA, transcriptional activity and the displacement of histones by protamines during spermiogenesis. The aim of the present study was to investigate the immunohistochemical distribution of hyperacetylated H4 during spermatogenesis in Polyodon spathula and to correlate these findings with the pattern of chromatin condensation in spermatids. In immature testis, the Sertoli cells showed more intense immunoreactivity for highly acetylated H4 than that of most primary spermatogonia. The testis of paddlefish at the beginning of spermatogenesis possessed early secondary spermatogonia and late secondary spermatogonia/preleptotene primary spermatocyte with intense nuclear immunoreactivity for highly acetylated H4. In seminiferous tubules in which secondary spermatogonia nuclei were intensely immunostained, Sertoli cell nuclei were unstained. Testes in late spermatogenesis contained tubules in which the immunohistochemical reaction for highly acetylated H4 was positive in the nuclei of preleptotene primary spermatocytes and secondary spermatocytes and in spermatids at the beginning of the elongation process. No immunostaining was found in round spermatids and spermatozoa. During the resting stage, immunostaining was confined to the nuclei of spermatogonia and the cells from the interstitial tissue. Transmission electron microscopy revealed that early spermatids had a round nucleus with filaments of fine chromatin that were dispersed or condensed in clumps. In later stages of maturation, the spermatids had slightly oval nuclei and homogeneous granular chromatin. The chromatin of advanced spermatids was organized into thick fibres. At the end of spermiogenesis, spermatozoan nuclei consisted of homogeneous highly compacted chromatin.