Spermiogenesis and chromatin condensation in the common tree shrew, Tupaia glis

We have investigated the cellular characteristics, especially chromatin condensation and the basic nuclear protein profile, during spermiogenesis in the common tree shrew, Tupaia glis. Spermatids could be classified into Golgi phase, cap phase, acrosome phase, and maturation phase. During the Golgi phase, chromatin was composed of 10-nm and 30-nm fibers with few 50-nm to 60-nm knobby fibers. The latter were then transformed into 70-nm knobby fibers during the cap phase. In the acrosome phase, all fibers were packed into the highest-order knobby fibers, each about 80-100 nm in width. These chromatin fibers became tightly packed in the maturation phase. In a mature spermatozoon, the discoid-shaped head was occupied by the acrosome and completely condensed chromatin. H3, the core histone, was detected by immunostaining in all nuclei of germ cell stages, except in spermatid steps 15-16 and spermatozoa. Protamine, the basic nuclear protein causing the tight packing of sperm chromatin, was detected by immunofluorescence in the nuclei of spermatids at steps 12-16 and spermatozoa. Cross-immunoreactivity of T. glis H3 and protamine to those of primates suggests the evolutionary resemblance of these nuclear basic proteins in primate germ cells.