Ultrastructural observations on nucleolar changes during mouse spermiogenesis.

When applying a new silver staining technique on developing mouse spermatids, it could be shown that strong argyrophilia in the "padlock" like nucleolus of very early spermatids is confined to its fibrillar and granular component, whereas the fibrillar centre is devoid of silver. During further steps the granular component disappears together with the fibrillar centre. The last remaining nucleolar part, the silver positive fibrillar component disintegrates at the beginning of nuclear elongation. Instead of it clusters of coiled fibers bordered with granules of approximately 40-60 nm in diameter, both silver positive, appear in the nucleoplasm. As chromatin condensation proceeds, these silver positive structures, now intimately attached to the centrally occurring focus of condensing chromatin decrease more and more in size and density. In later stages accumulations of silver positive material will appear in the posterior region of the nucleus, will leave it, and stays as silver positive "juxtanuclear body" in the nuclear pocket formed by the redundant nuclear envelope. As spermatid development continues, the "juxtanuclear body" disappears together with the nuclear pocket. The small silver positive fibrous clusters disintegrate too so that the mature sperm only contains the space in which they formerly existed, now called "nuclear vacuole". A possible connection between silver staining pattern and RNA synthesis is discussed.