Ultrastructural Observations on Nucleolar Changes during Mouse Spermiogenesis*

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Abstract
When applying a new silver staining technique on developing mouse spermatids, strong argyrophilia in the padlock like nucleolus of very early spermatids is confined to its fibrillar and granular component, whereas the fibrillar center is devoid of silver. During further steps the granular component disappears together with the fibrillar center. The last remaining nucleolar part, the silver positive fibrillar component desintegrates at the beginning of nuclear elongation. Instead of it clusters of coiled fibers bordered with granules of .apprx. 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 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. This small silver positive fibrous clusters desintegrate 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.