Giant pharyngeal gland nuclei of Ascaris: unusual cytology suggests a novel pathway for transport of ribosomal proteins to the nucleus.

We describe the unusual giant pharyngeal gland nuclei of the parasitic roundworm Ascaris lumbricoides suum and attempt to reconcile the relationships among its uncommon, and in some instances unique, collection of constituents. The nuclei were studied by light and electron microscopy, Feulgen cytophotometry, nuclear size analysis and histochemical methods, including those for detection of DNA, RNA, acidic and basic proteins, lipids and carbohydrates. A highly active nucleus is revealed: an extensive system of intranuclear annulate lamellae (IAL); membrane-bounded, acidophilic bodies which contain non-basic proteins; an abundance of free nucleoli; and an exceedingly large chromocenter containing a core of DNA surrounded by nucleoli (an apparent nucleolar organizing region, NOR). The relationships among the various nuclear constituents suggest that the acidophilic bodies consist of nucleolus-related proteins imported from the cytoplasm to the NOR, and that the annulate lamellae mediate this transport. Incidental findings include rounded cytoplasmic invaginations into the nucleus, thread-like structures with dense cores and surrounding small granules found among concentrations of nucleoli, and bundles of 12 nm filaments closely associated with these nucleoli. The significance of such huge, morphologically complex and highly polyploid nuclei, derived from chromatin-diminished progenitor cells early in development, and the possible interrelationship of these phenomena, remain obscure.