Published in: "The Cell Nucleus" (Busch H, ed.), Vol. 1, pp. 565-580, (1974), Academic Press, New York.

"Ultrastructure and Function of Heterochromatin and Euchromatin",

John H. Frenster,

Division of Oncology, Department of Medicine,
Stanford University School of Medicine, Stanford, California.

Fig. 1 (left, above). Electron micrograph of a normal human blood lymphocyte incubated in vitro in the absence of phytohemagglutinin for 48 hours (Stanley DA, et al, 1971). The cytoplasm is scanty and composed largely of monosomal ribosomes. The nucleus is compact with a major part of the DNA contained within condensed heterochromatin masses arranged directly underneath the nuclear membrane. The nucleolus is small and surrounded by additional condensed heterochromatin. (X 16,000).

Fig. 2 (right, above). Electron micrograph of normal human blood lymphocyte incubated in vitro in the presence of phytohemagglutinin for 48 hr (see Fig. 1). The lymphocyte has undergone blastic transformation (Tokuyasu K, et al, 1968) with an increased cytoplasm composed largely of polysomal ribosomes. The nucleus has enlarged with a major part of the DNA contained within extended euchromatin fibrils dispersed throughout the nucleus. The nucleolus has enlarged and is free of any surrounding heterochromatin. The plasma membrane has increased its number of microvillus projections and phagocytic activity (Frenster JH, and Rogoway WM, 1970). (X 9,000).


Fig. 3 (left, above). Electron micrograph of a mitotic polysomal cell in the lymph node of an untreated patient with Hodgkin's disease (Archibald RB, and Frenster JH, 1973). The mitotic cell is twice as large as surrounding monosomal lymphocytes but has condensed all of it nuclear DNA into highly condensed mitotic chromosomes. The nuclear membrane is discontinuous and is re-forming within the cytoplasmic areas. No euchromatin is apparent in this mitotic cell (X 4,000).

Fig. 4 (right, above). Electron micrograph of a lymphocyte nucleus isolated from the thymus of a normal calf under isotonic conditions (Frenster JH, et al, 1963). No cytoplasm is apparent. The nucleus is compact with the major part of the DNA contained within condensed heterochromatin masses (compare with Fig. 1). The nucleolus is small and fine extended euchromatin fibrils are dispersed throughout the nucleus (X 10,000).

Fig. 5 (left, above). Electron micrograph of an isolated lymphocyte nucleus swollen after extraction of nuclear ribonucleoprotein particles and saline-soluble proteins under hypotonic conditions (Frenster JH, et al, 1960). The condensed heterochromatin masses remain dispersed radially (compare with Fig. 4), and the extended euchromatin fibrils are seen to be continuous with the condensed heterochromatin masses (X 5,000).

Fig. 6 (right, above). Electron micrograph of an isolated lymphocyte nucleus after swelling induced by hypotonic extraction (Frenster JH, 1965a). At this higher magnification, the extended euchromatin fibrils are seen to be continuous with collapsed fibrils within the condensed heterochromatin masses. The average caliber of the euchromatin fibril is 10 nm, and individual euchromatin fibrils can sometimes be followed for up to 1.0 um of their length. The sharp zone of transition between euchromatin and heterochromatin is of a fibril length of less than 10 nm (X 50,000).

Fig. 8 (left, above). Electron microscopic probe analysis of a proerythroblast in the living bone marrow obtained from a normal human subject (Frenster JH, 1972). Acridine orange binds to DNA templates, and after DNase I digestion (Frenster JH, 1971), the electron-dense reaction product is found exclusively within the euchromatin portion of the cell nucleus (X 5,000).

Fig 9, (right, above). Electron microscopic probe analysis of an abnormal myeloblast in the living bone marrow of an untreated patient with chronic granulocytic leukemia (Frenster JH, 1971). The electron-dense reaction product is found equally distributed within the euchromatin of both nuclei (X 5,000).

Fig. 10 (left, above). Electron microscopic probe analysis of a polysomal lymphocyte within the lymph node of an untreated patient with Hodgkin's disease. The electron-dense reaction product is confined to the euchromatin portion of the cell nucleus and is often in striking contrast to the paucity of reaction product observed in adjacent far-advanced neoplastic cells (Fig. 9b in: Frenster JH, et al, 1974) (X 5,000).

Fig. 12 (right, above). Electron micrograph of a mitotic polysomal cell within the lymph node of an untreated patient with Hodgkin's disease (Archibald RB, and Frenster JH, 1973). The mitotic chromosomes are highly condensed and are completely encompassed within two systems of pentalaminar nuclear membranes. The advanced stage of nuclear membrane restitution without any evidence of cytoplasmic division suggests the formation of a binucleated cell (X 5,000).

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Fig. 7 (left, above). Macromolecular interactions within heterochromatin and euchromatin (Frenster JH, 1965c). Within repressed chromatin, polycationic histones are tightly bound to DNA templates, stabilizing such templates against strand separations and gene transcription (Frenster JH, 1965b). Within active euchromatin, histone repressors are displaced from DNA templates by polyanionic de-repressors such as de-repressor RNA (Frenster JH, 1965b) allowing DNA strand separations and gene transcription.

Fig. 11 (right, above). Postulated feedback inhibition of de-repression at a single gene within euchromatin. De-repressor RNA (dRNA) is thought to bind to the anticoding strand (anti-o) of the operator gene , allowing synthesis of the transcription product RNA on the operator (o) and the structural gene (sg) DNA templates (Herstein PR, and Frenster JH, 1972). Excessive rates of transcription result in increased amounts of operator RNA (oRNA) via cleavage from the transcription product RNA. This tends to decrease further transcription by removal of dRNA from the DNA template via formation of homometric and heterometric duplex RNA between dRNA and either oRNA or transcription product RNA, respectively. Messenger RNA (mRNA) might be included as part of the heterometric duplex RNA (Frenster JH, and Herstein PR, 1973; Jelinek W, and Darnell JE, 1972).

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