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Pictures in cell biology Lampbrush chromosomes from sperm chromatin
FIGURE 1 Small segment of a lampbrush chromosome from the newt Notophthalmus stained with an antibody against splicing factors (anti-Sm). The lateral loops represent transcriptionally active regions of the chromosome. The small round granules are snurposomes, organelles thought to correspond to the speckles seen in somatic nuclei stained with this antibody. Bar, 20 m.
The largest known chromosomes occur in the nuclei of amphibian oocytes. These so-called lampbrush chromosomes (LBCs) consist of a central axis of inactive chromatin from which hundreds or thousands of transcriptionally active loops extend laterally (Fig. 1). Although LBCs were discovered over 100 years ago, little is known about the factors that cause interphase chromatin to transform into these extraordinary structures during oogenesis. Recently, Gall and Murphy1 found that typical LBCs can be produced ‘artificially’ by injecting Xenopus sperm heads into the nucleus of a Xenopus oocyte. The injected sperm head first swells, then transforms into a loose aggregate of chromosome-like threads and eventually gives rise to 18 giant chromosomes that closely resemble the endogenous LBCs. Heterologous injections also work: sperm heads from the leopard frog and the zebrafish form LBCs in Xenopus oocytes, as do Xenopus sperm heads injected into newt oocytes (Fig. 2). Somewhat surprisingly, the injected sperm heads form LBCs with some characteristics corresponding to the species in which they are injected, suggesting that both donor DNA and recipient nuclear factors govern the formation of LBCs. These results suggest that it will be possible to produce LBCs from mammals and other organisms that do not go through a typical lampbrush phase or whose nuclei are too small to be handled by current techniques. Human LBCs would be particularly valuable for high-resolution analysis of transcriptionally active genes by in situ hybridization and immunofluorescence.
Reference 1 Gall, J. G. and Murphy, C. (1998) Mol. Biol. Cell 9, 733–747
FIGURE 2 (a) Swollen Xenopus sperm head one day after injection into the nucleus of a newt oocyte, stained with an antibody against RNA polymerase II. The inset shows a sperm head before injection, stained with the DNA-specific dye DAPI. (b) Two Xenopus lampbrush chromosomes derived from a sperm head injected two days previously into the nucleus of a newt oocyte. Each sperm head produces 18 such chromosomes. Immunofluorescent staining is for RNA polymerase II. Bar, 20 m.
trends in CELL BIOLOGY (Vol. 8) May 1998
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0962-8924/98/$19.00 PII: S0962-8924(98)01260-4
Contributed by Joseph Gall and Christine Murphy, Dept of Embryology, Carnegie Institute of Washington, 115 W. University Parkway, Baltimore, MD 21210-3399, USA. E-mail: gall@mail1. ciwemb.edu
207
Pictures in cell biology Lampbrush chromosomes from sperm chromatin
FIGURE 1 Small segment of a lampbrush chromosome from the newt Notophthalmus stained with an antibody against splicing factors (anti-Sm). The lateral loops represent transcriptionally active regions of the chromosome. The small round granules are snurposomes, organelles thought to correspond to the speckles seen in somatic nuclei stained with this antibody. Bar, 20 m.
The largest known chromosomes occur in the nuclei of amphibian oocytes. These so-called lampbrush chromosomes (LBCs) consist of a central axis of inactive chromatin from which hundreds or thousands of transcriptionally active loops extend laterally (Fig. 1). Although LBCs were discovered over 100 years ago, little is known about the factors that cause interphase chromatin to transform into these extraordinary structures during oogenesis. Recently, Gall and Murphy1 found that typical LBCs can be produced ‘artificially’ by injecting Xenopus sperm heads into the nucleus of a Xenopus oocyte. The injected sperm head first swells, then transforms into a loose aggregate of chromosome-like threads and eventually gives rise to 18 giant chromosomes that closely resemble the endogenous LBCs. Heterologous injections also work: sperm heads from the leopard frog and the zebrafish form LBCs in Xenopus oocytes, as do Xenopus sperm heads injected into newt oocytes (Fig. 2). Somewhat surprisingly, the injected sperm heads form LBCs with some characteristics corresponding to the species in which they are injected, suggesting that both donor DNA and recipient nuclear factors govern the formation of LBCs. These results suggest that it will be possible to produce LBCs from mammals and other organisms that do not go through a typical lampbrush phase or whose nuclei are too small to be handled by current techniques. Human LBCs would be particularly valuable for high-resolution analysis of transcriptionally active genes by in situ hybridization and immunofluorescence.
Reference 1 Gall, J. G. and Murphy, C. (1998) Mol. Biol. Cell 9, 733–747
FIGURE 2 (a) Swollen Xenopus sperm head one day after injection into the nucleus of a newt oocyte, stained with an antibody against RNA polymerase II. The inset shows a sperm head before injection, stained with the DNA-specific dye DAPI. (b) Two Xenopus lampbrush chromosomes derived from a sperm head injected two days previously into the nucleus of a newt oocyte. Each sperm head produces 18 such chromosomes. Immunofluorescent staining is for RNA polymerase II. Bar, 20 m.
trends in CELL BIOLOGY (Vol. 8) May 1998
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0962-8924/98/$19.00 PII: S0962-8924(98)01260-4
Contributed by Joseph Gall and Christine Murphy, Dept of Embryology, Carnegie Institute of Washington, 115 W. University Parkway, Baltimore, MD 21210-3399, USA. E-mail: gall@mail1. ciwemb.edu
207