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American Journal of Medical Genetics 83:334–337 (1999)
Letter to the Editor
First Transmission Electron Micrograph of
Continuous Mitotic Spindle Fibers Between Polar
Area and Chromosome Ends
To the Editor:
During the study of the ultrastructure of the fragile
X chromosome reported in these proceedings [Wen et
al., 1999], one striking preparation demonstrated an
apparent mitotic spindle apparatus not visible in the
Q-banded metaphase (Fig. 1) composed of microtubules
with diameters of 200 to 260 Å (Figs. 2 and 3). Two
microtubules per spindle fiber were observed (Fig. 3).
An apparent polar area or centriole was also observed.
These apparent microtubules were thought to be distinct from chromatin fibers based on their characteristic size and configuration and also because none of the
apparent spindle apparatus was visible in the DNAspecific Q-banded preparation (Fig. 1). This appears to
be the first such transmission electron microscope
(TEM) observation of the mitotic spindle apparatus
with continuous microtubules forming loops and connecting the ends of the short arms of both X and number 4 chromosomes as illustrated in Fig. 4.
The apparent mitotic spindle apparatus in Figs. 2 to
4, to our knowledge, is the first TEM observation of a
monoplanar longitudinal section of metaphase chromosomes connected from their short arms to microtubules. Furthermore, it appears more likely that what
we have observed in Fig. 3 is part of the mitotic spindle
apparatus rather than chromatin fibers stretching
from the chromosome [Hliscs et al., 1997] because of
the following: 1) chromatin fibers appear to be flexible
and have a ‘‘zig-zag’’ configuration, whereas spindle
microtubule fibers appear more rigid and straight
[Horowitz et al., 1994; Brinkley, 1997]; 2) chromatin
fiber diameter has been established at 300 Å [Ostashevsky and Lange, 1994; Horowitz et al., 1994; Bartolome et al., 1994; Santisteban, 1994; Martin et al.,
Contract grant sponsor: New York State Office of Mental Retardation and Developmental Disabilities; Contract grant sponsor: the Maternal and Child Health Program (Title V, Social Security Act), Health Resources and Services Administration, Department of Health and Human Services; Contract grant number:
*Correspondence to: Dr. Guang Wen, Institute for Basic Research in Developmental Disabilities, 1050 Forest Hill Road,
Staten Island, NY 10314. E-mail:
Received 22 August 1997; Accepted 9 November 1998
© 1999 Wiley-Liss, Inc.
1995], whereas microtubules have been shown to be
200 to 260 Å [Brinkley, 1997]; and 3) the Q-banded
metaphase did not exhibit this spindle-like configuration coming from the X and number 4 chromosomes.
We thank Dr. James Ray of Quest Laboratories for
constructive criticism during the preparation of this
manuscript. This work was supported in part by the
New York State Office of Mental Retardation and Developmental Disabilities and in part by grant
MCJ360587 from the Maternal and Child Health Program (Title V, Social Security Act), Health Resources
and Services Administration, Department of Health
and Human Services.
Bartolome S, Bermudez A, Daban JR. 1994. Internal structure of the 30 nm
chromatin fiber. J Cell Sci 107:2983–2992.
Brinkley W. 1997. Microtubules: A brief historical perspective. J Struct
Biol 118:84–86.
Hliscs R, Muhlig P, Claussen U. 1997. The spreading of metaphases is a
slow process which leads to a stretching of chromosomes. Cytogenet
Cell Genet 76:167–171.
Horowitz RA, Agar DA, Sedat JW, Woodcock CL. 1994. The threedimensional architecture of chromatin in situ: electron tomography
reveals fibers composed of a continuously variable zig-zag nuleosomal
ribbon. J Cell Biol 125:1–10.
Martin LD, Vesenka JP, Henderson E, Dobbs DL. 1995. Visualization of
nucleosomal substructure in native chromatin by atomic force microscopy. Biochem 34:4610–4616.
Ostashevsky JY, Lange CS. 1994. The 30 nm chromatin fiber as a flexible
polymer. J Biolmol Struct Dyn 11:813–820.
Santisteban MS. 1994. Structure of chromatin I: Levels of DNA organization in the nucleus: nucleosome and chromatin fibers. Pathol Biol 42:
Wen GY, Jenkins EC, Goldberg EM, Genovese M, Brown WT, Wisniewski
HM. 1999. Ultrastructure of the fragile X chromosome: new observations on the fragile site. Am J Med Genet 83:331–333.
Guang Y. Wen*
Edmund C. Jenkins
Erick M. Goldberg
Marilyn Genovese
W. Ted Brown
Henryk M. Wisniewski
Institute for Basic Research in Developmental
Staten Island, New York
Letter to the Editor
Fig. 1. Fluorescent light micrograph of a control pre-identified X chromosome (arrow) after Q-banding (DNA-specific) showing no indication of spindle
fibers (×3,048).
Fig. 2. Electron micrograph of the same X chromosome as in Fig. 1, showing an apparent part of the mitotic spindle apparatus connecting both X and
chromosome 4 at their distal ends to an apparent polar region (centriole) (×8,285).
Wen et al.
Fig. 3. Higher magnification electron micrograph of the same mitotic spindle apparatus shown in Figure 2, revealing continuous microtubule fibers
between X and chromosome 4 short arm telomeres and the centriole (×18,657). Two parallel microtubules (see insert, ×123,820) extended toward the
chromosomes and formed a loop that appears to have moved backward toward the centriole. This process repeated and formed many loops. The spindle
microtubules appeared to directly implant or connect without interruption with the short-arm telomeric chromosome ends, nearest the polar area.
Letter to the Editor
Fig. 4. Schematic reconstruction of Figures 1 to 3 and other high magnification electron micrographs (not shown) illustrating the interaction between
chromosomes X and 4, and the mitotic spindle apparatus that consists of two parallel microtubules that run longitudinally between centriole and
chromosomes forming several loops, which may function as a connecting hook that implants into the telomeric ends of the chromosome short arms. The
centromere of chromosome 4, as indicated in this figure, was identified in another serial section (not shown) of the same number 4 chromosome shown
in Figures 1 and 2. Note: p, short arm; q, long arm; c, centromere; cn, centriole; numbers 1 to 6 represent loops in the spindle apparatus (Figs. 2 and 3).
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