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Mitosis in non-striated muscle cells.

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Department of Zoo7ogy, Colzcmbia University
I n a fairly complete search of the literature I have been
unable to find any record of observations of the details of
mitotic cell division in unstriated muscle fibers. Division of
these spindle-shaped cells differs in some respects from the
type characteristic of the more common cylindrical and
spherical cells and these differences seem to justify a brief
description of the phenomena.
This study was made on the circular muscle fibers of the
wall of the stomach of Ambystoma opacum larvae of approximately 22 mm. total length. Animals were fixed entire in
Helly’s formal-Zenker fluid, sectioned at 6 p, and stained for
a prolonged period in iron-hematoxylin. The slides were differentiated to show spindle structure rather than chromosome details, and the latter were somewhat obscured by the
excessive stain.
The resting cell is very elongate and spindle-shaped, with
its greatest diameter near the middle region, where the nucleus is located. The fiber is somewhat flattened in a plane
parallel to the stomach wall. Probably one-fourth to onethird of the total length of the fiber is shown in figure 1. The
myofibrillae are present everywhere in the fiber except beside
the nucleus, where there is practically no cytoplasm, and in
a narrow cone-shaped area at either end of the nucleus. These
fibrillae undergo no apparent change during the process of
cell division.
During the prophase, as in all cells, the nucleus swells, increasing in size up to the time of disappearance of the nuclear
membrane (fig. 2 ) . The expanded region of the fiber produced by this swelling persists throughout most of the subsequent division stages as a zone in which the mitotic process
takes place.
The early metaphase spindle is nearly perpendicular to the
long axis of the fiber and to its broad surface (i-e., to the
surface parallel to the stomach wall) (fig. 3). This orientation of the spindle is foreshadowed in the late prophase by
the position of the central bodies, which lie in clear zones at
opposite sides of the nucleus (fig. 2). The spindle is very
short and broad, with a distinct spherical central body at
each end. There is no trace of astral structure. The chromosomes are attached to fibers at the periphery of the spindle,
so that the main mass of the equatorial plate lies outside
it. This is the typical Amphibian spindle as it has been
often carefully described (Flemming, '82). I n later metaphase stages there is a lengthening and narrowing of the
spindle, which are accompanied by a change in orientation
so that its long axis nearly coincides with that of the muscle
fiber (figs. 4 and 5). The most obvious explanation of this
movement seems to be that it results from the fact that the
elongation reaches such an extent that the spindle is longer
than the diameter of the cell. The spindle rotation involves
a similar change of position of the central part of the equatorial chromosome plate, since it is to that part of the plate
that the spindle fibers are attached, as described above, but
the unattached peripheral part of the chromosome plate
remains in its original early metaphase position (fig. 5). The
anaphase movement of the chromosomes begins from this
position so that figure 5 marks the end of the metaphase
The chromosome separation takes place toward opposite
ends of the cell and the middle anaphase and subsequent
stages give no hint of the original position of the metaphase
spindle. The anaphase movement seems to be mainly due to
elongation of the region of the spindle between the chromosome plates, the so-called Stemmkorper (BglBr, '27) (figs. 6
and 7). But there is also certainly a shortening of the polar
part of the spindle (compare figs. 5 and 7). By the anaphase
the central body has become a short rod (figs. 6 and 7). During the early telophase there is a rotation of each daughter
chromosome plate through an arc of approximately 90" (figs.
8, 9, and 10). The beginning of this is shown in figure 8.
An examination of other sections of this cell and of other
cells shows that the rotation occurs at both poles, but it is
frequently the case, as in this cell, that the two plates turn
in very different directions. I have found no examples in
which this rotation does not occur and I consider it an
essential part of the telophase and intimately concerned with
the polarity of the resulting daughter cells. I n discussing
a similar telokinesis in the epithelial cells of the pancreas
it has been suggested that it is a result, of continued elongation of the reg-ion of the spindle between the chromosome
plates (Pollister, '29). The same explanation probably holds
for this case with the added factor that the cell narrows
rapidly at either end of the swollen region where the mitotic
activity occurs.
As figure 8 shows the polar part of the spindle and the
central body are carried around with the chromosome plates,
probably because the two are still held together as in the
metaphase. Although this part of the spindle is not visible
in later stage than this early telophase, it seems safe to
assume that the completion of rotation, as in figure 10, brings
this part of the achromatic apparatus to a position adjacent
to the cell membrane and nearly opposite a point midway
along the length of the chromosome plate, and so opposite the
middle of the resting nucleus.
If one allows the assumption (admittedly more or less
gratuitous in the case of a cell of this type) that the rodshaped central body of the telophase gives rise directly by
division to the central bodies that are seen at the poles of
the ensuing metaphase, one can see a possible explanation of
the otherwise rather puzzling position of the early metaphase
spindle, It originates in a position perpendicular to the
long axis of the cell and of the nucleus because this involves
a much less extensive migration of the two central bodies
resulting from the division of the rod-shaped central body in
the previous telophase. It follows that the position of the
early metaphase spindle is ultimately traceable to the rotation
of the chromosome plates in the previous telophase.
The phenomena of the late telophase are distinctive enough
to be worth some attention. The cell constriction takes place
in such a manner as to divide the muscle fiber into what is
perhaps best described as two half-fibers, since in each the
nucleus is at one end (fig. 9). At first the cell constriction is
a wide cleft (fig. 9) and the two adjacent swollen ends of the
daughter fibers are rather widely separated from one another
though connection by the spindle remnant, which has been
compressed by the cell constriction into what will later form
the midbody. As the chromosomes become ragged and run
together and the nuclear membrane is developed the nucleus
and the whole tip of the cell begin to elongate. This elongation appears to involve a flow of the cytoplasm away from
the zone of myofibrillae so that the daughter cells are brought
into closer contact (figs. 10 and 11). Another result of this
movement of the daughter cells is a twisting of the midbody
(figs. 8 and 11). The cells are on contact only at their peripheries and around the midbody there is always a clear
space separating the sister fibers. As the elongation continues the daughter cells slide obliquely over one another
to form spindle-shaped cells that are in contact laterally
(fig. 12). This seems significant in view of the fact that
muscle fibers overlapping in this way can probably function
much more efficiently than if they were in contact only at
their narrow ends. The remnant of the spindle within each
daughter fiber is reduced t o a narrow thread which can be
identified until the nuclei have returned to a state nearly
like that of the resting cell (figs. 12 and 13). During these
reconstruction stages it is often possible to see in the oblique
position of this spindle remnant evidence of the earlier
telophase rotation of the chromosome plate (see especially
figs. 10 and 12). The midbody part of the spindle likewise
is identifiable throughout this period. In the later stages it
is reduced to a short rod (fig.12) and ultimately to a tiny bead
attached to the thread-like spindle remnant (fig. 13).
B i i ~ b , K. 1927 Beitrage zur Kenntnis des Meohanismus der indirekten Kernteilung. Nahrwissenschaften, Bd. 15.
W. 1882 Zellsubstanz, Kern, und Zelltheilung. Leipzig.
A. W. 1929 Notes on cell division in the pancrens of the dogfish.
Anat. Ree., vol. 44.
All the figures are from material fixed i n Helly’s fluid and stained with ironhematoxylin. The drawings were outlined, as f a r as possible, with the Abbe
camera lucida at the table level. Other details were filled in free-hand. The
initial magnification was approximately 2100 diameters. The figures have
been reduced one-third in reproduction.
1 Portion of a resting cell.
2 Late prophase, just after breakdown of the nuclear membrane. The
central bodies lie in clear zones at opposite sides of the middle of the nucleus.
3 Early metaphase stage, showing the orientation of the spindle perpendicular
t o the long axis of the cell. Some of the chromosomes have been omitted to
emphasize the details of the spindle.
4 Later metaphase, where the spindle has elongated slightly. In this figure
J s o , not all the chromosomes have been drawn.
5 Late metaphase stage, when the spindle has completed its elongation and
rotation. This cell was overstained so that details of the chromosomes and the
central bodies are obscured.
6 Early anaphase stage. Note that the central body is a short rod.
7 Late anaphase stage.
8 Early telophase, before cell constriction. The upper chromosome plate
has begun to rotate toward the left.
9 Later telophase stage, the rotation practically completed in the upper
cell. Cell constriction completed.
1 0 Later telophase. Elongation of the sister cells has closed up the gap
between them and brought about a twisting of the midbody.
11 Later telophase, just after the development of the nuclear membrane.
1 2 Later telophase. Continued elongation of the sister cells has caused them
t o slide obliquely over one another. Note the position of the thread-like spindle
remnants as a result of the earlier rotation of the chromosome plates. Midbody
reduced to a short rod.
13 Very late telophase, nuclear reconstruction practically completed. Note
spindle remnant and small bead-like midbody.
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muscle, mitosis, non, striated, cells
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