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The effect of contraction on the volume of the smooth muscle nucleus.

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THE EFFECT O F CONTRACTION ON THE VOLUME O F
THE SMOOTH MUSCLE NUCLEUS.
UY
CAROLINE McGILL.
Zii8trtictor in Aiiatoiny, Uniccrsity of Uissouri.
a recent paper (American Journal of Snatomy, Vol. IX,
KO. 4, 1909) I have described the histological changes which occur
in the smooth muscle nucleus during the contraction of the tissue.
Briefly these changes are as follows : The nucleus decreases markedly
in length and increases in thickness. The chromatin, which is distributcd uniformly through thc resting nucleus in a fine reticulum,
as the nucleus shortens streams towards the poles, and there forms
into coarse, deeply-staining strands. The nucleus thus appears to
take an active part in the process of contraction.
RJ most authors, contraction in smooth n~usclehas been assumed
to be due to an increase in the roliime of the myofibrilb brought
about by the absorptioii of water from the interfibrillar substance.
The observations reported in the present paper were made to determine whether there is any change in the ~ o l u m eof the smooth mnscle
nucleus during contraction, accompanying the marked changes in
structure described in the preceding paragraph.
The material chosen was the intestinal muscle of Nccturus. IIere
the nuclei are very large, in extreme extension reaching a length of
eight? micra. They are large enough for fairly accurate reconstruction. Pieccs of the miiscle mere fixed in Zenker's fluid, embedded
in paraffi and cut serially. Some of the series were cut at four
micra, others at five micra in thickness. The sections were stained
either by Delafield's haematoxylin or i l l Heidenhain's iron haematoxylin.
Measurements of the 1-olumes of a large number of nuclei, both
from contracted and expanded muscle, were made. Two methods
were used in determining the volume. The first method is as fol111
(633)
Caroline McGill.
634
lows: I n a given area of a longitudinal section of smooth muscle, the
nuclei showing the greatest size were taken as nuclei cut in median
longitudinal section. These were projected and drawn by means of n
camera lucida at a magnification of 1200 diameters. Assuming that
the nuclei were round in cross section, the volume was calculated.
This was done by breaking each niicleus u p into several frustra of
cones, finding the volume of each and adding the several volumes.
The following table is from a series of nuclei taken as representatives
from a large number of measurements. I n this table the magnified
size has been reduced to thc actual size. The length and width of
the nucleus give a fair index of the amount of contraction. The
first of the series therefore represents a niicleus fully contracted,
the last is fully extended.
No,
Length in Mirra.
Width i n Micra.
Volume in Cubic Micra.
1
28
18
236.74
2
30
17
2447.65
3
35
15
2938.52
4
43
9
1763.30
5
55
9
1940.82
6
62
9
2537.64
7
70
”
f
2013.68
8
72
7
2024.72
9
76
7
2031.12
10
82
5
1766.24
In the expanded muscle the nuclei are almost round in cross
section, but in contracted muscle they are often considerably flattened. This probably accounts for the apparently slightly smaller
volume in the expanded muscle nucleus shown in the above table.
Therefore, as a check on this first series of measurements by (probably) a more accurate method, the volumes of another series of
nuclei were found by reconstruction. The size of the nucleus made
it easy to do this. Thin serial sections in cross section of the
muscle fibres were made from both contracted and expanded muscle.
The Effect of Contraction.
635
Definite areas in series from these sections were projected and drawn
by aid of the camera lucida a t a magnification of 1500 diameters.
I n these series the nuclei were easily traced throughout their entire
length. From the serial drawings the volumes of the nuclei were
found either by means of a planimeter or by reconstruction by Born’s
wax plate method (the volume in the latter case being measured by
water displacement). The following table gives a fern representative volumcs taken from a large number of measurements by the
planimeter method. As in the preceding table, the magnified size
has been reduced to the actual size.
Volume in Cubic Micra.
Number.
Length in Micru.
1
27
2370.00
2
32
2400.00
3
37
241 1.90
4
43
2240.00
5
47
2370.00
6
47
2134.00
7
53
2340.00
8
64
2472.00
9
70
21 12.00
10
77
2408.00
The wax models of the nuclei showed very similar volumes, so a
table is not given.
The above table shows little difference betxeen the roliimes of
the resting and of the contracted smooth muscle nucleus. From thc
data it seems probable that during the contraction of the smooth
muscle nucleus of Necturus there is no change in rolnme.
Reccil-ed for publication September 9, InOD.
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