First published online October 7, 2005
Journal of Experimental Biology 208, 3819-3834 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01765
The modulus of elasticity of fibrillin-containing elastic fibres in the mesoglea of the hydromedusa Polyorchis penicillatus
William M. Megill1,2,3,*,
John M. Gosline1,2 and
Robert W. Blake1,2
1 Department of Zoology, University of British Columbia, Vancouver, BC, V6T
1Z4, Canada
2 Bamfield Marine Sciences Centre, Bamfield, BC, V0R 1B0, Canada
3 Centre for Biomimetic and Natural Technologies, Mechanical Engineering
Department, University of Bath, Bath, BA2 7AY, UK
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Fig. 2. Mechanical testing apparatus. (A) Isolated mesoglea preparation. A slab of
mesoglea was laid on the moving stage of an Instron testing machine.
Cyanoacrylate adhesive was applied to both plates. Orientation labels: A,
anterior; P, posterior; M, medial; L, lateral; C, circumferential. (B) Intact
animal preparation. Glue was applied to the bottom of the upper plate and the
top of the lower plate. (Inset) Section looking along the long axis of the
jellyfish in the mount. Plates were aligned over the ad-radius (as shown) or
per-radius.
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Fig. 3. Scaling of bell geometry with bell height. Shoulder height
(hs/h) and fineness
(r0/h) ratios are plotted on the left-hand
vertical axis, while the wall thickness ratio ( /h) is plotted on
the right-hand axis. Shoulder height and fineness ratios decrease with
increasing bell height. The slopes of both lines were significantly different
from zero (P<0.05 for both). The wall thickness ratio did not
scale with bell height.
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Fig. 4. Collage of micrographs showing a complete cross section of a jellyfish.
Micrographs were taken using a video capture system and a 25x objective
on an interference contrast microscope. Fibres can be traced from the inner
edge of the exumbrellar epidermis to the gastrodermal lamella. Note the high
degree of branching of the fibres at their ends, and the intertwining of the
fibres with the tissue of the exumbrellar epithelium and the gastrodermal
lamella. Also note the coiled, slack appearance of the fibres in the medial
half of the bell mesoglea and the absence of organisation in the fibres of the
joint mesoglea. The black rimmed circular structures in the micrographs are
air bubbles introduced during the thawing and transfer of the sample from the
microtome to a microscope slide. (Inset) Tracings of the six fibres from
gastrodermal lamella to exumbrella. Air bubbles are shown for orientation. The
path lengths of the tracings were calculated, and a correction (described in
text) applied to derive a reasonable minimum estimate of the unstressed length
of the fibres. Note that the tracings follow a single branch of the fibre in
the highly branched region near the gastrodermal lamella.
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Fig. 5. (A–C) Micrographs used to measure the density of jellyfish
microfibrils. Micrographs taken with a video capture system using a 40x
objective on an interference contrast microscope. Samples were allowed to dry
down for an hour before measurements were taken, to facilitate focussing the
microscope. Fibres were branched at both ends, more so at the medial end, near
the gastrodermal lamella. (A) Near the exumbrellar surface. (B) Mid bell. (C)
Near the gastrodermal lamella. Fibre density was determined by counting the
number of fibres crossing the black line across the micrograph. Micrograph
width: 640 pixels=128 µm. Depth of sample: 500 µm. (D) Digital
micrographs taken with a 100x objective on the interference contrast
microscope show that fibre diameter increases slightly with body size. For
improved accuracy, measurements were made with a 15x filar micrometer
eyepiece mounted on the microscope. (Inset) Approximate locations and
orientation of samples are indicated by the coloured boxes: red, A; blue, B;
green, C; magenta, D.
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Fig. 6. Fibre diameter as a function of bell height. Data are plotted for the
inter- (IR), ad- (AR) and per- (PR) radius regions. Regressions for all
regions of diameter against bell height were significantly different from
zero, but there was no difference between the slopes for the three regions.
Data are therefore pooled and a single regression is plotted, along with its
95% confidence intervals (broken lines). The slope of the line is
df=1.35+0.05h (µm). For the `standard'
jellyfish of 20 mm bell height, the predicted fibre diameter is
2.86±1.03 µm.
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Fig. 7. Fibre density as a function of bell height. The density of radial fibres
decreased with body size in all areas of the bell. There was no significant
difference in the slopes between regions, so data were pooled and a single
regression plotted, together with its 95% confidence intervals (broken lines).
The slope of the line is n=322–5.5h
(mm–2). The fibre density of the `standard' jellyfish (bell
height = 20 mm) is predicted to be 212±34 mm–2.
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Fig. 8. (A) Typical stress–strain behaviour of a slab of isolated mesoglea in
the radial direction. The figure is constructed from the first load and unload
cycle to avoid errors due to tissue damage and degradation during the
experiment. Negative strains represent compressive loading, while positive
strains indicate tension beyond the resting thickness. The sample was
compressed below its resting thickness during mounting, with the result that
the fibres, normally pre-strained in vivo, were slack. The joint
mesoglea was removed in this preparation, so it was possible to measure the
stiffness, Em (broken line), of the bell mesoglea alone.
The stiffness is the slope of a straight line fit to the data in the region
between the extension and compression shoulders. The solid line shows the
parallel stiffness, EL, which includes contributions from
matrix and fibres. Again, the stiffness was determined from the slope of a
line fit to the straight line region near the ultimate strain, defined as the
strain just before the tissue began to yield. For most jellyfish, the ultimate
strain was approximately 35%, which corresponds well to the 36% radial strain
observed by Gladfelter (1972 )
during swimming. Data for eight jellyfish tested in this manner are summarised
in Table 3 (the data shown in
this figure are from Jellyfish 19). Zero strain was determined by regressing
from the large strain data, as discussed in the Materials and methods.
Following Lillie et al.
(1998), stiffnesses were
determined using the loading curve rather than the unloading curve. The
hysteresis in the isolated preparation is probably due in large part to water
loss during the experiment. (B) Typical stress–strain behaviour of
intact mesoglea. Negative strains represent compressive loading, while
positive strains indicate tension beyond the native thickness. Because there
was no loss of joint mesogleal tissue in this preparation, the slope of the
broken line is the upper limit (as discussed in the text) of the stiffness of
the joint mesoglea, Ejm, while the slope of the solid line
is again the radial tensile stiffness parallel to the fibres and includes
contributions from the matrix and fibres. Data for three jellyfish tested in
this manner are summarised in Table
3 (data in this figure are from Jellyfish 25). As in A, zero
strain was determined from a regression through the large strain data.
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Fig. 10. Stress–strain data for individual fibrillin microfibrils, reproduced
from Sherratt et al. (2003). The diamond is their data point, and the solid
line their linear interpretation of the mechanical behaviour. The broken curve
is another valid interpolation, suggested by the molecular structure of
fibrillin (Sherratt et al., 2003), shown in the inset (reproduced with
permission from Elsevier). The initial toe region of the J-shaped curve
corresponds to molecular unfolding of flexible parts of the fibrillin protein,
while the final, much higher stiffness region probably arises from the
deformation of rigid, globular domains in the protein.
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© The Company of Biologists Ltd 2005
