The ilio-marsupialis muscle in the dasyurid marsupial Sminthopsis douglasi: form, function and fibre-type profiles in females with and without suckling young
1 Department of Zoology, La Trobe University, Bundoora, Victoria 3086,
Australia
2 School of Life Sciences and Technology, Victoria University, Melbourne,
Victoria 8001, Australia
* Author for correspondence (e-mail: p.woolley{at}latrobe.edu.au)
Accepted 16 September 2002
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Summary |
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Key words: ilio-marsupialis muscle, anatomy, function, fibre-type composition, pouch, development of young, Sminthopsis douglasi, dasyurid marsupial
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Introduction |
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In the present study, observations have been made on the ilio-marsupialis muscles of Sminthopsis douglasi, a dasyurid marsupial with a Type 3 pouch. There are eight nipples in the pouch of this species and this determines the maximum litter size because the young do not relinquish the nipple until approximately day 40 of the 70-day suckling period. In the event that the female does not have a full complement of young, the mammary glands associated with unsuckled nipples do not enlarge and produce milk. For approximately 30 days, the young are covered by the pouch skin fold, after which they become progressively more exposed (Fig. 2). By 40 days of age, they hang out of the pouch but are usually held close to the body of the mother when she is moving around. From this age, they begin to be left in the nest while the mother is out feeding (P. A. Woolley, unpublished laboratory observations). Sminthopsis douglasi is nocturnal and insectivorous but little is known of its activity patterns in the wild. It would seem to be advantageous for the mother to be able to forage without the burden of large young. The mechanism by which the mother dislodges the young is not known, but young in captivity have been observed to relinquish the nipples following repeated rapid movements (vibration) of the abdomen (P. A. Woolley and M. Coleman, unpublished observations).
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From this brief description of events in the development of the suckling
young, it seems likely that the demands placed on the ilio-marsupialis muscle
may change over the suckling period. As mammalian skeletal muscles in general
display plasticity (Pette and Staron,
1997,
2001
;
Pette, 1998
), changing demands
on the ilio-marsupialis muscle would be expected to induce changes in the
fibre-type composition. Therefore, bearing in mind that dynamic muscular
activity is usually associated with fast-twitch muscle fibres, and
postural-type activity is usually associated with slow-twitch,
fatigue-resistant muscle fibres (Jones and
Round, 1990
), it was of interest to investigate whether or not
changes occurred in the type of muscle fibres present in the ilio-marsupialis
muscles of Sminthopsis douglasi in the course of the development of
suckling young. Identification of such changes might also help to better
understand the function of this muscle.
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Materials and methods |
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Fibre typing
It has been shown that muscle fibres from eutherian mammals
(Fink et al., 1990;
Lynch et al., 1995
) and
dasyurid marsupials (Wilson and
Stephenson, 1990
) can be classified according to their sensitivity
to the activators strontium (Sr2+) and calcium (Ca2+)
into three groups: fast-twitch, slow-twitch and composite. Compared with
slow-twitch fibres, fast-twitch fibres display a greatly reduced sensitivity
to activation by Sr2+ ions and a slightly decreased sensitivity to
activation by Ca2+ ions. Composite fibres display high
Sr2+ sensitivity at lower levels of activation (similar to
slow-twitch fibres) but low Sr2+ sensitivity at higher levels of
activation (similar to fast-twitch fibres). This physiological method of fibre
classification has been validated previously by an electrophoretic method for
the determination of the myosin heavy chains in the same fibres for which
activation by Sr2+ and Ca2+ was measured
(Bortolotto et al., 2000
).
Between two and nine fibres per animal were examined. Single fibres were
randomly isolated from freshly dissected branches of the ilio-marsupialis
muscle associated with a particular nipple, and their Sr2+ and
Ca2+ activation characteristics were determined after brief
treatment with Triton X-100, a non-ionic detergent, to remove the membrane
barriers and enable access to the contractile apparatus
(Bortolotto et al., 2000). The
fibres were then activated in Sr2+- and Ca2+-buffered
solutions and classified as fast, slow or composite, and the proportions of
the different types of fibres were calculated. This method has been shown
previously to produce a fibre-type profile that is very close to that obtained
from the sampling of the entire muscle
(Bortolotto et al., 2000
).
Samples of the ilio-marsupialis muscle associated with individual nipples were taken from non-lactating and lactating females and grouped as follows:
The mass of the young, and of the mammary tissue, was obtained for some of the above females and for others at particular times throughout the lactation period as part of a separate study.
The proportions of the three types of fibres present in the ilio-marsupialis muscles in non-lactating females, and in lactating females at different stages of lactation, were analysed by Fisher exact test. All statistical analyses were two-tailed, and significance was accepted at the P<0.05 level.
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Results |
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Muscle fibre-type profiles
Fig. 4 shows the stages at
which the groups of muscle samples were taken and the appearance of the pouch
together with, in females suckling young, major events during the suckling
period. The number of fibres examined and the number of animals they were
obtained from together with the average percentage of fibre type per animal
(fA) and the percentage of fibre type per total fibres
(fT) in each of the five groups are given in
Table 1. It is important to
note that, despite the variation in the number of fibres investigated per
animal (2-9), the average percentage of fA was close to
the fraction of fT examined for each group. This indicates
that the inter-animal variability in the fibre composition of the
ilio-marsupialis muscle is relatively small. The largest difference between
fA and fT values was found in Group 5
(unsuckled nipples), where fibres were taken from animals at very different
stages of lactation. This may account for the higher inter-animal variability.
Nevertheless, even in this group, there was consistency in the sense that no
slow-twitch fibres were found in any of the four animals examined. Given the
close values between fT and fA in
Table 1, only the
fT values for the fast-twitch, slow-twitch and composite
fibres are presented in Fig.
5.
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In Group 1, the majority of fibres (71%) were classified as composite, with only 22% showing fast and 7% showing slow activation characteristics. The fibre-type profile of this group represents the steady-state (control) condition of the ilio-marsupialis muscle in females not suckling young. In muscles associated with suckled nipples (Groups 2, 3 and 4), the proportion of composite fibres was less than in Group 1. In Group 2, there was a greater proportion of fast fibres (75% versus 22%, P<0.05) and a lower proportion of composite fibres (25% versus 71%, P<0.05) than in Group 1. Further shifts in fibre-type composition were seen in Groups 3 and 4. In Group 3, there was a significant increase in the proportion of slow fibres (39% versus 0%, P<0.05) and, consequently, a lower proportion of fast fibres (39% versus 75%, P<0.05) than in Group 2. In Group 4, when weaning of the young is in progress, the proportion of composite fibres was lowest (6%), with most fibres being classified as either fast (69%) or slow (25%). These results show that the principal effect on the ilio-marsupialis muscle during progression of the lactation period is the transformation of composite fibres associated with suckled nipples into either fast-twitch or slow-twitch fibres. In our view, the fibre type transformation in both directions is not caused by one stimulus, i.e. suckling, but rather by changes in the load placed on the ilio-marsupialis muscle associated with growth of the young and changes in their behaviour (see Discussion). The change first seen was from composite to fast-twitch fibres and, later in the lactation period, it was also to slow-twitch fibres. Fibres in Group 5 (associated with unsuckled nipples in the pouch of females with young suckling other nipples) showed a similar fibre-type profile to those in Group 1 (Fig. 5). No significant difference (P=0.39) was found between the types of fibres identified in Groups 1 and 5, which suggests that lactation per se has no effect on fibretype profile; rather, it is the demand on the muscles associated with individual nipples that is related to changes in fibre-type profile.
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Discussion |
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In the absence of suckling young, the ilio-marsupialis muscle consists
predominantly of composite fibres (71%). By comparison, only 28% of the fibres
from the soleus muscle of the stripe-faced dunnart Sminthopsis
macroura, a smaller congener of the species used in this study, were
composite (Wilson and Stephenson,
1990). A high proportion of composite fibres in a muscle normally
indicates a more complex functional role of that muscle
(Stephenson, 2001
). When the
nipples are not suckled, the branches of the muscles are small, suggesting
little or no functional role other than perhaps to maintain the tone of the
pouch skin fold. The high proportion of composite fibres in these muscles
would enable them to adapt to changes in demand during the suckling
period.
In the presence of suckling young, the fibre-type profile of the ilio-marsupialis muscle changed. For approximately the first 40 days of the lactation period, the young are suckled continuously (the attachment phase), and during this time individuals increase in mass from 15 mg at birth to as much as 2 g. The mass of the mammary tissue also increases, from approximately 300 mg to 2 g. Towards the end of the attachment phase, the pouch skin fold no longer covers and provides protection and support for the young. They become increasingly exposed and finally hang down from the nipples (Figs 1, 4). Late in the attachment phase (Group 2; 35-39 days), it was found that the percentage of composite fibres had decreased and the percentage of fast-twitch fibres had concomitantly increased. At this stage, the mother would have to carry her young (total mass of approximately 16 g, or one-third of her own mass) in the pouch when out foraging. To prevent them from dragging on the ground, the mother would need to be able to hold them close to her body but perhaps only for short periods while actually moving in search of food. The shift at this stage to fast-twitch fibres, which are usually associated with dynamic muscular activity, would presumably facilitate intermittent retraction of the young, but more observations on the behaviour of animals at this time are required to support this explanation.
Growth of the young accelerates after day 40, with 51-53-day-old young (Group 3) each weighing as much as 6 g, and 68-70-day-old young (Group 4) weighing 15-20 g. These older young are not carried in the pouch when the mother is out foraging, but there is an increase in the mass of mammary tissue up to approximately 10 g (15-20% of maternal body mass at the beginning of lactation). The most pronounced change in fibre-type composition of the ilio-marsupialis muscles associated with suckled nipples in Groups 3 and 4 is an increase in the proportion of slow-twitch (fatigue-resistant) fibres. This may occur in response to the need to support a greater mass of mammary tissue and, although there may be little need to hold the young close to her, at least some of the mass of the young while they are suckling in the nest. Studies on captive females with young of these ages have shown that the young will relinquish the nipples following vibration, for 5-60 s, of the mother's abdominal muscles and possibly also the iliomarsupialis muscles. The major difference between fibre-type profiles in the ilio-marsupialis muscles of Groups 3 and 4 is the transformation of composite (uncommitted) fibres into fast-twitch fibres. The presence of a high proportion of fast-twitch fibres supports a dynamic function for them at these stages.
Griffiths and Slater (1988)
considered that the role of the iliomarsupialis muscle in the `pouchless'
dasyurid marsupials that they examined was to haul the young up tightly into
the pouch area and so protect them from injury. In Antechinus
stuartii (the species in which they illustrated that anaesthesia reduced
the ability of the mother to hold her young close to the body) and other
`pouchless' dasyurids, the young are exposed (i.e. not enclosed by pouch skin)
from the time they are born. It would be of interest to examine the fibre-type
composition of the iliomarsupialis muscles of `pouchless' species to determine
what changes occur throughout the suckling period for comparison with species
such as S. douglasi. Such a study might help to provide a better
understanding of the function of the iliomarsupialis muscles in `pouched' and
`pouchless' marsupials.
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Acknowledgments |
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Footnotes |
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References |
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