Function and Evolution of the Vertebrate Axis
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INTRODUCTION TO SYMPOSIUM:
FUNCTION AND EVOLUTION OF THE VERTEBRATE AXIS
T. Koob, Shriner"s Hospital for Children, Tampa, FL
MECHANICAL DESIGN OF EMBRYONIC
NOTOCHORDS. M.A.R. Koehl*, K.J. Quillin,
and C. A. Pell. Univ. of California, Berkeley and
Duke Univ. [email protected]
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The notochord can play an important mechanical
role in shape changes during early morphogenesis
of vertebrates. For example, osmotic inflation of
notochords elongates and straightens the axis of
frog early tail-bud embryos. In Xenopus laevis, the
sheath of cross-helically arranged fibers around the
notochord limits the shape changes it undergoes
when inflating, causing the notochord to stiffen and
straighten (Adams, Keller & Koehl 1990). We
used physical models of stage 21X. laevis
notochords to explore the mechanical consequences
of different arrangements of the sheath fibers on the
behavior of such curved hydraulic cylinders. All
the models straightened upon inflation regardless of
initial fiber angle (6 = angle of fiber to long axis of
cylinder), and all e converged on 54°, the angle at
which the volume that can be contained in the sheath
is greatest. Notochord models with 6 > 54°
lengthened and narrowed as they straightened;
although they could push, the forces they exerted
were limited by their tendency to buckle, which
increased the greater the e. In contrast, models with
6 < 54° shortened and widened as they straightened
and showed pronounced increases in flexural
stiffness. The mean 6 of X. laevis early tail-bud
notochords is 54°.
IMPLICATIONS OF NOTOCHORD
ULTRASTRUCTURE ON THE FUNCTION
AND EVOLUTION OF THE VERTEBRATE
AXIAL SKELETON.
R.J. Schmitz Univ. of Wise. Stevens Point.
[email protected]
The cellular ultrastructure of the notochord has
been examined and compared in five vertebrate
groups. Cylindrical notochords are found in adult
lampreys, sturgeons and lungfishes. Surrounded
by a fibrous sheath (FS), the cellular notochord
medulla consists of vacuole cells (VC). The VC
are connected by desmosomes, and each vacuole
is enclosed by a dense cytoplasmic mesh of
keratin-like intermediate filaments (IF). IF mesh
thickness and vacuole size is variable from group
to group. The peripheral basal cells (BC) appear
to transform into vacuolated cells as they loose
contact with the FS. Rough endoplasmic
reticulum in the BC of the lamprey and sturgeon
suggests that these cells synthesize the
components of the FS. In teleosts, the notochord
is constricted by an amphicoelus centrum, and
there are numerous ultrastructural changes in the
notochord cells in the intervertebral space. In the
axolotl, the intervertebral space is filled with cell
dense fibrous cartilage and a vacuolated notochord
remnant with a thin IF meshwork and reduced
desmosomes. This study suggests that as the
vertebrate axis evolved and took on different
functions, the ultrastructure and function of the
notochord cells also changed.
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NOTOCHORD FUNCTION IN HAGFISH: HOW
STRUCTURE AND COMPOSITION IMPACT
BENDING PROPERTIES.
T.J. Koob*, J.A. Trotter and J.H. Long, Jr.
Shriners Hospital for Children, Tampa, University
of New Mexico, Albuquerque and Vassar College.
[email protected]
Bending tests indicate that the hagfish notochord
absorbs energy used to bend the body during
swimming. We investigated the physicochemical
and structural bases for the notochord bending
properties. The notochord in Myxine glutinosa is
a pressurized tube consisting of a central core of
large, vesiculated epithelial cells with extensive
cytoskeletal elements thoroughly linked by
desmosomes. A thin but dense collagenous sheath
surrounds and bounds the central cellular core.
Matrix proteoglycans are present only in the
sheath and core-limiting basement membrane. No
extracellular matrix is present in the core. Isolated
notochord segments exhibit swelling and
mechanical properties governed by the osmolarity
of the surrounding solution and the fixed charge
density of the core contents. A decrease in
osmolarity increases notochord stiffness whereas
an increase in osmolarity lowers notochord
stiffness. Removal of the core eliminates the
response to osmotic challenges. In contrast to
intervertebral joints in which the extracellular
matrix of the nucleus pulposus governs the
properties of the disc, the structural congruity of
the cells coupled with the osmotic properties of the
intracellular components govern the bending
properties of the notochord and thereby the whole
body bending properties in hagfish.
504
SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY
Function and Evolution of the Vertebrate Axis
506
505
RELATIONSHIP BETWEEN STRUCTURE AND
MECHANICAL FUNCTION OF THE TISSUES
OF THE INTER VERTEBRAL JOINT.
Hukins, D.W.L. Univ. Aberdeen, Scotland.
[email protected]
The adult mammalian spine will be used to illustrate how the gross anatomy and internal structures
of intervertebral disc, vertebral end-plates, spinal
ligaments and vertebral bodies are related to their
mechanical functions. Emphasis will be on the
structure and function of the human spine but the
talk will also include information obtained from
the rabbit and the sheep. The function and movement of the nucleus pulposus within the annulus
fibrosus of the disc will be described. Internal
movements within the disc have been investigated
by magnetic resonance imaging, computer modelling (finite element analysis) and by a combination of mechanical testing and video recording.
The annulus and end-plates are able to contain the
nucleus because of the pattern of orientations of
their collagen fibrils. X-ray diffrac-tion has been
used to measure the orientations of collagen fibrils
in these tissues. The outer lamellae of the annulus
merge into the vertebral bodies. Vertebral bodies
are light, because they are composed mainly of
cancellous bone, but are stiffened by an outer shell
of cortical bone. The vertebrae are also linked by
ligaments which may provide additional support
for the intervertebral disc. X-ray diffraction and
scanning electron microscopy studies of collagen
fibril orientations show how ligament structure
may be related to function. The zygapophyseal
joints are synovial joints which limit torsion about
the axis of the human lumbar spine.
ABSTRACTS
THE NUCLEUS OF THE INTERVERTEBRAL
DISC FROM DEVELOPMENT TO
DEGENERATION. Urban, J.P.G. Oxford Univ.
[email protected]
The nucleus of the intervertebral disc in humans
shows the most dramatic changes with age of any
cartilaginous tissue. In the foetus and infant, the
nucleus contains actively dividing and biosynthetically active notochordal cells. Proteoglycans
and other matrix components have a high osmotic
pressure, imbibe water and maintain a hydrated
structure which, though it has little mechanical
strength, has a high swelling pressure which
maintains disc turgor. In some species, the notochordal cells and the mucoid nucleus pulposus
persist throughout adult life. However at 4 years of
age in humans, the notochordal cells begin to disappear to be replaced -by chondrocytic cells of
unknown origin. These cells continue to produce
proteoglycans but also synthesise significant
amounts of collagen. The nucleus becomes firmer
and less hydrated and loses its transparent appearance. The cell density of the adult nucleus is very
low with cells occupying less than 0.5% of tissue
volume; each cell thus has to turnover and maintain a large domain of extracellular matrix. The
density of living cells decreases with age, possibly
because of problems with nutrient supply to this
large avascular tissue. Proteoglycan concentration
also falls, and nucleus hydration decreases markedly, the disc discolours through non-enzymic
glycation products and in many cases clefts and
fissures form. In most adults, the disc nucleus
degenerates eventually to a stage where it can no
longer fulfil its mechanical role.
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