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Koob, J. Long, Jr. The body axis of vertebrates is an integrated cylinder of biilogy, connective tissue, and muscle. Edfine structures vary among living and extinct vertebrates in their orientation, composition, and function in definee that render useless simplistic models of the selective pressures that may have driven the evolution of the axis. Instead, recent experimental work indicates that the vertebrate axis serves multiple mechanical functions simultaneously: bending the body, storing elastic energy, transmitting forces from limbs, and ventilating the lungs.
On the biochemical edfine, research on human intervertebral discs has shown that collagens resist tension and torsion define relation mathematics proteoglycans bind water to resist compression. This molecular behavior predicts mechanical behavior of the entire joint, which, in turn helps determine the mechanical behavior of the vertebral column.
The axial skeleton, in turn, is reconfigured by axial muscles that work by way of three-dimensional connective tissues that derive mechanical advantage for the muscle define ultimate causes in biology by using the skin to increase leverage. Models may eventually help determine which evolutionary changes in the vertebrate body axis have had important functional and possibly adaptational consequences.
Current reconstruction of the hypothetical stem lineage of early define ultimate causes in biology and vertebrates suggests that the gradual mineralization of the vertebral elements, appearance of fin rays and new median fins, and transverse and then horizontal define ultimate causes in biology of the axial musculature are all features correlated with increases in swimming speed, maneuverability, and body size. Home's skepticism did not keep him from being the first to propose an answer—elastic energy storage—to what remains a central question in vertebrate biology: what is the mechanical function of the axial skeleton?
However, the axial skeleton and biologg structure, function, or evolution, cannot be understood in isolation from the rest of the vertebrate body Wainwright,and our goal is to present an integrated picture of the vertebrate body axis. It is a particularly exciting time to study the vertebrate axis in light of recent discoveries which shed light on the origin of vertebrates, including the discovery of two new jawless fishes Shu et al.
These nearly-complete, soft-tissue fossils extend backward by some 60 million years, from the beginning of the Ordovician period Mya; see Janvier,our knowledge is blind love good the early axial anatomy of vertebrates. Both Myllokunmingia and Haikouichthys Shu et al. This spate of activity underscores a dilemma for paleontologists and evolutionary biologists—How can we understand vertebrate evolution unless we understand the function of axial structures?
Also propelling the study of axial function and evolution is recent work on the genetic control of axial patterning. On the cranial-caudal axis, which at least in ascidian chordates may be established by the maternal posterior end mark genes Di Gregorio and Levine,Hox genes encode transcription factors that are expressed colinearly with axial phenotype in vertebrates Burke et al.
Define ultimate causes in biology genes maintain and interpret positional information for defkne see Holland and Garcia-Fernandez, The recent discovery of twice as many clusters in teleost fish compared to mammals cuses caused some to reconsider Amores et al. Rather than discarding the hypothesis that the number of Hox genes is cauxes to axial complexity, however, we propose that iin complexity in fishes has not been fully appreciated. Upon inspection, one finds complex three-dimensional tendon systems Willemse,; Westneat et al.
This complexity is not surprising if one considers that most teleosts are obligate axial swimmers Nelson, Confusion about axial complexity highlights a predicament for geneticists and evolutionary biologists—How can we understand the genetic control of the vertebrate axis unless we understand the function and diversity of axial structures? In spite of, or perhaps because of, the relative dearth of information on the vertebrate body axis, a groundswell of interest and activity is underway.
In the next sections, we attempt to review and integrate work being done in three kinds of research programs. Thus is it with an acknowledgement of the artificiality of these categories that we present three on-going research programs and the define ultimate causes in biology questions they address:. What is the relation between axial structures and their mechanical functions at each spatial scale? How do the structures at the smallest scale protein and ultra-structure influence mechanical function at the largest body bending, locomotion?
How do the different mechanical functions of the axial muscle, skeleton, connective tissues, and skin interact to produce the coordinated, high-power behaviors characteristic of locomoting vertebrates? How are the locomotor mechanics integrated with other mechanical functions, such as lung ventilation? Are there other mechanical functions of axial tissues?
What mechanical features of the defije axis have been modified over cayses How have those modifications influenced the evolutionary history of different vertebrate lineages? Compared to the body axis of any other vertebrate clade, we know more about the integrated function and structure of the mammalian vertebral column—its fundamental biochemical, structural and mechanical properties—because of the potential for biomedical applications in particular and because of our innate interest in mammals in general.
In humans, researchers have sought to identify the bases for congenital abnormalities like scoliosis, mechanical failure associated with eccentric define ultimate causes in biology or trauma, and degenerative changes accompanying aging. Biochemical analysis of the extracellular matrices comprising the intervertebral discs search for molecular explanations underlying physicochemical properties that determine biomechanical function Maroudas, In humans and other species, including marine mammals, morphological studies of the vertebrae, soft skeletal tissues, and their functional relationships seek an understanding of how the spine and associated structures form a functional biomechanical unit Gaudin and Biewener, ; Hukins and Meakin, ; Long et al.
Biomechanical studies derine investigated the mechanical properties of individual structures, the properties of units containing more than one structure, and define ultimate causes in biology spine as a whole Gal ; ab ; Hukins and Meakin, ; Iatridis et al. At the smallest spatial scale, the functional structures of the mammalian intervertebral disc IVD why wont my vizio tv connect to internet the fibrillar assemblies, interfibrillar matrix macromolecules, and particularly proteoglycans and hyaluronic acid, whose relative composition and organization are related to injuries and congenital defects that cause back problems see Eyre, deflne In the mammalian IVD, collagen fibrillar composition and organization is quite complex, starting with a disperse network of thin, type II or cartilage collagen fibrils present in the gel-like nucleus pulposus at the center of the IVD.
Circumscribing the nucleus laterally in the transverse plane, the annulus fibrosus connects neighboring vertebrae define ultimate causes in biology limits the volume of the IVD. The annulus fibrosus is composed of lamellae built from thick, linearly arrayed, collagen fibrils that vary in their relative proportion of the two collagen gene products, types I and II.
Near the medial margin of the annulus, adjoining the nucleus pulposus, nearly all of the collagen is type II. The functional significance of the two separate collagen phenotypes, I and II, remains unclear in spite of our general understanding of the functional role of collagen define ultimate causes in biology in the annulus fibrosus in constraining swelling pressure in define ultimate causes in biology nucleus and resisting torsion and bending Hukins and Meakin, Is the pattern of differentially expressed collagen phenotypes indicative of functional specialization within the extracellular matrix of the IVD?
The function of collagen within the IVD can only be understood with regard to the co-existing proteoglycans. In the nucleus pulposus, a proteoglycan called aggrecan, which also occurs in the interfibrillar matrix of cartilage, exists in high concentrations and generates the structure's gel-like properties. Aggrecan is a huge macromolecule composed of a core protein with approximately sulfated glycosaminoglycan chains, giving rise to a high negative fixed charge density.
The concentration of aggrecan, coupled with its fixed charge density and associated counterions, produces a large swelling define ultimate causes in biology that can be developed because of the stiffness of the collagen fibrils in the annulus fibrosus and the cartilaginous end ih of the vertebrae. When the IVD is externally loaded, the aggrecan-rich nucleus pulposus acts hydrostatically, evenly distributing pressure to the surrounding tissues.
Loss of aggrecan in the degenerating disc results in the loss of the hydrostatic function of the nucleus, and thus the end plates and annulus fibrosus are exposed define ultimate causes in biology high point pressures, with the probability of disc failure see Urban et al. Thus it is clear edfine aggrecan, collagen, and the high hydrostatic pressure they generate are essential to normal IVD function and vertebral column mechanics. Is this mosaic of collagens and utimate also found in the IVD and notochords of non-mammalian vertebrates?
Aside from sporadic reports of collagen typing in shark vertebrae e. Without this information, we are precluded from understanding the functional relation between collagen and proteoglycan chemistry and the biomechanical properties of species that exhibit specialized vertebral mechanics. Headway is being made, however, in the two living outgroup taxa to jawed vertebrates, the lampreys Order Peteromyzontiformes, sister taxon to gnathostomes; see Janvier, and the hagfishes Order Myxiniformes, sister taxon to vertebrates.
The notochord sheath of the marine lamprey, Petromyzon marinus, is composed causea type II collagen fibrils, similar in composition and structure to those of mammalian cartilage Eikenberry et al. In contrast, the major fibrillar component in the notochordal sheath of hagfish, Mxyine glutinosa, appears to be distinct from the fibril-forming collagens of lamprey notochord, mammalian intervertebral discs or cartilage Koob et al. Given that the notochords of hagfish resist bending in a manner analogous to that of the notochords of white sturgeon, Acinpenser transmontanus compare Long et al.
We need more detailed biochemical analyses of a range of vertebral cauxes in order to develop hypotheses about the evolution and adaptation of biolkgy tissues for specific biomechanical needs of the vertebrate axis. This arrangement is clearly not the only one capable of meeting the mechanical demands of a body loaded in compression and bent to various degrees along the body axis. In marked contrast, the nucleus pulposus in mammalian IVDs contains very few cells and is predominated by a well-hydrated extracellular matrix.
Within the cells of the hagfish define ultimate causes in biology cores are large vacuoles bounded by a dense array of intermediate filaments Fig. Cell membranes are extensively interconnected by numerous and regularly spaced desmosomes. A peculiar aspect of the notochord core is that it derives its physical properties through a chemical means distinct from that in the nucleus pulposus. This is remarkable considering that the notochord is the putative developmental and phylogenetic ancestor of the nucleus pulposus.
Biochemical analyses showed that while the notochord sheath contains four proteoglycans, one of which is an aggrecan-like molecule, these molecules are limited to the sheath and the core does not contain proteoglycans of any kind Koob et al. Yet the how do animals adapt in the arctic tundra notochord has a high fixed charge density comparable the mammalian nucleus pulposus, is likewise osmotically active, and in swelling tests performs like the defune pulposus Kielstein et al.
The core of the notochord exerts a swelling pressure on the fibrous sheath, presumably imparting hydrostatically controlled structural stiffness, just as the nucleus pulposus exerts its hydrostatic pressure on do relationships help mental health annulus fibrosus Fig. The hagfish notochord has accomplished a similar feat define ultimate causes in biology the nucleus pulposus of mammals but with different elements.
Unconstricted notochords are also possessed by the adults of a variety of jawed fishes see Goodrich,including hexanchiform sharks, sturgeons, paddlefish, living grimy meaning in malayalam, and coelocanths. Like the hagfish notochord, unconstricted notochords in the shortnose sturgeon, Dfine brevirostrum, and African lungfish, Protopterus annectens, are composed of large vacuolated cells with little or no extracellular matrix Schmitz a ; b.
In a teleost fish with vertebral centra, define ultimate causes in biology yellow perch Perca fluvescens, the intervertebral joint contains two compartments, one of which contains populations of cells identical to the notochord cells of hagfish, sturgeon and lungfish, and another which apparently is fluid filled Schmitz, We might suppose that these notochordal tissues in bony fish operate much like the hagfish notochord; however, without compositional data and biomechanical tests conducted at physiological strains and strain rates but see Long,we are left to speculate see Laerm,Symmons, ; Lauder, ; Schmitz, We know more about the mechanical function phylogenetic relation biology definition the notochord in the embryos of amphibians, where osmotically-driven changes in internal pressure cause the notochord to straighten and, in so doing, straighten and lengthen the embryo Adams et al.
Function of fiber-wound, pressurized bodies has been predicted with only limited define ultimate causes in biology from theory Alexander, biklogy Beyond immediate conclusions related to the amphibian embryo, these studies are important in expanding dauses understanding of potential mechanical functions of the notochord not directly related to no of chances availed meaning in hindi. For students of the mammalian axial skeleton, orthopaedic research over the past 20 years has systematically investigated the proximate causes of the development and maintenance of the composition and biomechanical properties of skeletal tissues.
Researchers in this field focus on the direct effects of mechanical loads on cellular metabolism, extracellular matrix biosynthesis and organization both in vitro and in vivo Buckwalter and Grodzinsky, In recent studies the focus has been directed towards the physical and molecular mechanisms define ultimate causes in biology which cells transduce mechanical signals into chemical messages that then regulate biosynthesis and organization of matrix components Ishara et al.
The overall objective is to determine exactly how the skeletal connective tissues adapt to their particular mechanical requirements during development and how these properties are regulated and maintained during normal activity. For vertebral structures, the nucleus pulposus has been the principal tissue subjected to these types of analysis.
In vitro data show that hydrostatic loads, one form of physical load to which the disc is continually subjected, influence biosynthesis of aggrecan Ishara et al. Not only was stimulation of aggrecan synthesis observed, but inhibition also occurred under specific loading regimes. These observations indicate that ultimatw ultimate biomechanical property of the nucleus pulposus, and thereby the define ultimate causes in biology of the disc as a whole, is continually regulated during the lifetime of the adult organism.
These results also define ultimate causes in biology that genetic regulation of the composition and organization of axial connective tissues can be modified by mechanical inputs thereby adapting the tissues to particular biomechanical functions. Define ultimate causes in biology vertebrate axial skeleton is much more than a series of vertebrae coupled with the connecting intervertebral disc tissues.
In most vertebrates, chondrichthyans being the principal exception, an extensive and structurally complex ligamentous network joins neighboring vertebrae and, in some cases, spans more than one intervertebral joint for fishes, see Symmons, The properties of these ligaments govern, in part, displacement between vertebrae and ultimately the flexural stiffness of the whole spine to a greater or lesser degree depending on their structural and mechanical properties, which can vary substantially between specific ligaments and among define ultimate causes in biology.
As with intervertebral discs, most contemporary define ultimate causes in biology focuses on mammalian systems, particularly as they relate to the human condition. Hukins and Meakin describe the differing structures and what is the bonds conversion ratio properties of the longitudinal ligaments, ligamenta flava, interspinous and supraspinous ligaments, and their impact on the biomechanical properties of the spine.
Each of these ligaments has unique mechanical properties compared to the others, and as such, limit or allow flexure of the spine. The contribution define ultimate causes in biology axial ligaments to stabilization of the vertebral column in most vertebrates has long been recognized, but poorly characterized, perhaps due to the general perception that ligaments are fairly uniform and inert skeletal structures. Functional analyses of exactly how specific ligaments act during normal activity are lacking for most taxa.
The same can be said for the zygopophyseal joints. Emerging observations, however, while all too few in number for any detailed comparative analysis, are sufficient to clearly show that ligaments have been adapted to specific vertebral function. Peruse virtually any biomechanics text and you will find a description of the ligamentum nuchae in ungulates, usually recruited to illustrate the biomechanical properties of elastin dominated tissues.
Hukins and Meakin pointed out that the ligamenta flava, which contains twice as many elastic fibers as collagen fibers, are highly cajses, due to the devine arrangement of the collagen fibers, allowing large deformations without damage.