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It has been shown that the outer root sheath—inner root sheath boundary is not as smooth as was previously thought and that the Flugelzellen of Huxley's layer contain trichohyalin granules. Also, the β-keratose fraction obtained from oxidized wool and hair has been shown to be of dual origin—partly intra- and partly intercellular. Finally, a suggestion is put forward concerning the phosphorus content of wool. The presence of fibrillar material in the keratinizing hair cuticle has been demonstrated for the first time, and this makes possible a restatement of the concepts of fibrous protein formation in the hair follicle.

Results obtained by physical methods of examination, particularly X-ray diffraction, polarizing and electron microscopy and more recently differential thermal analysis, are reviewed. The different structures of the elastic wall of the disc (annulus fibrosus) and the gel-like centre of the disc (nucleus pulposus) are described in relation to their functions. An important component of aged nucleus pulposus is a substance giving an X-ray diffraction spacing of 4·7 A, presumably a denatured protein in the β-form, but its origin is still obscure. The value of differential thermal analysis in the study of collagen of the disc and other connective tissues is illustrated.

THE shrinkage temperature of the collagen of dura mater or fascia lata has been shown to increase with age1. Shrinkage of these tissues and tendon or skin can be watched and a micro-melting point apparatus was used for their determinations. The transition can also be detected from force–temperature curves, and use of this method has shown an increase of shrinkage temperature with age for tendon collagen, except when measurements were made at pH less than 4.5. If the fibres were maintained at constant length during the heating, the shrinkage temperature showed little variation with age (or pH)2–4.

Publisher Summary This chapter discusses the structure and function of the human intervertebral disk and the three major components: (1) the bony end plates, (2) the gel structure of the nucleus pulposus, and (3) the elastic wall of the anulus fibrosus. It describes idealized disk mechanism and the factors associated with the elastic behavior of collagen in the wall of the disc. On the assumption that the nucleus pulposus behaves as an incompressible gel, the basic elasticity of the disk can be understood. From the end plates of the vertebral bodies, the pressure is transmitted to the nucleus. If there is no change in the volume of gel, then a uniform pressure is developed on the disk wall. The crystalline component of the wall has a high aqueous content, and the sheets of collagen fibrils can slide over one another without opening the structure appreciably. The cross-striation in the anulus has the merit of producing a tissue system with the strength per unit thickness comparable in all directions of strain. However, it must be realized that any variations in the aqueous content of the crystalline lattice must have an effect on the ultimate strength of the individual sheets of collagen. Any differences in wall thickness must play a major part in determining the extension of the wall even if a uniform pressure is applied to the inner collagen sheets.

There is evidence that degradation of the proteoglycans in the intervertebral disc occurs on aging and similar changes appear to be associated with herniation or prolapse (Davidson and Woodhall, 1959; Lyons et al., 1964, 1966). It is not known whether a defect in biosynthesis or increased catabolism is responsible for the deterioration. Human discs obtained at surgery have been shown to contain β-N-acetylglucos-aminidase and β-glucuronidase (Platt, 1970), which were probably derived from lysosomes but this was not proved.

THE nucleus pulposus of the normal human intervertebral disk consists of collagen fibrils dispersed in a gel of polysaccharide and protein complexes. During ageing collagen fibrils are deposited as a matrix showing increasing orientation with age as seen from X-ray diffraction investigations1. In the case of specimens of more than 70 years of age, the normal photograph shows with the diagram of oriented collagen a clearly defined ring of spacing 4.7 A., which is similar to that associated with a denatured β-protein1 The presence of other components in the disk, for example, calcium phosphate, etc., will be discussed in a later publication. It was postulated from consideration of earlier work1 that the soluble protein molecules normally found in a folded form in the nucleus had unfolded and formed a hydrogen-bonded lattice which gave rise to this spacing.

A cellulose-protein complex is reported as a normal although minor constituent of mammalian connective tissue; higher concentrations have been observed in certain pathological human skin conditions. Experiments on the degradation of collagen by treatment with alkaline buffers have afforded histochemical evidence for the production of highly anisotropic fibres. Chemical and physical studies show that these fibres consist of a protein-polysaccharide complex, the polysaccharide fraction of which is indistinguishable from native cellulose, arranged in helical form round a protein template. The question of fibrogenesis is discussed in the light of synthetic studies whereby anisotropic fibres having similar properties to those of native mammalian cellulose fibres can be obtained by the interaction of gelatin, chondroitin sulphate and calcium ions.

THE nucleus pulposus of normal human intervertebral disk contains the two acidic mucopolysaccharides, chondroitin sulphate and keratosulphate1–3. During ageing an increase of collagen occurs4 leading to a decreased ratio of mucopolysaccharide to collagen, although the absolute amount of mucopolysaccharide remains constant4.

In attempting to explain the different physical characteristics of collagen in various tissues, a number of possibilities have been proposed. These include variations in glycosaminoglycans, more firmly associated “structural glycoproteins” (Robert and Robert, 1967) and collagen hexose contents (Spiro, 1969; Grant et al., 1969). The location of the hexose on specific hydroxylysine residues may hinder the formation of intermolecular cross-links (Morgan et al.. 1970), possibly imposing a limit on fibril diameter. Analyses of different layers of bovine skin indicate that coarser fibres are characterised by an increased dermatan sulfate: hyaluronic acid ratio (Hoffman et al., 1957). This arises mainly from a deficiency in hyaluronic acid because, contrary to previous statements, the dermatan sulfate remains approximately constant (Pearson, 1970). Keratan sulfate may be implicated in the formation of very thin fibrils found in more transparent tissues like cornea and nucleus pulposus.

Meyer and Rapport1 described the isolation from hyaline cartilage of two chondroitin sulphates, A and C. The only differences so far reported2 between these are in the specific optical rotation and alcohol solubility of their calcium, salts. Orr3 found that chondroitin sulphate from cartilage gives an infra-red spectrum which suggests that the product is a mixture of two isomers, A and B (see below). By counter-current electrophoresis he was able to obtain the faster component of this mixture, which exhibited in the 700–1,000 cm.−1 region 3 bands, at 928, 855 and 725 cm.−1, respectively, and was called the A isomer. The B isomer, on the other hand, was isolated alone in a well-defined form from the nucleus pulposus and showed two bands at 825 and 775 cm.−1, respectively. Meyer et al. 2 pointed out the similarity between Orr's B isomer and chondroitin sulphate C. On the other hand, both A types show the same optical rotation and origin. The following results seem to give a different picture of the mucopolysaccharides of cartilage and nucleus pulposus.

CHANGES in the glycosaminoglycans of the human intervertebral disk with ageing have been reported by a number of authors (see Antonopoulos1 for recent work). On the other hand, there have been few investigations of the protein components, although they make up the larger part of the tissue. The collagen content has been determined in the central portion of the disk2 (the nucleus pulposus) at different ages. Similar analyses have been made on the nucleus pulposus and on the surrounding annulus fibrosus3. X-ray diffraction4 has indicated that at least one of the non-collagenous proteins present in the nucleus pulposus increases with age, but except for a brief reference by Hallen5, chemical data on these components are lacking. In the present investigation the effects of ageing on the non-collagenous protein nitrogen and on the tyrosine, most of which also derives from the non-collagenous proteins6, were investigated in addition to the collagen.