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Ground Substance of the Connective Tissues A Postgrad Med J: first published as 10.1136/pgmj.41.477.382 on 1 July 1965. Downloaded from POSTGRAD. MED. J. (1965), 41, 382 RECENT TRENDS IN THE BIOCHEMISTRY OF THE GROUND SUBSTANCE OF THE CONNECTIVE TISSUES A. G. LLOYD, B.Sc., Ph.D., A.R.I.C. Senior Lecturer in Biochemistry and Member of the M.R.C. Research Group for Studies on Connective Tissue and Lung, University College, Cardiff. THE UBIQUITY of members of the anatomical to the surrounding tissue, the fibres usually group loosely defined as connective tissues is being described as collagenous. As yet there a matter of long-standing record. The same is no clear indication whether such fibroses comment applies to the convention that, despite arise according to the partially established the differing biological roles which they are pattern of inflammatory response in which the required to play and their characteristically earliest recognisable biochemical changes in- different mechanical character at macroscopic clude alterations in the ground substance of level, the individual members of the connective connective tissues followed only subsequently tissue group exhibit certain basic similarities by de novo formation of additional fibrous which first become apparent at microscopic material. It is for this reason that the research level. A common biochemical approach is to group housed in the Department of Biochemis- develop these features of similarity to a point try is designed to assist in the clarification where an idealised composite of all connective of fundamental biochemical changes associated tissues can be devised. Use of such a model with general lung dust diseases from the aspect allows initial broad generalisations to be drawn. of disturbed connective tissue metabolism. Suchby copyright. On the other hand, the same model must sub- an approach must obviously take into account sequently be employed to establish not only the many recent developments concerning the morphological but fundamental biochemical nature and properties of the structural elements differences which exist between the tissues. of the connective tissues and in particular the A model connective tissue is usually con- intrinsic complexity of the ground substance. ceived as containing relatively few cells per unit volume compared to a highly cellular The Ground Substance organization such as liver and additionally differentiated from the latter by the presence There is now no doubt that histologicalhttp://pmj.bmj.com/ of a significant quantity of extracellular examination of connective tissues using con- material. This material is characterised by the ventional techniques provides a wholly incom- presence of organised fibrillar elements, com- plete and frequently artificial picture of the posed variously of the biochemically distinct complexity of the ground substance. It has even proteins collagen, elastin and reticulin, these been established that long-used methods of being embedded in an amorphous medium tissue fixation may even facilitate the extrac- termed the substance. tion of substance constituents ground ground (see on September 23, 2021 by guest. Protected In this context, it is fitting that laboratories Szirmai, 1963). Thus, even for relatively gross in the Cardiff area should be connected in histological observations such methods must studies on all aspects of pulmonary dust dis- be replaced by the use of modified tissue fixa- ease and in particular of coal miner's pneumo- tives (Quintarelli, Scott and Dellovo, 1964a) coniosis, progressive massive fibrosis and or alternatively frozen section techniques in silicosis. The need for such work is presented which 'fixation' is virtually carried out in the with considerable impact in the observations dye bath (Szirmai, 1963). Methods have be- of Gough 1(1947). 'Much 'has already been come available for differentiating some of the established about the statistical incidence and ground substance components by selective pathological characteristics of such dust dis- physico-chemical blocking and unblocking in eases, but their true aetiology is still a matter the presence of quaternary ammonium com- for conjecture (Policard, 1962). However, from pounds (Kelly, Bloom and Scott, 1963), by the simplified histological standpoint the human selective staining in the presence of solutions lung lesion in such conditions shows character- of varying pH or electrolyte concentration istically an increased 'fibrosis' when compared (Szirmai, 1963; Saunders, 1964; Quintarelli, Postgrad Med J: first published as 10.1136/pgmj.41.477.382 on 1 July 1965. Downloaded from July, 1965 LLOYD: Biochemistry of Ground Substance of Connective Tissues 383 Scott and Dellovo, 1964b) and by selective the polymer chains. As both carboxyl and enzymic digestion ,(Zugibe, 1962 a and b). sulphate groups are fully ionized at physiologi- However, such techniques continue to be the cal pH the resultant highly negative character province of the specialist in the field, rather is likely to have considerable bearing on the than methods of routine applicability. As such, functions attributed to the polymers in the much reliance must still be placed on native state. approaches based on the extraction of connect- The acidic glycosaminoglycans recognised to ive tissue components followed by their date fall into four classes, differentiated by characterisation using physical, chemical and features of chemical fine structure: biochemical methods. Even in this regard A. Polysaccharides having anionic character studies initiated during the middle of the last conferred solely by hexuronic acid constituents century and subsequently prosecuted with e.g., hyaluronic acid and chondroitin. increasing vigour are still the subject of con- B. Polysaccharides having anionic character tinuous contemporary reappraisal. resulting from the dual presence of hexuronic acid and O-sulphate groups e.g., chondroitin Polysaccharide-Protein Complexes of the 4-sulphate (syn. chondroitin sulphate-A), Ground Substance chondroitin 6-sulphate (syn. chondroitin sul- The Polysaccharides phate-C) and dermatan sulphate (syn. chond- Early studies on these materials invariably roitin sulphate-B, P-heparin). involved treatment of connective tissues under C. Polysaccharides having O-sulphate groups relatively vigorous conditions such as pro- as sole functional anionic entities e.g., keratan longed digestion with proteolytic enzymes sulphate (syn. keratosulphate). followed by extraction with strong salt solutions D. Polysaccharides characterised by the pre- or even dilute alkali. The resultant extracts sence of hexuronic acid, O-sulphate and N- yielded, by several methods of purification, sulphate residues e.g., heparan sulphates (syn.by copyright. varying amounts of one or more of a group of heparitin sulphates). Heparin and o-heparin carbohydrate high polymers. These have been may also be included in this class for the sake variously and synonymously termed acid muco- of completeness. polysaccharides, acid aminopolysaccharides or The compositions of each of these materials is more recently acid glycosaminoglycans (Jeanloz, shown in Table 1 and the major repeating 1960). In defining the purity of such materials periods of the polysaccharides from Classes A, early workers attached much significance to B and C are summarised in Figs. 1, 2 and 3 the absence of amino acid and polypeptide respectively. These should be considered to be residues from the polysaccharide preparations structures derived on the principle of best http://pmj.bmj.com/ and when demonstrated such 'contamination' analytical evidence. While there is little doubt was frequently attributed to inefficiency of that they account for the format of the major purification procedures. Structural studies (re- portion of the polysaccharide chains there are viewed by Jeanloz, 1963) on these compounds indications that some regions of the polymers reveal a pattern of similar molecular organiza- are not constructed in this way. Such evidence tion. Thus, in the main are will be dealt with later. It should also be molecular they relatively high weight polysaccharides having noted that variability of sulphate analysis is on September 23, 2021 by guest. Protected linear chains containing hexosamine residues. not an uncommon occurrence and that acid The latter may be either D-glucosamine or glycosaminoglycan preparations with sulphur D-galactosamine each occurring in a state in to nitrogen ratios both greater and less than which the amine grouping is blocked. The unity have been isolated. Conclusive data hexosamines alternate regularly in the polymer allowing the assignment of wholly acceptable chains with other monosaccharide residues repeating period structures for heparan which may be hexuronic acids, such as D- sulphates (and for heparin and o-heparin) glucuronic acid and L-iduronic acid or a has yet to be obtained (see Muir, 1964). hexose such as D-galactose. Aside from the significant contributions All of the polysaccharides are also poly- which they have made in structural work on anions, the negative charge arising either from the acid glycosaminoglycans, elegant studies the carboxyl group of the above uronic acids made by Professor Karl Meyer and his col- or from the presence of ester bound sulphate, leagues lay a foundation for the concept that or both. Accepted structures show the poten- connective tissues fulfilling distinctive biological tially charged groups arranged regularly along functions are also provided with differing Postgrad Med J: first published as 10.1136/pgmj.41.477.382
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