Studies on renal mucopolysaccharides

This thesis is an account of studies made on the distribution and nature of acid mucopolysaccharides in the mammalian renal papilla. The papilla was found to be the only region of the kidney which has an appreciable extracellular space with acid mucopolysaccharide enmeshed in connective tissue fibres. Animals are unable to secrete urine which is either more concentrated or more dilute than plasma unless the tonicity of this region is greater than that of plasma. In the first part of this work (Chapter 2) the extent of extracellular acid mucopolysaccharide was correlated with the ability of the species to conserve water.
It was found that the papilla could be sharply delineated by staining the acid mucopolysaccharide within it. This had the advantage over the less distinct cut surface appearances in that it was possible accurately to measure the length of the papilla in relation to that of the cortex and medulla in stained sections. An empirical relationship between the development of the papilla and the ability to secrete hypertonic urine was obtained for a number of species:
Umax = 1.74 (M′/C′) + 0.6,
where Umax is the maximum concentration in Osmols/litre that can be achieved in the urine; M′ is the length of the papilla, measured as the distance of the region containing extracellular acid mucopolysaccharide from the papillary tip towards the renal cortex; and C′ is the continuation of this line through the medulla and cortex. The correlation coefficient for the six species for which figures for maximum urinary concentration were known was 0.9. The significance of this expression is discussed in Chapter 2 in relation to a similar equation put forward by Black in 1965.
The morphology of the connective tissue in the papilla of the guinea pig was examined by light and electron microscopy (Chapter 3) and by histochemistry (Chapter 4). The histological results were correlated with the electrophoretic and staining characteristics of acid mucopolysaccharides extracted from this region.
There was very little collagen of the adult type to be found in the papilla. The main fibrillar component was reticulin which was well demonstrated by silver impregnation and the periodic acid - Schiff technique. Within the reticulin mesh another class of PAS positive fibril was found which, in addition, stained positively for anionic groups. These fibrils could not be impregnated with silver. In the electron microscope the second class of fibril was seen not to be banded and to be smaller than the reticulin fibrils.
The histochemical results (Chapter 4) showed that the largest part of the amorphous extracellular acid mucopolysaccharide in the papilla was hyaluronic acid. In addition, two quantitatively small fractions were observed in the biochemical extracts which were associated with the extracellular acid mucopolysaccharide. Both these fractions stained metachromatically with thiazine dyes and were shown to incorporate radioactive sulphate. In sections metachromatic extracellular material was confined to the tip of the papilla: In this region of high in vivo osmotic pressures the interstitial cells also contained monochromatic granules. The exact nature of these two fractions was not elucidated, nor was an attempt made to identify the intracellular granules with its synthesis or destruction.
The results from the morphometry of the kidneys of fourteen species (Table 2/1) showed that desert species have long papillae which contain a limited amount of connective tissue. The metachromatic staining of extracellular acid mucopolysaccharides extends further towards the cortex in those species which can secrete highly concentrated urine. On the other hand, the papillae of animals which habitually produce dilute urine were squat in outline and contained abundant connective tissue. The tip of the papilla in these species was not found to stain set metachromatically. The extent of metachromasia was thus found to be related to the development of high tonicity in vivo, and thus to be most prominent in that part of the papilla where the sodium concentration is greatest in life.
It was observed histologically that acid mucopolysaccharide was associated with the lateral spaces between collecting tubule cells in the guinea pig. It had been suggested by Ginetzinsky in 1958 that the material found between these cells in the rat constituted the barrier between fluid in the collecting tubule lumen and the interstitium. He postulated that antidiuretic hormone released hyaluronidase from the apex of the collecting tubule cell and that the enzyme then digested the intercellular cement permitting tubular fluid to equilibrate osmotically with the interstitium.
The material between collecting tubule cells the guinea pig was extracted and information about its nature was obtained from its behaviour in electrophoresis and from its staining characteristics. It was found to be a very soluble acid mucopolysaccharide which contained sulphate and PAS positive groups, both of which features distinguish it from hyaluronic acid. The histological technique had thus to be modified to preserve as much as possible of the material in the thin (3 μ) wax sections required for the optimal demonstration of this region in the light microscope. The fixative developed to give the most satisfactory retention of acid mucopolysaccharide at the same time as preserving cytological structure reasonably well was as follows:..
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