Sweating has a variety of functions in mammals including pheromone action, excretion of waste products and maintenance of the skin surface ecosystem. In a small number of mammalian species, which includes humans and the Equidae, it also has an important role in thermoregulation. This review is focused specifically on the thermoregulatory role of sweat in Equidae and the causes of sweating failure (anhidrosis). The first part describes the glandular appearance, sweat composition, and output rates; and considers the latest theories on the glandular control and secretory mechanisms. It is concluded that the glands are not directly innervated but are controlled by the interplay of neural, humoral and paracrine factors. The secretory mechanism is not as simple as previously thought and is mediated by the dynamic interaction of activating pathways, including autocrine control not only of the secretory process but probably also of secretory cell reproduction, growth, and death.

Ultrastructural examination of sweat glands from the human loin before and during heat-induced activity indicated that the sweat is formed from the contents of disrupted cells as well as from the products of secretion. The principal secretory processes appear to be fluid transport and exocytosis of vesicles. However, configurations suggesting microapocrine secretion were also observed. It is concluded that the mechanisms involved in sweat production in man are fundamentally similar to those in animals and the terms 'apocrine' and 'eccrine' should be discarded. The myoepithelial cells which were contracted at the onset of sweating appeared to be under less tension after 3 h of continuous activity.

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K E Y w ORDS. Cryoquenchant, propane, freeze substitution, Freon 22, ice crystal damage, cooling rates. SUMMARY Three .approaches were taken with the aim of defining the optimum conditions for rapid cryopreservation in liquid quenchants. In a theoretical approach, two mathematical models were used. The first is of value in defining the absolute maximum rates of cooling which could be achieved at various depths in the tissues. The second highlights the poor thermal properties of liquid coolants and therefore emphasizes the essential requirement for vigorous quenchant mixing and rapid specimen entry.Experimental work with thermocouples showed that fastest cooling rates occur at the leading edge of the object entering coolant. Of five liquid quenchants investigated, cooling rates were in the order, propane > Freon 22 > Freon 12 > liquid nitrogen slush > liquid nitrogen. Other considerations, however, may affect the choice of quenchant. For a given quenchant, cooling rate is maximal near the equilibrium freezing point. The consequences of quenching in the presence of thermal gradients either within the coolant or in the gas layer above it are shown. Cooling rate was found to be approximately proportional to entry velocity at least up to N 2 m s-1 in our system. Stereological analysis of rapidly quenched, freeze-substituted tissue samples, of geometry which imposed an approximately unidirectional heat flow, revealed four zones : (i) a narrow surface layer ( N 10 pm) of low image contrast and apparent absence of ice crystals; (ii) a zone of enhanced contrast with ice crystals whose size increased rapidly with depth from the surface (the 'slope'); (iii) a sharply defined zone (the 'ridge') of maximum ice crystal size beyond which there is (iv) an extensive 'plateau' with smaller ice crystals and no marked increase in size with depth. The 'ridge' of maximal ice-crystal damage was consistently found but varied considerably in depth from the surface ( N 25-120 pm) between samples. The existence of the deeper plateau region of relatively uniform ice-crystal-size may be of significance in X-ray microanalytical studies of physiological processes at some depth from the sample surface.In terms of our present understanding of the quenching process, the conditions for optimal cryofixation of small tissue samples are listed.

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Using arginase and hydroxyproline as biochemical markers, the yields and homogeneity of separated epithelial and stromal tissues from surgically removed benign hyperplastic prostate glands have been assessed. On the basis of these markers, about 30% of epithelial and 95% of stromal tissues were recovered. Dehydroepiandrosterone sulphate sulphatase activity was found predominantly in the epithelium, whereas testosterone 5alpha-reductase activity was predominantly in the stroma.

The present finding of the absence of structural defects in the glands indicates that future studies should concentrate on the investigation of neurohumoral or secretory cell metabolic abnormalities.

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