title: Studies in the secretory function of the skin of ruminants, with special reference to the sebaceous glands of cattle
Creators: name: Smith, Miranda Evelyn
Publisher: name: University of Glasgow
Year: 1975

description: The work described in this thesis was initiated to investigate the physiology of bovine sebaceous glands, to endeavour to elucidate some of the factors which affect their output and mode of activity and hence to establish the function of bovine sebum. A method to determine sebum output in the thoracico-lumbar region of Ayrshire calves was developed. Sebum was collected from pre-cleaned areas of skin after a given period of time, usually 3 h, using a specially designed cup in conjunction with the solvent methanol. The collected sebum was then extracted from the methanol and weighed on a micro-balance. 2. No evidence was found to support the hypothesis that sebum flows from areas of high to low concentration on the skin of cattle indicating that the integrity of the surface lipid layer is dependent on local sebum production. 3. After cleaning the skin with methanol, the rate of sebum output decreased with time and after 24 h there was no appreciable increase in the weight of sebum on the skin surface. Thus the sebum layer when removed experimentally takes over 24 h to be completely replaced. Sebum output measured over a period of 3 h appeared to be lower in winter than throughout the rest of the year but this reduction just failed to be significant. 5. Sebum output over a period of 3 h was not statistically different between animals of different ages, but tended to be higher in castrated males than in females. 6. Short-term changes in the environment had no consistent effect on sebum output. However, on prolonged exposure to a warm environment there was a significant increase in sebum output. 7. The composition of bovine sebum was similar to that of other species in that it contained phospholipids, free cholesterol, unesterified fatty acids, triglycerides, diester waxes, cholesteryl esters and squalene. However, more triglyceride was present than was found in a previous study on cow skin surface lipid (Nicolaides et al, 1968). Most of the linoleic acid present in cattle sebum was found in the triglyceride fraction. 8. Short-term exposure to different air temperatures and humidities had no appreciable effect on the fatty acid composition of bovine sebum. At a low humidity, however, the cutaneous output of palmitic acid was higher. On prolonged exposure to a warm environment, a higher output of sebum occurred together with an increase in the percentage of linoleic acid in the sebum. 9. Although the precise function of bovine sebum is unknown it is concluded that it has similar properties to that from other species. Thus the role of bovine sebum is to form a natural barrier - layer which aids in the thermoregulation of the animal, the prevention of water loss from the epidermis and acts as a deterrent against skin disease. The bovine sebaceous glands, unlike the sweat glands, are not of major importance in the regulation of body temperature in the heat. 10. Sebum output and sebaceous gland volume and cell number per m2 of skin were measured after stimulation of the glands by cleaning the skin with methanol and compared with estimates of glandular mitotic activity. None of these quantities was altered by a single or by repeated stimulation even though the number of cells necessary to produce the sebum by a holocrine mechanism greatly exceeded the estimated level of cell production. Sebum output was not associated with the DNA content of skin washings but sweat output was; the DNA was probably derived from sweat. It was concluded that the mode of secretion of cattle sebaceous glands is unlikely to be holocrine. It is suggested that sebum secretion may be a more complex mechanism than previously supposed. Sebum may not be solely a product of cell degeneration but could conceivably be produced by a process involving lipogenesis and secretion from live cells.