The formation and the fine structure of the components (cuticle, porous calcitic shell, shell membranes and limiting membrane) of the integument of the hen's egg were studied in detail with Scanning Electron Microscopy or, when appropriate, Transmission Electron Microscopy, in order to build up a detailed picture of the physical, antimicrobial systems of the egg pre- and post-oviposition. This phase of the study revealed that (i) the limiting membrane is present on the egg in the magnum, (ii) the interstitial spaces of the shell membranes in the oviduct are filled with amorphous material which probably condenses to form the mantle on the individual fibres of the membranes and (iii) the cuticle at oviposition is immature and mechanically weak. The immature cuticle is an ineffective barrier to bacterial penetration for a few minutes post-oviposition. When the components of the integument were considered as resistances in series, the mature cuticle - especially that on brown eggs - was shown to be the major barrier to the movement of water in the liquid but not the vapour state. Consequently this structure plays an important role in protecting the egg contents from infection. Work needs to be done in order to overcome this resistance - differences in partial pressure caused by immersing warm eggs in a cold solution overcame the cuticle's resistance. The efficacy of this method was enhanced by reducing the surface tension or "degassing" the water. Of the many methods used to assess cuticle quality, staining was the only one that would appear to be of any use at the moment in a breeding programme that sought improvements in the quality and hence effectiveness of the cuticle as a barrier to infection. The porosity of the shell, as measured by GH2O, was not associated with water uptake by eggs. Nevertheless the pore contributed to the waterproofing, and hence antimicrobial defence of the egg, by imposing such a resistance to water movement that the osmotic potential of the albumen to pull water across the shell membranes was negated. The shell membranes proved to be ineffective barriers to microbial movement. The contribution of the limiting membrane to the egg's defence could not be assessed with the techniques used in this study. The environment within the shell membranes was selective for Gram-negative bacteria. Indeed the membranes appeared to offer a haven for microorganisms that had passed across the shell, especially in infected eggs held under conditions recommended for hatching eggs before transfer to incubation temperature. In practice viable organisms persisted in these structures whereas contaminants of the underlying albumen died out especially with incubation temperatures approaching the body temperature of the hen. Heavy microbial challenges (>106/egg) to the shell membranes resulted in heavy contamination of the albumen within two days of infection. It was deduced that contaminants of the albumen failed to multiply unless they made contact with the yolk. The above observations were discussed in the context of practices that should be considered by hatchery managers who seek to interrupt the transfer of Salmonella from the breeding to the rearing flock by egg treatment. The identification of the immature cuticle state directs attention to the management and hygiene of nest boxes and the primary role of the cuticle in the eggs antimicrobial defence. It suggests that breeding programmes ought to be concerned with the quality of the cuticle throughout the laying cycle. The influence of temperature on the behaviour of contaminants in the shell membranes suggest also that treatment of eggs with germicides to control Salmonella transmission needs to be done.
|Date of Award||1985|