The results presented in this thesis confirm that in brown eggs laid by
the domestic hen the intensity of colour depends on the amount of
protoporphyrin-IX in the egg shell. This pigment which constitutes about
42% of shell porphyrin, is concentrated in the cuticle, the thin
proteinaceous layer which covers the shell.. Although the calcareous shell
also contains pigment, its concentration, due to the larger mass, is vastly
reduced in comparison to the cuticle, and therefore its contribution to the
depth of colour is negligible. For this reason, the cuticle was used in
this study to indicate the degree of shell colour intensity.
Variability in shell colour between eggs laid by individual hens is
extremely large. In addition, in eggs from most hens, there is a decrease
in shell colour as the laying cycle progresses. The loss of colour is
attributed to a reduction in porphyrin content of the egg shell which is
expressed in the cuticle. Overall, the lack of persistency in colour
appears to be a less serious problem than was previously supposed, but it
may still merit consideration in breeding programmes for dark shell colour.
Furthermore, it was observed that darkly coloured eggs contained more shell
than lighter ones, and it is suggested that selection for dark shells may
be of practical value when wishing to improve shell strength.
While the cuticle was deposited on the shell at an even rate during the
final stages of shell formation, within the shell gland pouch, this is not
so for the pigment associated with it. By 3 hours before oviposition, 77%
of the cuticle has been formed. Pigmentation, on the other hand is slow up
to this point, after which time around 81% is added to the cuticle during
the final 3 hours prior to oviposition. During this time, the depletion of
shell gland tissue porphyrin corresponds to the uptake of porphyrin by the
Oral administration of a porphyrin-inhibiting drug, Nicarbazin, reduced
egg shell porphyrin content by more than 75%. The loss of pigment more
closely corresponds to the magnitude in the reduction in shell gland tissue
porphyrin, during treatment, than to the porphyrin content in the blood.
Finally, the biochemical processes of egg shell pigmentation and this
relationship with cuticle formation are discussed.