The egg – the chick’s
insurance
policy
by Professor Sally E. Solomon, senior
research fellow, University of Glasgow
Veterinary School, Scotland.
our mother built this house, she
stocked it with food and water and
what did you do? – ate the lot, hung
around for three weeks and trashed the
place when you left! Stroppy teenager?
Candidate for reality TV? No, the normal
chick from embryo to hatch.
The maternal investment in both the chick
embryonic chamber and its contents is
prodigious. It is to the credit of the species
that even under conditions of severe stress
that the bird will attempt to achieve its purpose in terms of the reproductive process –
the production of an egg.
The title of this paper includes the words
‘insurance policy’. In human terms this can
be interpreted as a pre-investment against
eventualities in times of need. With reference to the chick, perhaps the time of need
manifests itself to the greatest degree in
those days immediately following hatching.
Therefore if during development, the chick
is nutritionally satisfied, protected adequately from bacterial ingress, buffered
against mechanical trauma and enabled to
escape easily from its hard shell - then it
should be able to withstand the immediate
insults of the external environment.
Y
Egg formation
In the sexually mature hen the active ovary
contains a plentiful supply of ova which, as
they accumulate lipoproteins manufactured
in the liver, swell in sequence. This phenomenon is termed ‘follicular hierarchy’.
Under normal circumstances only one
ovum leaves the ovary at any one time but
double ovulations happen and such eggs
from layer birds are a welcome addition to
the breakfast table.
In broiler breeders, however, feed restriction is normal practice to minimise the
occurrence of such a phenomenon.
At fertilisation, the mature ovum released
from the ovary enters the funnel shaped
portion of the upper oviduct (the infundibulum). Here it is penetrated by motile sperm
which have moved from the lower part of
The active ovary.
the reproductive tract and lodged themselves temporarily in the infundibular sperm
storage glands.
These nutrient rich glands maintain sperm
viability for some time and ensure the
sequential fertilisation of ova with each
cycle. The very low density lipoproteins
which are found within the yolk reach the
oocyte via the circulation. During embryonic
development lipid is transferred from the
yolk to the embryo by active metabolism
within the yolk sac membrane.
Albumen
The fertilised ovum surrounded by its protective capsule then enters the longest
region of the oviduct, the magnum. Over a
period of approximately four hours it
becomes enveloped in a multilayered semiShortened fibres in an egg with watery
white.
International Poultry Production — Volume 17 Number 6
gelatinous protein/fluid composite. This
mixture has many properties ranging from
bacterial inhibition (both chemical and physical) to the provision of an adequately firm
base for shell membrane deposition through
its complement of ovomucin.
Egg white is the main source of water for
the embryo, although during early embryonic growth, the water volume in the yolk
also increases. The yolk is maintained in a
central position within the egg white by
virtue of an anchoring mechanism called the
chalazae.
As the egg rotates on its journey down
the magnum these are formed from the
inner part of the egg white which is next to
the yolk. The range of proteins present
within egg white is still being studied. Many
are specific to the albumen whilst others are
also found in other parts of the egg, for
example lysozyme which is found both in
egg white and within the shell itself.
Egg white is multilayered and in the freshly
laid egg the long protein strands are fairly
regularly arranged. This pattern can be disrupted by disease as seen when ‘watery
white’ occurs, then the fibres are shortened
and random in their organisation.
During embryo development the pH of
albumen changes, a feature which is considered to encourage the dissolution of the
shell. At the same time the egg white
becomes less viscous caused by a natural
breakdown in the morphology of the fibres
of thick white.
The shell membranes
The lower magnum is covered with cells
containing highly sulphated mucosubstances.
As the yolk and albumen move from this
area through the narrow aglandular zone
and into the isthmus, they adopt the characteristic egg shape.
Over a period of 60-75 minutes both the
gland cells and surface epithelial cells lining
the isthmus secrete the membranes. The
paired shell membranes are structurally distinct layers. The inner surface of the inner
layer is a continuous, amorphous sheet and,
as such, acts as a barrier to prevent migration of the egg contents.
Continued on page 9
7
Part of the inner surface of the inner layer. At the edge of the
shell this region is smooth.
Continued from page 7
The outer membrane is described as a
fibrous reticulum: each intersecting fibre has
a protein core and a carbohydrate mantle.
This close interwoven structure ensures
that as the shell forms, calcium salts are prevented from moving inwards. Together the
membranes constitute the greatest proportion of protein in the shell. Data show that
membrane thickness decreases with increasing bird age and also varies during the laying
year.
In addition to their role in the process of
shell formation the membranes, with their
unique constituent type X collagen, must
respond to the process of ‘plumping’. The
addition of this nutrient-rich fluid, prior to
the main phase of shell formation stretches
the shell membranes and puts them under
tension.
At the same time, this process exposes the
countless nucleation sites which originate on
the outer surface of the outer membrane to
the process of calcification.
The shell
Over a period of 20 hours in the shell gland
pouch the egg is plumped, calcified within an
organic framework, pigmented and overlaid
with an organic cuticle. Plumping fluid originates from the gland cells lining the pouch.
Failure to plump will leave the membranes
in a flaccid or wrinkled state which will ultimately affect the appearance and texture of
the shell when fully formed.
The process of shell formation is identical
in both layers and breeders, although the
functional requirements of the breeder egg
put a greater demand on shell structure.
The ‘true’ shell (the part of the shell without its membranes attached) is multilayered.
It is an organic/inorganic complex with the
inorganic part consisting of calcium carbonate in its calcite modification.
The calcium used for shell formation
comes from the diet and from the cyclical
breakdown of medullary bone.
The organic component which originates
in the oviduct varies in its composition
The fibrous reticulum with attached calcium salts.
according to its location within the shell and
has been implicated both in directing crystal
growth and signalling the end of calcification.
Nucleation sites on the outer shell membrane attract calcium ions from the oviducal
fluid in which the egg is bathed and this
forms the mammillary layer.
As individual mammillae grow and fuse
they form the palisade columns which make
up most of the true shell. On top of this a
vertical crystal layer is formed and this has
been shown to contain the bone protein
osteopontin.
Prior to the egg leaving the hen via the
cloaca the shell acquires the pigmented cuticle. The cuticle partially blocks the thousands of gas exchange pores which originate
within the mammillary layer and extend
through the entire thickness of the shell.
According to the literature pore numbers
are matched to metabolic demands, and
recent work has revealed the presence of
antibacterial proteins in the cuticle.
Over a period of 21 days the embryo will
grow inside the egg using up the nutritional
resources, then the shell will thin and the
fully formed chick will emerge. The egg,
however, as a chamber is neither consistent
in its form nor in its ability to satisfy the
needs of the embryo.
If liver function is impaired it will modify
The cuticle with a central open gas
exchange pore.
International Poultry Production — Volume 17 Number 6
yolk development, likewise disease and
stress can cause breakdown of the oviduct
and so alter albumen quality and the
integrity of the shell membranes; and if
these foundation layers are substandard
then shell quality will be negatively affected.
The policy – all risk?
In the worst case scenario every part of the
embryonic chamber may be compromised
and the chick will fail to hatch – but it does
not take complete system failure to impair
development!
Each aspect of egg formation is designed
to provide either nutrition and/or protection, such that even one defect can reduce
the value of this ‘insurance policy’
Returning to the beginning – can the functional properties of the shell and its contents
be positively influenced by dietary intervention and can such intervention influence a
wide range of egg parameters or are there
limits to the extent and nature of improvement?
In recent years, evidence has accumulated
to support the role of antioxidants in creating a beneficial cellular environment.
Carotenoids have been implicated as protective agents against oxidative damage during hatching and early post hatch.
Organic selenium in the form of Sel-Plex
organic selenium (Saccharomyces cerevisiae
CNCMI-3060, Alltech) has also been shown
to play a crucial role in antioxidant defence.
Its role in egg formation has been the subject of critical review and improvements
have been recorded in sperm quality and
sperm penetration, Haugh units and shell
membrane elasticity. In a recent study of
shell quality in breeder birds over one laying
year, shell structure was maintained at the
end of lay and hatchability improved when
birds were fed Sel-Plex.
The premium for this antioxidant insurance policy may be a little higher than budgeted for but with regard to selenium the
cover is extensive and if the hatched chick
experiences less oxidative stress pre and
post hatch, the cost is more than justified. ■
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