Page 551 - Anatomy and Physiology of Farm Animals, 8th Edition

536 / Anatomy and Physiology of Farm Animals

Germinal disc
Egg white layers (albumen):
VetBooks.ir Outer (thin), 23% Yolk
Dense (thick), 58%
Inner (liquid), 16% Yolk membrane
Chalaziferous
(thick), 3%
Air cell

Shell
Chalazae
Figure 30-12. Basic internal structure of a typical egg. Source: adapted from Reece, 2009. Reproduced
with permission of John Wiley & Sons, Inc.

Egg candling, shining a beam of light Approximately 95% of the total weight of an
through the intact egg to view the interior, eggshell is calcium carbonate. Plasma cal-
is used to determine the size of the air cell, cium levels decrease during the period of
and this is one measure of egg quality. eggshell formation and then rebound before
The mineral component of the eggshell, the next period of eggshell formation begins.
primarily calcium carbonate, is deposited During periods of active eggshell formation,
within the mammillary core and matrix calcium is added to the blood by intestinal
layers. The mammillary cores and matrix absorption and mobilization of medullary
layers contain proteins, carbohydrates, bone. If dietary calcium is adequate, the
and mucopolysaccharides, which provide a majority of the necessary calcium can be
framework for the deposition of calcium obtained during this period by intestinal
carbonate crystals (i.e., calcification). absorption. However, a net loss of medul-
Several different proteins that contribute lary bone calcium will occur during the
to this framework have been identified, hours of active eggshell formation, even
and experiments suggest that the appro- with adequate calcium intakes. Replacement
priate production of these proteins by the of the lost medullary bone can be done with
shell gland is a key factor in determining calcium gained by intestinal absorption
shell quality. Gene expression studies indi- during the hours that active eggshell forma-
cate that the production of these proteins tion is not occurring. Vitamin D stimulates
is influenced by hormonal factors, such as both the intestinal absorption of calcium
progesterone, and that rates of gene expres- and transport of calcium by the shell gland
sion vary greatly as the egg passes through (uterus). Parathyroid hormone also pro-
the shell gland. The outermost cuticle layer motes calcium transport by the shell gland
contains lipid, prevents evaporation, is for eggshell formation.
water repellent, and functions as a barrier Osteoclasts are responsible for the
to microorganisms. mobilization of calcium from medullary
Eggshell formation in the uterus (or bone, so the increased mobilization
shell gland) of the domestic chicken during eggshell formation reflects an
requires about 17 to 20 hours. During this increased osteoclastic activity. Osteoclasts
period, approximately 2 to 2.5 g of calcium also mobilize calcium from lamellar cor-
are transported from the blood by the tical bone throughout the skeleton, and a
uterus and deposited in the eggshell net loss of calcium from cortical bone
as calcium carbonate crystals (calcite). does occur in high‐producing laying