Eggs are developed in the oviduct and uterus. 2-3 hours prior to ovulation, the hormone levels of progesterone and luteinizing hormone increase for a short time. The first ovulation of a chicken happens in the morning right after sunrise. Each subsequent ovulation occurs ~15 minutes later. After the delay has increased to several hours, the hen will not lay an egg for a day or two. Then she ovulates at sunrise again, and a new egg-laying period begins.
|The formation of an egg. (c) Unknown|
Ovulation is the release of an ovum. Nearly 500 000 immature ova exist in the ovaries at the time of hatching. 300-500 of them ever mature. Maturation of each ovum takes months, during which the ovum grows from 8 to 37 mm and 0,08 g to 15-18 g. Each day one ova fully matures into a follicle. The follicle develops into a yolk, which consists of darker and lighter layers, a latebra and a germinal disc. Yolk is mostly water, protein (16 %) and fats (33 %), and it is surrounded by a yolk sac. Originally the contents of the yolk were produced in the liver of the hen, and transported into the ovaries by blood. The formation of egg yolk is called vitellogenesis.
During ovulation the yolk is released and dropped into the funnel-shaped beginning of the oviduct. The funnel is called infundibulum. Infundibulum is the site of fertilization, if the chicken has been artificially or naturally inseminated. The yolk remains approximately 18 minutes in the infundibulum, before it moves forward in the oviduct.
The next phase of the oviduct is the magnum, a site where the egg-white is developed around the yolk. Egg-white consists of two thick and two thin layers. Unlike yolk, egg-white is mostly water and proteins. The thick layers form two chalazae, a pair of twisted "strings" which keep the yolk in the center of the egg. This protects the yolk from touching the egg shells and thus from being contaminated by bacteria and microbes. Both thick and thin layers of egg-white protect the yolk, which, if fertilized, would develop into a chick.
After magnum the yolk and egg-white move on to isthmus. In the isthmus two protective, thin layers are created on top of the egg-white. The inner shell membrane is attached to the egg-white, and the outer will be attached to the egg shell. An air bubble forms between the two membranes. As the egg is laid and stored for consumption, it will dry, and the contents shrink. The air bubble can expand, so the shrinking membranes do not break the egg shell.
So far the egg formation has lasted approximately 5 hours. The last phase is forming the shell. The shell is formed in the uterus. It is uncertain whether the egg shell is pigmented in the uterus or already in the isthmus, but generally brown chicken breeds lay brown eggs. Formation of the hard shell takes nearly 20 hours. When the egg is finished, it is moved from the uterus to the vagina. Oviposition, or laying the egg, occurs in 10 minutes. Only an hour after the oviposition a new ovum matures and the next yolk is released into the infundibulum.
Structure of the egg
|(c) Encyclopaedia Britannica|
The yellow color is caused by carotenoids received solely from chicken feed. The pigments in the feed can be either natural (paprika, pumpkin, shellfishes, etc) or synthetic. The color of the yolk is simply an esthetic quality factor, and has no impact on the nutritional value of the egg.
|Contents of yolk and egg-white.|
The outermost layer of an egg is the hard shell, which is covered by a waxy layer called a cuticle. Cuticle protects the air ducts, which are formed between hard calcite shards. The cuticle is only 10 µm thick and very sensitive. If an egg is washed, polished or otherwise handled, the cuticle will rub off and the egg will spoil. All eggs are collected and sold to consumers as they are, without any washing. Otherwise the shelf-life of eggs could be reduced to less than a day. If the cuticle is intact, eggs can be stored in cool temperatures for months without any spoilage. The egg shell allows oxygen inside the egg, and carbon dioxide and water out from the egg.
Quality of eggsThe quality of eggs is controlled in many ways. The components of quality can be divided into three sections:
- Inner quality: blood or meat spots, size of the air bubble, quality of the yolk, firmness of the albumen (Haugh-value)
- Outer quality: egg size, cleanliness, qualities of the shell, form of the egg
- Other quality factors: microbiological quality, taste, odor, nutritional content
|Blood spot. (c) The poultry site|
Physical methods for determining the quality of an egg include measuring the thickness of the albumen in an broken egg and measuring the egg's corrected weight by sinking the egg into salt solutions of different concentrations. The shape of the egg is described as the ratio of its length (from the blunt end to the sharp end) and height.
|Sandpaper egg (c) The poultry site|
Read more online from Optimum Egg Quality: A practical approach in the Poultry Site.