1. Why does forming a thin eggshell take about 20 hours?
Chickens are said to lay an egg every 24 to 25 hours, producing roughly 300 eggs per year [9]. The average time for one shell to form is about 20 hours [1]. This means the hen spends most of her day building a new shell inside her body. Reports indicate that calcium carbonate is actively deposited between approximately 10 and 22 hours after ovulation [2]. During this window, 5 to 6 grams of calcium carbonate settle in the uterus.
If you multiply 5 to 6 grams by 300 eggs, that is a simple calculation of about 1.5 to 1.8 kg per year [9]. This is an estimate based on the annual count, not an exact figure, but it shows the amount of material behind the thin shell. The speed is also surprising. Mineralization in the uterus is said to be one of the fastest forms of mineral formation in the biological world [3]. It corresponds to a rate that would use up all the calcium in blood plasma in about 12 minutes. Note that 12 minutes is not the time it takes to make a shell; it is just a rough guide for how fast calcium moves through the blood.
2. The shell membrane forms before the shell itself
When you peel a boiled egg, you find a thin skin on the inside of the shell. This is the shell membrane. Electron microscope observations report that this membrane is made in the isthmus of the oviduct and is completed just before it moves to the uterus [4]. Fibers overlap gradually, shaping themselves at the start, middle, and end of the isthmus. This means that in the uterus, where the shell is built, crystals accumulate on top of a finished membrane.
Does this order have a specific advantage? The sources do not state why this order is beneficial. What is known is simply that the sequence is: first the membrane, then the shell. So, where do the crystals begin to grow on the membrane?
3. How shell crystals start growing from small points on the membrane
It is reported that shell crystals begin to grow from clumps of organic matter (mammillary cores) gathered on the shell membrane [5]. From there, a cone-shaped mammillary layer forms, and above it, a layer of column-like crystals (the palisade layer) extends. Most of the shell consists of calcium carbonate crystals (calcite), while the rest is organic matter like proteins that stabilize the crystals [1]. Proteins are said to link the crystals and regulate how they grow.
Shell formation is explained in three main stages: first, creating crystal nuclei; second, active growth where crystals build up at a steady rate; and finally, a stage where crystal growth is suppressed. It is reported that proteins working in the uterine fluid change with each stage [6]. However, most of the shell building proceeds during times when calcium from food is scarce.
4. Where does the calcium for the shell come from at night?
A single shell uses about 2 grams of calcium [7]. Since hens do not eat at night, little new calcium enters from food [7]. The missing amount needs to be supplemented. One source is bone. Laying hens develop a temporary bone called medullary bone, and it is said that 20 to 40 percent of the shell’s calcium comes from this bone [7]. Medullary bone has many blood vessels, irregular crystal orientations, and is quickly formed and dissolved while eggs are being made. It appears when estrogen levels rise during sexual maturity [7].
Another source is the gizzard. During the night, coarse limestone grains remaining in the gizzard are slowly dissolved by stomach acid and absorbed as calcium in the intestine, according to reports [7]. Calcium reaching the blood passes through uterine cells to the shell-building site. Cells primarily take it up using energy [7]. The detailed mechanisms are too complex for this article.
5. The cuticle: a thin protein film on the outside of the shell
After laying, the outside of the shell is covered by a thin protein film called the cuticle. It is only a few micrometers thick (1 micrometer is one-thousandth of a millimeter) and covers more than 7,000 pores [10]. The cuticle serves to prevent the entry and exit of bacteria, dust, and moisture. Pigments for the shell are reported to be released from surface cells in the uterus into uterine fluid, then deposited onto the shell [8]. Even white-egg hens may have pigment materials. For brown eggs, the amount of pigment released in the uterus is said to be regulated.
6. How to see the membrane and shell thinness with a boiled egg
You can verify this by making a boiled egg. Let it cool completely before peeling it. Look for the inner membrane. Can you peel it off gently with your fingers? The egg is hot, so please wait until it cools down to handle it safely.
Also, look closely at the broken edges of the shell. You can feel how thin it is, around 0.3 mm [9]. Be careful not to cut your fingers on the sharp broken shell pieces.