How do chicken eggs get fertilised?

"How do chicken eggs get fertilised, when there's a shell around them?", asks Amelia. Developmental biologist Afnan Azizi from KCL and The Francis Crick Institute cracks open the science of chicken development...

Afnan - The egg shell is made up of calcium carbonate or calcite, the same stuff that limestone and marble are made of. The whole function of the egg shell is to protect the developing embryo of the chicken from danger, especially from bacteria and parasites. So, a rooster’s sperm definitely can’t get through the shell to fertilize the egg. But it can before the shell forms. The chicken’s reproductive tract is about 75 cm long and each part of it is specialized to form the egg to the final shape that we see it. Like the female members of many other animal species, including humans, each chicken is born with all the eggs that she’ll ever lay, fertilized or not.

Just to clarify when we say an egg “cell”, in biology, we mean the cell that is contributed by the mother and fuses to the sperm to create a zygote. And the zygote is the very first cell that has the genetic information from the mom and the dad. It then divides many, many times to make a new animal. So, in a chicken’s ovaries, at any given time, egg cells and yolks.
And the yolk is a food storage system because if the egg cells fertilise, the mother hen can’t actually feed the growing embryo like human mothers can. So at each reproductive cycle, the most mature yolk with its attached egg cell is dropped into a specialized pouch called the infundibulum (rather a mouthful!) where it stays for up to half an hour. This is the only chance a sperm will get to fertilize the egg.

So if a rooster has mated with a chicken and its sperm has managed to swim the approximately 70 cm distance to combine with the egg cell, then you could get a fertilized chicken egg. After this, the rest of the process is the same for fertilized and unfertilized eggs. The yolk is covered by a layer of albumen, or egg white, and that is all wrapped in a couple of really thin membranes. The outer one has a network of regularly spaced protein fibers where the calcium carbonate crystalises in a process called calcification. A process that is very similar to what happens in bone development, but is much faster when making the egg shell. The chicken gets the calcium from its rib and leg bones (that have to then be replaced by calcium from food) and the carbonate from the carbon dioxide it would normally breathe out. The whole process takes about 26 hours and half an hour after a chicken lays an egg, the process can start again. Interesting fact: chickens can store a rooster’s sperms for up to two weeks and fertilize multiple eggs after mating only once.

Now it's good to know that majority of eggs we get from supermarkets and grocery stores aren’t fertilised because the farmers limit mating of egg laying chickens. But even if the egg were fertilized, we need an optimal temperature of 37 degrees Celsius to grow. And anything below room temperature would completely stop its development. So it's very unlikely to get a fertilised egg from most commercial sources. And even if one does make it to your kitchen, it's very unlikely to develop unless you put it in an incubator.