Human development days 1-4

Human development begins with fertilization, which is the moment when a sperm cell and an oocyte (or egg cell) fuse to form a zygote, the seed of what will eventually grow into a human baby.

During sex, semen containing about 200 million spermatozoa (or sperm) enters the vagina.

This seminal fluid is alkaline, which means it’s capable of neutralizing acidic vaginal fluids.

The sperm quickly make their way through the cervix and uterus and swim into the fallopian tubes, which are also called the uterine tubes.

Eventually, these millions of sperm enter the ampulla of the uterine tube and then the infundibulum, an opening which flowers out next to the ovary.

By this point, most of the 200 million sperm that entered the body during sex have died for numerous reasons: some got stuck in the vaginal mucus, others ended up lost in the cervix, and the rest were killed and absorbed by the white blood cells.

About a thousand lucky survivors are left to wait in the uterine tube for the egg to arrive.

As the sperm wait, they start to rub up against the walls of the uterine tube, and that helps them remove the protective glycoprotein coat and plasma membrane covering the acrosome, a cap-like structure covering what you might think of as the sperm’s head. This process is called capacitation.

Once these protective outer layers are gone, the sperm are able to secrete an enzyme called hyaluronidase which can break down hyaluronic acid, a major component of the extracellular matrix protecting the egg.

Now, the egg is the largest cell in the human body, big and round, the size of a grain of sand.

As you’ll soon see, it’s kind of like an onion, as it’s made up of many layers.

The sperm trying to enter and fertilize this big egg are the smallest cells in the human body—about 1/30th the size of the egg—and they’re long and thin.

The most intrepid sperm make their way past the extracellular matrix surrounding the egg to a deeper layer called the corona radiata, which is made up of follicular cells.

The sperm then make their way through the corona radiata to the zona pellucida, another layer of extracellular matrix made of glycoproteins, which protects the egg. Only about 500 sperm cells make it this far!

The zona pellucida is also called the jelly coat, since it’s a clear, jelly-like covering wrapped around the egg.

The jelly coat/zona pellucida lies over another layer, this one made up of a protein called zona pellucida sperm-binding protein 3, or ZP3 protein for short.

As sperm close in on the zona pellucida, they undergo a process called the acrosome reaction, which happens in two parts.

First, the sperm release acrosin, a hydrolytic enzyme that bores a hole in the jelly-like coating of the zona pellucida.

After that, the sperm start assembling actin proteins, which fold out like a large protein crane, anchoring and binding the sperm to the ZP3 proteins.

Once the sperm is anchored to the surface of the egg, the plasma membrane overlying the sperm and the egg begin to fuse together—this is called sperm-binding.

Now, the egg contains cortical granules which are like bags of enzymes, one being peroxidase.