Human development days 1-4

Definitions & Key takeaways

Human development starts with fertilization on day one. This involves the fusion of an oocyte and a spermatozoon to form a single-celled zygote. During the next 36 hours after fertilization, mitotic division or cleavage takes place, leading to two cells (known as blastomeres). Series of cleavages continue, with the second cleavage giving four blastomeres, and eight blastomeres after the third cleavage.

Around day three following fertilization, we have a mulberry-shaped 16-celled mass known as a morula. At day four to five after fertilization, the embryo now has around 100 cells. It has a single layer of large and flat cells originating from its outer cell mass, which will later give rise to the placenta. There is also another part called embryoblast made up of 10 to 30 pluripotent cells, which originate from the inner cell mass. Later on, the embryoblast becomes the fetus.

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Human development begins with fertilization, which is the moment when a sperm and an oocyte, or egg, fuse to form a zygote, the seed of what will eventually grow into a human baby.
During sex, semen containing about 200 million spermatozoa, aka sperm, goes into the vagina. And that seminal fluid is alkaline, meaning it helps neutralizes the acidic vaginal fluids.
The sperm quickly make their way through the cervix and uterus, and into the fallopian tube, also called the uterine tube.
And eventually they enter the ampulla of the uterine tube and then the infundibulum, which flowers out right near the ovaries.
By this point, most of the 200 million sperm that entered the body during sex have died, either getting stuck in the vaginal mucus, lost in the cervix, or killed and absorbed by the white blood cells.
About a thousand lucky survivors 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.
That process is called capacitation. Once those protective layers are gone, the sperm can 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, whereas sperm are the smallest cells in the human body - only about 1/30th the size of the egg - and are long and thin.
The sperm makes its way past the to the corona radiata, a layer of follicular cells. It then makes its way to the zona pellucida, a layer of extracellular matrix of glycoproteins that protects the egg.
Only about 500 sperm cells make it to this layer. The zona pellucida is also called the jelly coat since it’s a clear jelly like covering over the egg, that lies over a layer of proteins known as zona pellucida sperm-binding protein 3, or ZP3 proteins for short.
As a sperm gets very close to the zona pellucida layer, an acrosomal reaction occurs which has two parts. First, the sperm releases acrosin which is a hydrolytic enzyme that bores a hole in the jelly-like layer of the zona pellucida.
After that, the sperm starts assembling actin proteins that fold out like a large protein crane that anchor and bind to the ZP3 proteins.
Once the sperm is anchored to the surface of the egg, the plasma membrane overlying the sperm and the egg fuse - this is called sperm-binding.
Now, in the egg, cortical granules which are like bags of enzymes like peroxidases. When a sperm binds to the egg, it’s not quite clear how, but somehow calcium levels start to rise in the egg’s cytosol.
This rising levels of calcium signals the cortical reaction which is when the granules fuse with the egg’s cell membrane, releasing enzymes into the space between the surface of the egg and the fertilization envelope.
Like a chemistry experiment - the granule enzymes get mixed in with glycosaminoglycans, water, and calcium ions to form a gel-like barrier called the hyaline layer.
This prevents polyspermy, and it ensures that only one sperm can fertilize the egg. So that’s how a single sperm out of the hundreds of millions successfully enter the egg - those are about the odds of winning a major lottery!
While this is all happening, the egg undergoes it’s meoisis 2, leaving one polar body which has barely any cytoplasm and undergoes apoptosis.
The surviving daughter cell ultimately forms the mature female ovum, or egg. The nucleus of this egg is called the female pronucleus, and it contains 23 chromosomes.
Now that lottery-winning winning sperm sheds its sugary protein coat completely, leaving it behind on the egg’s surface, and uses its tail and mitochondria to make one final short swim over to the female pronucleus, which is the nucleus of the egg, settling down beside it.
Once it’s at rest, the sperm’s tail and mitochondria detach and degenerate, and the sperm’s nucleus expands a bit as it becomes the male pronucleus.
Like dancers on a stage, the male and female pronuclei slowly move toward each other in the center of the egg. As they move together, a groove forms between them, and their individual nuclear envelopes dissolve.
The pronuclei release their genetic information, merging into a single nucleus leaving no barrier between the male and female chromosomes.
A structure made of microtubules called the mitotic spindle forms across the diameter of the cell, and it weaves the chromosomes into complementary pairs, aligning them at the equator of the cell.
Now the cell contains both maternal and paternal genetic information, it’s become diploid - a zygote has formed. This moment in embryonic development is called syngamy, where there’s a single cell.
but it only lasts a few hours before the process of cell division begins. All right, as a quick recap… fertilization occurs when a sperm successfully navigates through the vagina, cervix, uterus, and uterine tubes to meet the egg.
From there it makes its way through the corona radiata, the zona pellucida, and the oocyte cell membrane - to fuse with the egg.
The genetic materials contained within the nuclei of the sperm and egg combine to produce a brand new diploid cell called a zygote.