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Ovulation to Implantation: Oocyte's Path through Uterine Tubes

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  • 0:07 Uterine Tubes
  • 2:50 Fertilization
  • 4:26 Cell Division
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

Follow the female's egg as it leaves the ovary and travels down the uterine tube on its way to the uterus - all with hopes of meeting up with a male's sperm along the way.

Uterine Tubes

Step right up! Come one, come all. The greatest ride on earth - follow the female's egg as it leaves the ovary and makes its long-awaited trip to the uterus. How will it end? Will the egg be fertilized and grow into a baby, or will it miss its chance? Which path will it take?

Everybody ready? First stop: the ovary. This is where it all begins - where the female's egg begins its trip through the female reproductive tract. At puberty, the ovary starts recruiting immature eggs, sending them down the path of maturation. Once mature, the egg is released from the ovary during ovulation. Notice I said 'egg' - singular? That's because usually only one egg is released per month. And, even though this egg is mature, its DNA is paused in metaphase of meiosis II. That's the stage when all the chromosomes are lined up in the middle of the cell.

Fingerlike projections called fimbriae are located at the uterine tube opening
Uterine Fimbriae.

Around day 14 of the ovarian cycle, the egg, also called an ovum, or an oocyte, is released from the ovary during ovulation, and it falls into the opening of the uterine tube. This opening (that part right there) has all these tiny fingerlike projections called fimbriae. The fimbriae aren't actually attached to the ovary, so as the egg leaves the ovary, the cilia (those are those really small, tiny, hairlike projections) on the fimbriae have to catch the egg and guide it into the opening. Sounds a little risky, doesn't it? But that's just another one of those wonders of biology.

Once inside the uterine tube, the egg starts its trek down the uterine tube. It's not an easy journey, though, seeing as the egg has no real way of facilitating its own movement. Therefore, it has to depend on the cilia and smooth muscles of the uterine tubes to move it along its path. Along the way, it can take one of two paths. It can either be fertilized by sperm traveling up the uterine tube, or it doesn't get fertilized.

Let's move on and first take a look at what happens if the egg doesn't get fertilized. If the egg fails to meet up with an eligible sperm, then about 24 hours after it has been released from the ovary, it'll start to deteriorate and it'll break down before it finishes its trek to the uterus. Okay, that's pretty simple, right? Now, let's move on and look at what happens if the egg does meet up with an eligible sperm.

Cilia help propel the egg through the uterine tube
Uterine Tube Cilia

Fertilization

Here is where it gets interesting. Our egg moves down the pathway towards fertilization, searching out the most eligible sperm for the job. Now, if fertilization is to occur, the sperm have to be on time! They need to arrive around 12-24 hours after the egg has entered the uterine tube. It's the race of a lifetime, the race to be first sperm to reach the egg, the one to cross the finish line first and fertilize the egg!

The first sperm to reach her burrows as quickly as he can through her outer membrane. When this happens, the membrane surrounding the egg, called the zona pellucida, undergoes changes that prevent other sperm from entering the egg. That way only one sperm can fertilize the egg.

It takes about 30 minutes for a sperm to penetrate the egg, give or take a few. This triggers the egg to complete meiosis II. Next, the membrane of the sperm and egg then fuse, allowing their DNA to combine. This begins the cell replication process. Without successful combination of the male and female DNA, cell replication couldn't occur. And, seeing as we humans are made up of billions and trillions of cells, this is a very important process. Believe it or not, all the cells in your body stem from the single-celled zygote produced when the DNA of the sperm and the egg combine.

Cell Division

So, I bet you can guess what comes next - cellular division! That's right, once the zygote is formed, the process of cell division begins. First, the zygote divides into two connected cells about 30 hours after fertilization. From there cell division continues. Each of those two cells divides into to two more cells, creating a total of four. Then those four each divide to produce eight, and so on, with each division occurring about every 12 hours. But, even though it's increasing in cell number, the overall size of our fertilized egg stays the same.

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