Human development days 1-4

62,407views

00:00 / 00:00

High Yield Notes

8 pages

Flashcards

Human development days 1-4

of complete

Questions

USMLE® Step 1 style questions USMLE

of complete

An experiment is performed to understand the steps of fertilization. In one study, a researcher removes cortical granules from the egg. Which of the following is expected to be observed during the fertilization process of this egg?  

External Links

Transcript

Watch video only

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.

Summary

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.

Elsevier

Copyright © 2024 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Cookies are used by this site.

USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.

RELX