Embrology - Day 0 7 Fertilization, Zygote, Blastocyst

Summary

This video is the first in a series exploring fertilization. It details the female reproductive cycle, ovulation, and the journey of a secondary oocyte. If not fertilized within 24 hours, menstruation occurs. If fertilized, the egg undergoes meiosis, forming a zygote that develops into a morula and then a blastocyst, shedding its zona pellucida before implantation into the uterus lining.

Takeaways

• 🧬 Ovaries are female gonads that produce eggs, which are released during ovulation in the menstrual cycle.
• 🩸 If the egg is not fertilized within 24 hours, menstruation occurs, where the endometrium is shed and expelled from the uterus.
• 🔬 The secondary oocyte is surrounded by the zona pellucida (ZP), a protective membrane with sperm recognition proteins called ZP3.
• ⚙️ Sperm must undergo capacitation and initiate the acrosome reaction to fertilize the egg by docking with ZP3 proteins.
• 🧪 Once fertilized, the secondary oocyte undergoes Meiosis II (Mei2), creating an ovum and a polar body, and the sperm genes combine to form a zygote.
• 👶 Cleavage is the process where the zygote divides from one cell to two, two to four, and so on, without increasing the total cytoplasm.
• 🧫 By day 4, cleavage results in a 16-cell mass called the morula, and by day 5, it becomes a 32-cell blastocyst.
• 🛡 The zona pellucida continues to protect the blastocyst until it sheds just before implantation on day 7.
• 🧩 A blastocyst, before implantation, is a hollow sphere with a mass of cells on one side, which binds to the endometrium for implantation.
• 🌱 Implantation of the blastocyst occurs after the zona pellucida is shed, enabling the embryo to attach to the uterine lining.

Q & A

What are the primary functions of the ovaries in the female reproductive system?

The ovaries are the female gonads responsible for producing eggs (oocytes) and releasing them during ovulation. They also produce female hormones like estrogen and progesterone.

What is ovulation and when does it occur in the menstrual cycle?

Ovulation is the process of releasing an egg (secondary oocyte) from the ovary, typically occurring mid-cycle, around day 14 of a 28-day menstrual cycle.

What is the zona pellucida (ZP) and what role does it play in fertilization?

The zona pellucida (ZP) is a thick protective membrane surrounding the secondary oocyte. It contains sperm recognition proteins (ZP3) that bind sperm, triggering the acrosome reaction needed for fertilization.

What happens if the egg is not fertilized within 24 hours?

If the egg is not fertilized within 24 hours, the menstrual cycle proceeds to menstruation, where the denatured endometrial lining is shed and exits through the vagina.

What is capacitation and how does it prepare sperm for fertilization?

Capacitation is the biochemical process that sperm undergo in the female reproductive tract, allowing them to become capable of fertilizing the egg. It involves changes to the sperm’s membrane and enhances its motility.

What is the acrosome reaction and why is it important?

The acrosome reaction is the release of enzymes from the sperm’s acrosome (a cap-like structure) after it binds to the ZP3 proteins of the egg. These enzymes help the sperm penetrate the zona pellucida to reach the egg for fertilization.

What is cleavage in the context of early embryonic development?

Cleavage is the rapid series of mitotic cell divisions that occur after fertilization, where the single-celled zygote splits into multiple smaller cells without increasing the overall cytoplasmic volume.

What is the morula and when does it form?

The morula is a solid ball of approximately 16 cells formed by day 4 of embryonic development after several rounds of cleavage. It will eventually develop into a blastocyst.

At what stage is the blastocyst formed and what distinguishes it from the morula?

The blastocyst is formed around day 5 of development when the morula cleaves into about 32 cells. Unlike the morula, the blastocyst has a hollow center and an outer cell layer that will participate in implantation.

What happens to the zona pellucida during implantation?

By day 7, the zona pellucida is shed from the blastocyst, which allows the blastocyst to attach to the endometrial lining of the uterus for implantation.