Human fertilization and early development | High school biology | Khan Academy

Summary

This video discusses human fertilization and early development, highlighting the competition among millions of sperm to fertilize an egg. It explains the formation of a zygote, the packaging of DNA in chromosomes, and the transition through stages from zygote to morula and then to blastocyst. The instructor details the changes occurring in the embryo, the distinction between embryo and fetus, and the growth trajectory leading to a viable baby. This fascinating journey emphasizes the complexity of human development from a single cell into a fully formed organism.

Takeaways

• 😀 Fertilization occurs when a sperm cell successfully meets and merges with an egg cell.
• 🏆 Sperm cells face intense competition, with hundreds of millions racing to fertilize the egg.
• 🧬 A zygote is formed after fertilization, containing a complete set of 46 chromosomes from both parents.
• 🔬 Gametes (sperm and egg cells) each contain 23 chromosomes, half the normal amount found in other cells.
• 📈 The zygote undergoes rapid cell division, starting with mitosis and increasing the number of cells exponentially.
• 🌱 After a few days, the zygote becomes a morula, which is a solid ball of cells.
• 🔄 The morula develops into a blastocyst, a hollow structure with 200-300 cells, within about five to nine days post-fertilization.
• 👶 The developing organism is called an embryo until approximately the 10th to 12th week, after which it is referred to as a fetus.
• 🌟 Key developmental features begin to emerge in the embryo around seven weeks, including limbs and facial structures.
• ⏳ Full-term pregnancy typically lasts 36 to 42 weeks, with viability (chance of survival) reaching about 50% around six months.

Q & A

What is fertilization in human beings?

Fertilization is the process where a sperm cell from a male human merges with an egg cell from a female human, resulting in the conception of a potential human organism.

How small are the sperm and egg cells involved in fertilization?

Sperm and egg cells are extremely small, with a distance of about one-fiftieth of a millimeter separating them at the moment of fertilization.

How many sperm cells compete to fertilize an egg?

Typically, several hundred million sperm cells are released during ejaculation, all competing to fertilize a single egg cell.

What are gametes, and how many chromosomes do they contain?

Gametes are the reproductive cells—sperm cells from the father and egg cells from the mother. Each gamete contains 23 chromosomes, half the number found in regular body cells.

What is a zygote?

A zygote is the fertilized egg that forms when the sperm and egg combine, containing a full set of 46 chromosomes, or 23 pairs.

What is the significance of pronuclei in fertilization?

Pronuclei are the nuclei of the sperm and egg before they fully fuse, marking the initial stage of genetic combination after fertilization.

What stages follow the zygote in early development?

After the zygote, the cell undergoes mitosis, splitting into two, then four, and eventually forming a morula (16 cells) and then a blastocyst (200-300 cells).

Why is gestational age measured from the last menstrual cycle?

Gestational age is measured from the first day of the mother's last menstrual cycle because it provides a consistent starting point for tracking development, even though conception occurs about two weeks later.

What are the differences between an embryo and a fetus?

An embryo refers to the early stages of development up to about 8 weeks post-conception, while a fetus is recognized from about 10 weeks onwards, when more recognizable human features begin to develop.

What is viability in the context of fetal development?

Viability refers to the point in fetal development, roughly around six months, where there is a significant chance that the baby could survive outside the womb if born prematurely.