Fertilization and the first week: from zygote to blastocyst
Fertilization is the fusion of the sperm nucleus with the egg nucleus to form the zygote. The egg is ready for fertilization for a short time after it is released on about day 14 (the book gives 1 to 2 days), and fertilization takes place in the upper third of the Fallopian tube. The book gives 300 to 500 million sperm in one release, and most are lost on the way, so only a few reach the egg. Together the sperm release the enzyme hyaluronidase, which dissolves part of the egg’s coat, but only one sperm enters, its head and neck. Sperm can stay alive for about 2 to 3 days in the female system. After one sperm has entered, the egg makes a coat that stops others entering. About 24 hours later the zygote divides by mitosis into 2 cells, and the next day into 4. Cleavage then speeds up and makes a solid mass of small cells, the morula. Cilia and muscle contractions push it along the Fallopian tube. It changes gradually into a hollow ball, the blastocyst, which reaches the uterus and settles, or implants, among the folds of its thick endometrium at the end of the first week. The rich blood supply of the endometrium feeds the embryo for the nine months.
| Description | Stage |
|---|---|
| solid mass of small cells | morula |
| hollow ball of cells | blastocyst |
| one cell with 46 chromosomes after fusion | zygote |
| settling in the endometrium | implantation |
Make a timeline poster, with a grown-up. Draw a line from day 0 to day 7. Mark: fertilization at day 0 (zygote), about day 1 (2 cells), about day 2 (4 cells), morula, blastocyst, and implantation at the end of the week. Under each, write the number of cells you would expect if the number doubled each time and one fact from this lesson. Keep to the biology of the lesson and ask a doctor if a question concerns a person’s own health.