Grades 9–12 · Organisms
How can an egg and sperm begin an embryo?
Human reproduction begins a new developmental sequence when an egg and a sperm combine through fertilization. Sperm develop in testes; eggs develop in ovaries. Each gamete normally contributes one set of 23 chromosomes, compared with the two sets found in most body cells.
In the diagramFertilization begins a new developmental sequenceGametes → zygote → cleavage → implantationFour smaller cells • 46 chromosomes eachThe stages are schematic; success is not guaranteed.
Following ovulation, an egg can enter a uterine tube, also called a fallopian tube. Fertilization usually occurs there. Fusion of the gamete membranes initiates changes in the egg, including completion of its meiotic division and changes that limit entry of additional sperm. The genetic contributions become organized into a zygote, normally with 46 chromosomes.
As the early embryo travels toward the uterus, cell divisions called cleavage produce smaller cells within approximately the original overall extent. DNA is copied before each division, so four cells do not mean that the original chromosomes were simply divided four ways. Each cell normally retains the two chromosome sets.
A fluid-filled blastocyst later forms, with an inner cell group and an outer cell layer. After leaving its surrounding coat, it can attach to and begin entering the prepared uterine lining. This is implantation, with outer cells contributing to structures that support pregnancy. The schematic shows the sequence, not a guarantee that fertilization or implantation will succeed.
About this explanation
Starting knowledge: Understand chromosomes, meiosis, DNA copying and mitosis.
Sources
- OpenStax, Anatomy and Physiology 2e — Fertilization
- OpenStax, Anatomy and Physiology 2e — Embryonic Development
- Eunice Kennedy Shriver National Institute of Child Health and Human Development — Menstruation