To create these strains of mice, scientists start at the beginning, with an egg cell.
Acquiring such a number of human egg cells would be difficult and expensive.
Before a female fetus is born, she has up to seven million egg cells.
They produce egg cells and are the sites for fertilization.
The length of time these egg cells are kept inside the female's body is called the gestation period.
A fertilized egg cell is a special case, but it?
Except why bother with girls, if you don't need mothers to lay those little egg cells in the first place?
A baby girl is born carrying all the egg cells she will produce through her life.
As a woman ages, so do the egg cells from which the ova are produced.
Financial gain is of course a further reason why egg cells are sold and embryos manipulated.