A biology teacher describes what happens to a human egg cell after it has been fertilised by a sperm cell in the oviduct, up to the point where the resulting embryo has implanted in the wall of the uterus. (a) State what is meant by the term fertilisation, and state where in the…

Answer: ((a)) Fertilisation is the fusion of the nucleus of a sperm cell (the male gamete) with the nucleus of an egg cell (the female gamete), producing a single cell (a zygote) with a full set of chromosomes. This normally takes place in the oviduct. ((b)) The zygote divides repeatedly, producing a growing ball of genetically identical cells, while it is moved slowly along the oviduct towards the uterus; by the time it reaches the uterus it has become a small embryo, ready to implant in the uterus lining. ((c)) Implantation is the process by which the embryo embeds itself into the thickened, blood-supplied lining of the uterus wall. Implantation is necessary because it attaches the embryo to the mother's blood supply, from which the placenta later develops, allowing the embryo to obtai…. Part (a): Defining fertilisation and where it occurs Fertilisation is the fusion of the nucleus of a sperm cell (the male gamete) with the nucleus of an egg cell (the female gamete). This fusion produces a single new cell, the zygote, whic…

IGCSE Biology 0610: Fertilisation, the Zygote and Implantation

Part (a): Defining fertilisation and where it occurs

Fertilisation is the fusion of the nucleus of a sperm cell (the male gamete) with the nucleus of an egg cell (the female gamete). This fusion produces a single new cell, the zygote, which has a full set of chromosomes made up of contributions from both parents. In humans, fertilisation normally takes place in the oviduct, shortly after an egg cell has been released from the ovary and sperm cells have swum up from the vagina.

Part (b): From zygote to embryo, on the way to the uterus

Once formed, the zygote does not remain a single cell. It begins to divide repeatedly, each division producing more cells that are genetically identical to one another, since this division is by ordinary cell division rather than involving any further fusion of gametes. This produces a small, solid ball of cells. While this division is taking place, the developing structure is carried slowly along the oviduct, away from the ovary and towards the uterus. By the time it arrives in the uterus, several days after fertilisation, it has developed from a single zygote into a small embryo, made up of many cells, ready to implant.

Part (c): Implantation and why it matters

Implantation is the process by which the embryo sinks into and embeds itself in the thickened, blood-supplied lining of the wall of the uterus.

Implantation is essential for the pregnancy to continue because it is only once the embryo is embedded in this lining that it becomes closely associated with the mother’s blood supply. This connection is what allows the placenta to develop, through which the embryo (and later the fetus) obtains the oxygen and nutrients it needs from the mother’s blood in order to keep growing. An embryo that fails to implant has no way of accessing these supplies from the mother, so it cannot continue developing and the pregnancy cannot proceed.

Final answers

(a) Fertilisation is the fusion of a sperm cell’s nucleus with an egg cell’s nucleus, forming a zygote; it normally occurs in the oviduct.
(b) The zygote divides repeatedly into a ball of genetically identical cells while moving along the oviduct to the uterus, becoming a small embryo.
(c) Implantation is the embryo embedding itself in the uterus lining; it is necessary because it connects the embryo to the mother’s blood supply (via the placenta) for oxygen and nutrients, without which the embryo cannot survive.