Preventing polyspermy, the blastocyst and implantation

Organisms (Continuity and change) · Reproduction · note 9 of 11

Spec D3.1.15, D3.1.16

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Preventing polyspermy, the blastocyst and implantationSpec D3.1.15, D3.1.16

In short

Polyspermy, fertilization of an egg by more than one sperm, is prevented by the cortical reaction. The acrosome reaction lets a sperm digest a path through the zona pellucida; when it fuses with the egg, cortical granules release enzymes that change the zona pellucida so other sperm cannot pass. The embryo then forms a blastocyst, which implants in the endometrium.

Mechanisms to prevent polyspermy

An egg with more than one sperm nucleus would have too many chromosomes and could not develop normally. Two reactions ensure only one sperm fertilizes each egg.

Acrosome reaction: when a sperm reaches the zona pellucida (the glycoprotein layer around the egg), its acrosome releases hydrolytic enzymes that digest a path through it, so the sperm can penetrate the zona pellucida and reach the egg cell membrane.
The sperm cell membrane fuses with the egg cell membrane.
Cortical reaction: this fusion causes cortical granules just under the egg membrane to release their contents by exocytosis. Enzymes from them alter the zona pellucida so that it hardens and its sperm-binding sites are destroyed, so other sperm cannot pass through.

Development of a blastocyst and implantation

The zygote divides repeatedly by mitosis as it is moved along the oviduct towards the uterus.
After about five days it has become a blastocyst: a hollow ball of cells with an outer layer (trophoblast), an inner cell mass that will form the embryo, and a fluid-filled cavity.
At about day 7 the blastocyst sinks into the thickened endometrium: this is implantation. Trophoblast cells invade the endometrium, giving the embryo nutrients and oxygen and later forming part of the placenta.

Left: a blastocyst at about 5 days, with the trophoblast (outer layer), inner cell mass and fluid-filled cavity. Right: implantation at about 7 days, with the blastocyst sunk into the endometrium and trophoblast cells invading it near maternal blood vessels. — The blastocyst forms after about 5 days and implants in the endometrium at about day 7. The inner cell mass becomes the embryo.

Exam tip:

The names of other embryo stages are not required. Know the blastocyst, its parts and implantation.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between sexual and asexual reproduction?

Asexual reproduction uses one parent and produces genetically identical offspring, while sexual reproduction fuses gametes to give offspring new gene combinations. Asexual reproduction suits a parent already adapted to an existing environment. Sexual reproduction produces the variation needed for a population to adapt when the environment changes, because meiosis and fertilization reshuffle alleles.

How do hormones control the menstrual cycle?

FSH from the pituitary stimulates a follicle to grow and secrete oestradiol, which thickens the endometrium. High oestradiol causes an LH surge by positive feedback, triggering ovulation. The corpus luteum then secretes progesterone, which maintains the endometrium and inhibits FSH and LH by negative feedback. When progesterone falls, menstruation begins.

Why are sperm small and eggs large?

Sperm are small because the male gamete has to travel to the female gamete, so it carries very little cytoplasm or food reserves. The egg does not move, so it can be large and store food for the early embryo. As a result, males produce far more gametes than females.

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