Fertilisation in Humans
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Direct answer
In humans, fertilisation occurs at the ampullary-isthmic junction of the fallopian tube, where a motile sperm meets the secondary oocyte released at ovulation. The acrosome at the sperm tip releases enzymes that let it pass through the corona radiata and zona pellucida, and its entry induces the zona pellucida to change so no second sperm can enter — the block to polyspermy. Sperm entry also triggers completion of meiosis II, after which the male and female pronuclei fuse to restore diploidy in the zygote, whose sex was fixed by the fertilising sperm.
What you must remember
- The journey: during insemination semen is deposited in the vagina; the sperm swim through the cervix and uterus and reach the fallopian tube, guided by motility and the tract's contractions.
- The site: the ampullary-isthmic junction, where the ovum is arrested in metaphase II; only a motile, capacitated sperm fertilises it.
- Acrosomal action: enzymes released from the acrosome digest a path through the follicular cells and zona pellucida so the sperm plasma membrane can fuse with the oocyte membrane.
- Polyspermy block: the sperm's entry alters the zona pellucida structure so additional sperm cannot penetrate, ensuring a single paternal genome.
- Completing meiosis: sperm entry finishes the oocyte's second meiotic division, extruding the second polar body and yielding the mature ovum.
- Syngamy: the sperm nucleus (male pronucleus) and ovum nucleus (female pronucleus) fuse — amphimixis — forming the diploid zygote; an X-bearing sperm makes it female (XX) and a Y-bearing sperm male (XY).
- Onward events: the zygote cleaves into a morula as it travels to the uterus, becomes a blastocyst, and implants in the endometrium, with the trophoblast going on to form the placenta.
Common confusion
The fertilisation site is the ampullary-isthmic junction, not the uterus — implantation, not fertilisation, occurs in the uterus. The sperm determines the genetic sex because the oocyte always contributes an X chromosome. And the zona pellucida block is what prevents polyspermy in humans — a post-entry physical change, not a species-specific incompatibility.
Exam-focused takeaway
NEET-UG pins the site of fertilisation, the acrosome's enzymatic role, the polyspermy block and the completion of meiosis II at sperm entry. Statement questions test the sex-determining role of the sperm, the morula-to-blastocyst sequence, and implantation in the endometrium. Assertion-reason stems commonly pair sperm motility with successful transport and pronuclear fusion with restoration of diploidy.
Frequently asked questions
Where does fertilisation occur in humans?
At the ampullary-isthmic junction of the fallopian tube, where the sperm meets the secondary oocyte moving from the ovary.
What prevents more than one sperm from entering the ovum?
Entry of the first sperm induces changes in the zona pellucida that block the entry of any additional sperm — the block to polyspermy.
What happens to the oocyte at the moment of sperm entry?
It completes its arrested second meiotic division, releases the second polar body, and becomes a mature, fertilisable ovum.
What is syngamy?
The fusion of the male and female pronuclei after sperm entry, restoring diploidy and forming the zygote.
Which parent determines the sex of the zygote?
The father — the sperm carries either an X or a Y chromosome, while every ovum carries an X.