Major Results of Fertilisation

Fertilisation achieves several fundamental outcomes at once, each essential to normal development. Together they define what the multi-step process accomplishes.

The first outcome is restoration of the diploid chromosome number. Each gamete contributes a haploid set of 23 chromosomes. Their union at syngamy re-establishes the diploid complement of 46 chromosomes, 23 homologous pairs, in the zygote. Every pair comprises one maternally derived and one paternally derived homologue.

This restoration is the structural basis of biparental inheritance. It depends on each gamete carrying the correct haploid number, which is reliably met only when meiosis during gametogenesis has proceeded without error.

Because every oocyte carries a single X chromosome while a spermatozoon carries either an X or a Y chromosome, in an approximately equal ratio, the genetic sex of the resulting zygote is determined entirely by which category of spermatozoon achieves fertilisation. Fusion with an X-bearing sperm produces a 46,XX, genetically female, zygote; fusion with a Y-bearing sperm produces a 46,XY, genetically male, zygote.

Fertilisation is therefore the specific event at which an individual's genetic sex is fixed, and this determination is, in the strictest sense, entirely paternal in origin, since the oocyte's own sex chromosome contribution is invariably X in every case.

Fertilisation is the final of three sequential mechanisms that generate the extensive genetic variation of sexually reproducing organisms. The first two, crossing over and independent assortment, occur during meiosis.

Random fertilisation is the unpredictable pairing of one gamete from each parent, out of the very large number of genetically distinct gametes each can produce. It combines two already variable haploid genomes into a single, unique diploid genome.

This is why full siblings, despite sharing the same two parents, are never genetically identical, except for monozygotic twins.

Fertilisation activates the oocyte. It changes the oocyte from a metabolically quiescent cell, arrested at metaphase II and awaiting a triggering signal, into a metabolically and developmentally active cell.

The calcium oscillations triggered by sperm entry drive a broad cascade of downstream changes:

• completion of oocyte meiosis
• the cortical granule reaction, which establishes the block to polyspermy
• general reactivation of the oocyte's metabolic and protein synthetic machinery, resuming processes held in a suppressed state throughout the prolonged period of meiotic quiescence

The final major outcome of fertilisation is initiation of cleavage. This is the series of rapid mitotic divisions that follow formation of the zygote's first mitotic spindle at syngamy.

It marks the transition from fertilisation as a discrete process to the embryo's own developmental sequence. The first cleavage division typically occurs within around 24 to 30 hours of fertilisation.

Every subsequent stage, including cleavage itself, compaction, blastocyst formation and implantation, continues uninterrupted from the developmental programme that fertilisation has switched on.

Initiation of cleavage therefore links fertilisation, a defined, completed event, most directly to embryogenesis, the extended developmental process it exists to begin.