Fertilisation, Chromosomes and Early Cleavage

How gametes reach the same region

Human fertilisation is normally internal. Semen introduced into the vagina during insemination contains sperm that can move through the cervix, uterine cavity and oviducts. Movement is supported by sperm flagellar motility and by conditions in the reproductive tract; the majority of deposited sperm never reaches an oocyte. The secondary oocyte released from an ovarian follicle is caught near the fimbriae and carried through the oviduct. The most common site of fertilisation is the ampullary region near the ampulla–isthmus junction. Fertilisation requires compatible, viable gametes to be present there within a suitable interval.

Sperm entering the female tract undergo capacitation, changes in surface composition and movement that prepare them for effective interaction with the oocyte coverings. The oocyte is still usually a secondary oocyte arrested in metaphase II. It is surrounded by extracellular material called zona pellucida and a group of granulosa cells called the corona radiata. The encounter is therefore between a specialised sperm and a complex oocyte–covering assembly, not simply two naked nuclei floating in liquid.

Recognition, penetration and the prevention of polyspermy

During fertilisation a sperm interacts with the oocyte's surrounding cells and zona pellucida. Molecules and enzymes associated with the sperm acrosome participate in passing through these coverings; sperm movement also contributes mechanically. When a sperm fuses with the oocyte plasma membrane, its entry activates intracellular calcium signalling and triggers changes to the oocyte's surrounding structures that prevent additional sperm from fusing. The coordinated block to polyspermy is essential because more than one sperm contributing a nucleus would usually yield an abnormal chromosome complement.

The second meiotic division of the secondary oocyte resumes and completes in response to activation. Because the division is asymmetrical, it produces a large mature haploid female gamete and a small second polar body. The paternal and maternal haploid nuclear sets subsequently come together in a coordinated series of events, establishing the diploid zygote. In the typical human case each gametic nucleus contributes 23 chromosomes, so the new zygote contains 46. Successful sperm–oocyte contact, completion of meiosis II and restoration of diploidy are distinct stages, even though they occur in close succession.

Chromosomal combinations at fertilisation

In the common XX/XY chromosomal system, an ovum contributes an X chromosome because the usual female karyotype is XX. Typical sperm can contribute either an X or a Y chromosome because the usual male karyotype is XY. Fusion of an X-bearing sperm with the egg usually yields an XX zygote; fusion of a Y-bearing sperm usually yields an XY zygote. If the two sperm types are produced in roughly equal proportions and have similar chances of fertilising the egg, each combination has an approximate probability of one-half. This is a statistical model, not a rule for an individual pregnancy.

The sperm's X or Y contribution is the variable in this simplified chromosomal combination, so the person producing the oocyte cannot be scientifically held responsible for whether a sperm carrying X or Y fertilises it. Natural biological variations exist, including atypical sex chromosome complements and different pathways of sexual development; they are beyond this chapter's main chromosomal model. The central genetic principle remains that maternal and paternal gametes each provide one haploid chromosome set and that fertilisation brings these sets together.

Cleavage: repeated mitosis without early growth of the whole embryo

Soon after fertilisation, the zygote begins mitotic divisions called cleavage as it moves through the oviduct toward the uterus. The cells produced are blastomeres. A zygote divides to a two-cell embryo, then to four-cell, eight-cell and larger early stages. During cleavage the total outer volume changes relatively little compared with the increasing cell number, so the individual blastomeres become smaller. Cleavage is not another meiotic event; the diploid chromosome complement is maintained through mitosis within each normally developing blastomere.

An embryo with roughly eight to sixteen or more tightly grouped blastomeres is often described as entering a morula stage. The morula resembles a compact cluster of cells and continues progressing toward the uterus. Subsequent rearrangement of cells and fluid entry produce the blastocyst, which has distinct inner and outer cell populations. The transition from single zygote to morula demonstrates why the genetic and developmental events must be distinguished: fusion establishes the diploid genome, while cleavage distributes replicated genomes among multiple embryonic cells.

Quick Quiz