4.0 Learning aims, what you should already know, introduction, delving deeper

Learning aims

  • Explaining the ovulation process
  • Knowing the necessary steps which lead to spermatozoa being ready
  • Describing how the enabling of the spermatozoa takes place
  • Describing how the spermatozoon penetrates into the oocyte
  • Knowing the process whereby a zygote is formed

What you should already know

  • How the female genital tract is formed (anatomy)
  • Development of the oocyte in the ovary
  • How the male genital tract is formed (anatomy)
  • Spermatogenesis



Quiz 08

For successful fertilization, a mature spermatozoon (sperm cell) must penetrate into the mature oocyte. At the same time this is also an encounter between one of the smallest human cells with one of the largest: The head of the sperm has a diameter of only 4mm, whereas the egg cell has one of 120mm (0.12 mm!) aufweist.

Fig. 1 - Oocyte with corona radiata Fig. 2 - Native spermatozoa  Legend

Cumulus oophorus

Spermatozoon tail
Spermatozoon head
Seminal plasma

Fig. 1
An Oocyte following ovulation, surrounded by a cloud of cumulus cells, the corona radiata, which form a cloak around it.
© Dr. A. Senn et al, CHUV Lausanne
Fig. 2
Spermatozoa in a native ejaculate. These must still mature in several steps during their way through the female genital tract before they are able to fertilize the oocyte.


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One can speak of a successful fertilization when the genetic material of the male spermatozoon has united with that of the female oocyte thereby creating a new, unique individual.

More info

What Swiss lawmakers say about this topic.

Fig. 3 - Impregnated oocyte Fig. 4 - Zygote  Legend


Male nucleus
Female nucleus closer to the
polar body
A polar body
Pellucid zone

Several polar bodies

Fig. 3
In this oocyte the the two nuclei are just about to fuse.
Since the spermatozoon penetrated into the oocyte, more than 18 - 20 hours have elapsed (the cumulus cells have been shaved away so the oocyte can be better seen).
© Dr. A. Senn et al, CHUV Lausanne
Fig. 4
After the nuclei have fused the spindle apparatus with the chromosomes in the anaphase appears (not shown). The zygote, the first cell of the embryo, immediately divides into two cells.
© Dr. A. Senn et al, CHUV Lausanne

Delving deeper

Key concepts

Terms that are important in the fertilization module.

  • When the spermatozoon leaves the testicle it is not yet capable of fertilizing an oocyte. What changes must it undergo to make it capable of uniting with the oocyte (capacitation)?
  • How does the oocyte get from the ovary into the fallopian tube?
  • What makes it possible for the spermatozoon to penetrate into the oocyte?
  • How is it made sure that a spermatozoon from another species is unable to deposit its genetic material?
  • When the fertilization is completed, the genes of the two parents have united. How does this union of the two genomes take place?

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