Human Anatomy
Gametogenesis and Fertilisation
Gametogenesis and fertilisation for MBBS: spermatogenesis, oogenesis, ovarian and menstrual cycles and the process of fertilisation, mapped to NMC codes AN77.1 to AN77.6.
MedNext Academy | 3 min read
Gametogenesis and Fertilisation
Gametogenesis and fertilisation for MBBS: spermatogenesis, oogenesis, ovarian and menstrual cycles and the process of fertilisation, mapped to NMC codes AN77.1 to AN77.6.
This chapter covers the formation of male and female gametes by meiosis, the synchronised ovarian and menstrual cycles, and the process and consequences of fertilisation. It also introduces the anatomical basis of contraception and factors affecting fertility.
High-yield: Gametogenesis and Fertilisation
- Gametogenesis is the formation of the gametes, spermatozoa in the male and oocytes in the female, by meiosis.
- Meiosis halves the chromosome number so that each gamete is haploid with 23 chromosomes.
- Spermatogenesis begins at puberty and continues throughout life, producing four functional sperm from each primary spermatocyte.
- Oogenesis begins in fetal life, arrests, and produces only one functional ovum from each primary oocyte plus polar bodies.
- The primary oocyte is arrested in prophase of the first meiotic division until puberty.
- The secondary oocyte is arrested in metaphase of the second meiotic division and completes it only if fertilised.
- The ovarian cycle has follicular, ovulatory and luteal phases driven by the pituitary gonadotrophins.
- Ovulation releases the secondary oocyte at about the fourteenth day of a 28-day cycle.
- The menstrual cycle of the endometrium runs through menstrual, proliferative and secretory phases.
- The proliferative phase is driven by oestrogen and the secretory phase by progesterone from the corpus luteum.
- Fertilisation normally occurs in the ampulla of the uterine tube within a day of ovulation.
- The acrosome reaction lets the sperm penetrate the zona pellucida, and the cortical reaction then blocks polyspermy.
- Fertilisation restores the diploid number, determines the sex and initiates cleavage of the zygote.
- Contraceptive methods act by preventing ovulation, blocking sperm transport or preventing implantation.
- Teratogens act most severely during organogenesis, and factors affecting fertility and sterility have social significance.
Spermatogenesis versus oogenesis
- **Timing:** Spermatogenesis starts at puberty and is lifelong; oogenesis begins in fetal life and arrests until puberty.
- **Products:** One primary spermatocyte gives four sperm; one primary oocyte gives one ovum and polar bodies.
- **Meiotic arrest:** Primary oocyte arrested in prophase one; secondary oocyte arrested in metaphase two until fertilisation.
- **Fertilisation:** Occurs in the ampulla; restores diploidy, determines sex, blocks polyspermy by the cortical reaction.
NMC competencies in this chapter
- **AN77.1:** Uterine changes during the menstrual cycle
- **AN77.2:** Synchrony between the ovarian and menstrual cycles
- **AN77.3:** Spermatogenesis and oogenesis
- **AN77.4:** Stages and consequences of fertilisation
- **AN77.5:** Anatomical principles underlying contraception
- **AN77.6:** Teratogenic influences, fertility and sterility, surrogacy and the significance of the sex ratio
Frequently Asked Questions
How do spermatogenesis and oogenesis differ?
Spermatogenesis starts at puberty, continues for life and yields four sperm from each primary spermatocyte. Oogenesis starts before birth, arrests, and yields one ovum and polar bodies from each primary oocyte.
Where does fertilisation normally occur?
In the ampulla of the uterine tube, usually within about a day of ovulation, when a sperm penetrates the secondary oocyte.
What prevents fertilisation by more than one sperm?
The cortical reaction. After the first sperm enters, cortical granules are released that alter the zona pellucida, forming a block to further sperm penetration, known as the block to polyspermy.
What are the consequences of fertilisation?
It restores the diploid chromosome number, determines the genetic sex of the embryo through the sperm, and triggers the first cleavage divisions of the zygote.
Continue reading
MBBSAll Human Anatomy chapters
Continue through the Human Anatomy chapter map.
Continue studying Human Anatomy
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans