Glossary: Sexual Reproduction, Development and Growth in Humans and Animals

This chapter's terms in context

Chapter 15 covers four terms describing human reproduction and the early stages of development: fertilisation, zygote, menstrual cycle and placenta. Fertilisation is the fusion of a haploid sperm nucleus with a haploid egg (ovum) nucleus, restoring the full diploid chromosome number in a single new cell; in humans, fertilisation normally occurs in the oviduct, and only one sperm normally fertilises one egg because a fertilisation membrane forms rapidly around the egg surface once the first sperm has entered, preventing further sperm from penetrating.

The single cell produced by fertilisation is called a zygote, and it is diploid, genetically distinct from both parents, and the very first cell of a new individual; it does not remain a single cell for long, dividing repeatedly by mitosis as it travels down the oviduct towards the uterus, first into a solid ball of cells and then into a hollow ball called a blastocyst that implants into the wall of the uterus. A common exam error is using 'zygote' and 'embryo' interchangeably; strictly, the zygote is specifically the single fertilised cell before its first division, while embryo describes the developing individual once cell division has begun.

The menstrual cycle is the roughly monthly cycle of hormonal and physical changes in a woman's reproductive system, coordinating the release of an egg from an ovary (ovulation) with the thickening of the uterine lining in preparation for a possible pregnancy; if fertilisation does not occur, the thickened lining breaks down and is shed as menstrual bleeding, and the cycle then begins again. Understanding this cycle as a coordinated hormonal timeline, rather than a list of disconnected events, is what SPM structured questions on this chapter usually reward, particularly questions relating the timing of ovulation to the most fertile period of the cycle.

The placenta is the specialised organ that develops in the uterus once implantation has occurred, forming the interface between the mother's blood supply and that of the developing foetus; it allows oxygen, glucose, amino acids and other nutrients to diffuse from the mother's blood into the foetal blood, and allows carbon dioxide and other waste products to diffuse in the opposite direction, while keeping the two blood supplies physically separate rather than mixing them directly. Students frequently and incorrectly state that maternal and foetal blood mix in the placenta; the correct SPM-level answer is that the two blood systems remain separate, with exchange occurring purely by diffusion across the placental membranes, a distinction examiners specifically look for.

Sequencing these four terms correctly, fertilisation produces a zygote, the zygote implants and grows with support from the placenta, and the menstrual cycle governs the timing that made fertilisation possible in the first place, gives a clear narrative for answering any extended question on human reproduction and development, rather than four isolated definitions.

Paper 2 questions on this chapter frequently ask a candidate to explain a stage in early human development using a labelled diagram of the female reproductive system or a simplified diagram of the placenta, rather than only asking for a written definition. A common scoring gap is correctly identifying structures on the diagram but failing to explain the reasoning behind an event, such as explaining why implantation must occur in the uterus specifically rather than elsewhere in the reproductive tract, or why a menstrual period indicates that fertilisation did not occur that cycle. Linking each structure on a diagram to the specific event happening at that location and stage, ovary to ovulation, oviduct to fertilisation, uterus to implantation, placenta to nutrient exchange, is the reasoning skill these diagram-based questions are actually testing.