Reproduction and Developmental Biology
This domain covers gamete formation, fertilisation, cleavage, gastrulation, organogenesis, placentation and metamorphosis. Questions often ask you to relate yolk distribution to cleavage or a hormone to a developmental event.
Core concepts
- Spermatogenesis produces four haploid sperm from one primary spermatocyte; oogenesis produces one ovum and polar bodies. Oogenesis begins before birth and arrests at prophase I; meiosis II completes only after fertilisation.
- Hormonal control: FSH stimulates follicle growth; LH triggers ovulation and forms the corpus luteum; progesterone maintains the endometrium.
- Fertilisation: sperm acrosome enzymes digest the zona pellucida; the cortical reaction blocks polyspermy.
- Cleavage: holoblastic in microlecithal and mesolecithal eggs; meroblastic in heavy-yolk eggs. Patterns include radial, spiral, bilateral and discoidal.
- Gastrulation forms ectoderm, mesoderm and endoderm; neurulation follows. The placenta has chorionic villi and secretes hCG, progesterone and oestrogen.
- Metamorphosis: amphibian metamorphosis is driven by thyroid hormones; insect moulting by ecdysone, with juvenile hormone maintaining larval traits.
Worked example
A frog egg and a chick egg are both fertilised, but the frog shows holoblastic cleavage and the chick shows meroblastic discoidal cleavage. Reasoning: frog yolk is moderate and concentrated toward one pole (mesolecithal), so cleavage furrows pass through the whole egg; chick yolk is massive, so cleavage is confined to the small cytoplasmic disc.
Traps
Fertilisation triggers completion of meiosis II in the oocyte. hCG maintains the corpus luteum early in pregnancy. Placental blood does not normally mix. Metamorphosis is not confined to amphibians. Cleavage does not increase embryo size.