Read from Fertilisation to Early Development
Follow development in order: fertilisation, cleavage, morula, blastocyst, implantation, then differentiation and organ formation.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Protect the fertilised egg
After one sperm fuses with the oocyte, the zona pellucida is modified to block additional sperm and prevent polyspermy. Its glycoprotein coat also prevents premature adhesion before the embryo reaches the uterus.
Track cleavage without losing the totals
Human cleavage is holoblastic. Repeated divisions produce smaller blastomeres, then a morula and blastocyst with outer trophoblast and inner cell mass. Each blastomere has a similar genome, while the total DNA across the embryo increases with cell number. The blastocyst is not wholly totipotent.
From layers to organs
Organogenesis occurs as germ-layer cells differentiate and express different genes. The early foetus develops limbs and digits by the end of the second month. Monozygotic twins arise when one early embryo splits; two separately fertilised ova produce dizygotic twins.
Add the support membranes
Extraembryonic membranes distribute essential support jobs: the yolk sac is a food source, the chorion provides the outer gas-exchange interface (and contributes to the placenta in mammals), the allantois stores metabolic waste and contributes to exchange, and the amnion encloses the embryo in protective fluid. Match the role before matching the option code.
OlympIQ exam lens
- ✓Zona pellucida helps block polyspermy and early adhesion.
- ✓Human cleavage is holoblastic, not meroblastic.
- ✓Morula precedes blastocyst; blastocyst has trophoblast and inner cell mass.
- ✓Differentiated gene expression drives organogenesis.
- ✓Monozygotic twins come from one zygote; dizygotic twins from two.
- ✓Yolk sac: food; chorion: exchange; allantois: waste; amnion: protective fluid.
Separate DNA per cell from total DNA in the whole embryo, and do not label every blastocyst cell totipotent.
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