Exercises: Gametes, fertilisation and development

Describe the structure of a mammalian sperm cell (head, midpiece, and flagellum) and egg cell (large size and yolk reserves), and relate each feature to its function: motility versus nourishment for the embryo.

Name the gonads and the exit ducts for gametes in the mammalian reproductive system, and indicate at which point in the female tract fertilization occurs.

Put the stages of fertilization in order — fusion of gamete nuclei, gamete encounter, egg activation, and sperm penetration — and explain in one sentence what happens during each.

Explain what meiosis is and why it is indispensable for producing gametes: what would happen to the species' chromosome number in each generation if gametes were diploid?

Distinguish between zygote, embryo and fetus in the development of a mammal, indicating the approximate limits of each stage and which processes dominate in them (division, organogenesis, growth).

What is the placenta and which three vital functions does it perform for the fetus (nutrition, respiration, excretion)? Explain why it is important that the blood of the mother and of the fetus do not mix directly.

Explain what the amniotic sac and the amniotic fluid are, and which functions they fulfill during pregnancy (mechanical protection, stable temperature, freedom of movement).

Distinguish between monozygotic (identical) twins and dizygotic (fraternal) twins: how each type originates, what genetic similarity they have, and why the former are always of the same sex and the latter not necessarily.

Look at the image of the mammalian gametes. a) Identify the numbered structures and explain the role of each during fertilization. b) Reason why the egg cell is thousands of times larger than the sperm cell and what each gamete contributes to the zygote. c) Indicate which barriers the sperm must cross before fusing with the egg nucleus.

The amniotic egg of reptiles and birds incorporates extraembryonic membranes: amnion, chorion, allantois, and yolk sac. Explain the function of each membrane and reason why this adaptation freed vertebrates from reproducing in water.

Following fertilization, the zygote undergoes cleavage and subsequently gastrulation. Explain what each process involves, what the germ layers (ectoderm, mesoderm, and endoderm) are, and name an adult organ derived from each.

Describe the process of human fertilization: where it occurs (the fallopian tube), what the acrosome reaction is, and which mechanisms prevent polyspermy (the entry of more than one sperm).

The first weeks of pregnancy are the most sensitive. Explain what organogenesis is, why teratogens (alcohol, some medications, infections such as rubella) damage mainly in the first trimester, and what fetal alcohol syndrome is.

A biological expedition discovers in an isolated cave two related amphibian species with very different reproductive strategies:

• Species A: Lays about 1,500 small eggs (diameter < 1.5 mm) that are yolk-poor in temporary ponds with abundant predators.

• Species B: Lays barely 12 large eggs (diameter > 6.0 mm), very rich in yolk, in protected damp crevices, from which fully formed individuals hatch without passing through a free tadpole stage.

1. a) Justify in terms of bioenergetic cost and survival why Species B has been able to dispense with metamorphosis and the aquatic larval stage, relating this to egg type (amount of yolk) and development type.
2. b) A researcher states: "Species A is evolutionarily more efficient because indirect development always implies a lower reproductive cost for the mother." Refute this statement by analyzing the adaptive advantages and disadvantages of each strategy depending on environmental stability.
3. c) Resolve the following apparent paradox: eutherian mammals have an egg virtually devoid of yolk (alecithal) and yet exhibit direct development without a larval stage. How have they resolved this evolutionary conflict?

Imagine a hypothetical evolutionary scenario: on an Earth-like planet, a group of aquatic animals first evolves an egg with a waterproof shell and extraembryonic membranes (amniotic) before having developed internal fertilization mechanisms, retaining external fertilization in the water.

1. a) Assess the biophysical feasibility of this scenario: why does the emergence of an egg with an impermeable protective shell prior to internal fertilization represent an insurmountable biological contradiction for the process of gametic penetration?
2. b) Some modern amphibians deposit their clutches on leaves or carry tadpoles on their skin out of open water, but none possess a truly amniotic egg. Based on this fact, justify why internal fertilization alone was not sufficient for vertebrates to become completely independent of damp habitats.
3. c) Analyze how gravity, desiccation, and gas exchange interact in the aerial environment to impose the observed chronological evolutionary sequence: 1st internal fertilization, 2nd amniotic egg.