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 fertilisation occurs.

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

Look at the image of the mammalian gametes. a) Identify the numbered structures and explain the role of each during fertilisation. 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 fertilisation, 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.

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 analysing 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 fertilisation mechanisms, retaining external fertilisation 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 fertilisation 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 fertilisation alone was not sufficient for vertebrates to become completely independent of damp habitats.
3. c) Analyse how gravity, desiccation, and gas exchange interact in the aerial environment to impose the observed chronological evolutionary sequence: 1st internal fertilisation, 2nd amniotic egg.

A conservation team seeks to save the northern white rhinoceros from extinction, a species of which only two females survive that are infertile for gestation, but for which frozen semen samples from several males and skin fibroblast cultures from twelve deceased individuals are preserved.

Two biotechnological strategies are evaluated:

• Strategy A: In vitro fertilisation (IVF) using intracytoplasmic sperm injection (ICSI) with oocytes retrieved from the living females and frozen semen, transferring the embryos to southern white rhino females as surrogate gestators.

• Strategy B: Somatic cell nuclear transfer (cloning) using the frozen fibroblasts and enucleated oocytes from the southern white rhino.

1. a) Compare both strategies with respect to future genetic variability: which one allows for greater allelic diversity in the resulting offspring, and why would cloning be detrimental as a sole long-term strategy?
2. b) From the standpoint of cell biology and embryonic development, what specific molecular barriers associated with epigenetic reprogramming of the donor nucleus explain the very high rate of miscarriages and congenital malformations typical of Strategy B compared to Strategy A?
3. c) Evaluate the bioethical limits of applying these technologies: if a company offered to commercialise the cloning of deceased pets, what biological and animal welfare objections would you base your advice against this practice on?